156b46ded3
This patch fixes several mistakes in the documentation of the
compressed cluster descriptor:
1) the documentation claims that the cluster descriptor contains the
number of sectors used to store the compressed data, but what it
actually contains is the number of sectors *minus one* or, in other
words, the number of additional sectors after the first one.
2) the width of the fields is incorrectly specified. The number of bits
used by each field is
x = 62 - (cluster_bits - 8) for the offset field
y = (cluster_bits - 8) for the size field
So the offset field's location is [0, x-1], not [0, x] as stated.
3) the size field does not contain the size of the compressed data,
but rather the number of sectors where that data is stored. The
compressed data starts at the exact point specified in the offset
field and ends when there's enough data to produce a cluster of
decompressed data. Both points can be in the middle of a sector,
allowing several compressed clusters to be stored next to one
another, sharing sectors if necessary.
Cc: qemu-stable@nongnu.org
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
693 lines
28 KiB
Plaintext
693 lines
28 KiB
Plaintext
== General ==
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A qcow2 image file is organized in units of constant size, which are called
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(host) clusters. A cluster is the unit in which all allocations are done,
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both for actual guest data and for image metadata.
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Likewise, the virtual disk as seen by the guest is divided into (guest)
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clusters of the same size.
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All numbers in qcow2 are stored in Big Endian byte order.
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== Header ==
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The first cluster of a qcow2 image contains the file header:
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Byte 0 - 3: magic
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QCOW magic string ("QFI\xfb")
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4 - 7: version
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Version number (valid values are 2 and 3)
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8 - 15: backing_file_offset
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Offset into the image file at which the backing file name
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is stored (NB: The string is not null terminated). 0 if the
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image doesn't have a backing file.
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16 - 19: backing_file_size
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Length of the backing file name in bytes. Must not be
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longer than 1023 bytes. Undefined if the image doesn't have
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a backing file.
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20 - 23: cluster_bits
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Number of bits that are used for addressing an offset
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within a cluster (1 << cluster_bits is the cluster size).
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Must not be less than 9 (i.e. 512 byte clusters).
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Note: qemu as of today has an implementation limit of 2 MB
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as the maximum cluster size and won't be able to open images
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with larger cluster sizes.
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24 - 31: size
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Virtual disk size in bytes
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32 - 35: crypt_method
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0 for no encryption
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1 for AES encryption
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2 for LUKS encryption
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36 - 39: l1_size
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Number of entries in the active L1 table
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40 - 47: l1_table_offset
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Offset into the image file at which the active L1 table
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starts. Must be aligned to a cluster boundary.
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48 - 55: refcount_table_offset
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Offset into the image file at which the refcount table
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starts. Must be aligned to a cluster boundary.
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56 - 59: refcount_table_clusters
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Number of clusters that the refcount table occupies
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60 - 63: nb_snapshots
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Number of snapshots contained in the image
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64 - 71: snapshots_offset
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Offset into the image file at which the snapshot table
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starts. Must be aligned to a cluster boundary.
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If the version is 3 or higher, the header has the following additional fields.
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For version 2, the values are assumed to be zero, unless specified otherwise
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in the description of a field.
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72 - 79: incompatible_features
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Bitmask of incompatible features. An implementation must
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fail to open an image if an unknown bit is set.
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Bit 0: Dirty bit. If this bit is set then refcounts
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may be inconsistent, make sure to scan L1/L2
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tables to repair refcounts before accessing the
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image.
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Bit 1: Corrupt bit. If this bit is set then any data
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structure may be corrupt and the image must not
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be written to (unless for regaining
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consistency).
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Bits 2-63: Reserved (set to 0)
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80 - 87: compatible_features
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Bitmask of compatible features. An implementation can
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safely ignore any unknown bits that are set.
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Bit 0: Lazy refcounts bit. If this bit is set then
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lazy refcount updates can be used. This means
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marking the image file dirty and postponing
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refcount metadata updates.
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Bits 1-63: Reserved (set to 0)
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88 - 95: autoclear_features
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Bitmask of auto-clear features. An implementation may only
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write to an image with unknown auto-clear features if it
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clears the respective bits from this field first.
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Bit 0: Bitmaps extension bit
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This bit indicates consistency for the bitmaps
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extension data.
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It is an error if this bit is set without the
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bitmaps extension present.
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If the bitmaps extension is present but this
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bit is unset, the bitmaps extension data must be
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considered inconsistent.
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Bits 1-63: Reserved (set to 0)
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96 - 99: refcount_order
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Describes the width of a reference count block entry (width
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in bits: refcount_bits = 1 << refcount_order). For version 2
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images, the order is always assumed to be 4
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(i.e. refcount_bits = 16).
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This value may not exceed 6 (i.e. refcount_bits = 64).
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100 - 103: header_length
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Length of the header structure in bytes. For version 2
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images, the length is always assumed to be 72 bytes.
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Directly after the image header, optional sections called header extensions can
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be stored. Each extension has a structure like the following:
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Byte 0 - 3: Header extension type:
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0x00000000 - End of the header extension area
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0xE2792ACA - Backing file format name
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0x6803f857 - Feature name table
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0x23852875 - Bitmaps extension
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0x0537be77 - Full disk encryption header pointer
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other - Unknown header extension, can be safely
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ignored
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4 - 7: Length of the header extension data
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8 - n: Header extension data
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n - m: Padding to round up the header extension size to the next
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multiple of 8.
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Unless stated otherwise, each header extension type shall appear at most once
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in the same image.
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If the image has a backing file then the backing file name should be stored in
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the remaining space between the end of the header extension area and the end of
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the first cluster. It is not allowed to store other data here, so that an
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implementation can safely modify the header and add extensions without harming
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data of compatible features that it doesn't support. Compatible features that
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need space for additional data can use a header extension.
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== Feature name table ==
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The feature name table is an optional header extension that contains the name
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for features used by the image. It can be used by applications that don't know
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the respective feature (e.g. because the feature was introduced only later) to
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display a useful error message.
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The number of entries in the feature name table is determined by the length of
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the header extension data. Each entry look like this:
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Byte 0: Type of feature (select feature bitmap)
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0: Incompatible feature
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1: Compatible feature
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2: Autoclear feature
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1: Bit number within the selected feature bitmap (valid
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values: 0-63)
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2 - 47: Feature name (padded with zeros, but not necessarily null
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terminated if it has full length)
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== Bitmaps extension ==
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The bitmaps extension is an optional header extension. It provides the ability
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to store bitmaps related to a virtual disk. For now, there is only one bitmap
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type: the dirty tracking bitmap, which tracks virtual disk changes from some
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point in time.
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The data of the extension should be considered consistent only if the
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corresponding auto-clear feature bit is set, see autoclear_features above.
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The fields of the bitmaps extension are:
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Byte 0 - 3: nb_bitmaps
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The number of bitmaps contained in the image. Must be
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greater than or equal to 1.
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Note: Qemu currently only supports up to 65535 bitmaps per
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image.
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4 - 7: Reserved, must be zero.
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8 - 15: bitmap_directory_size
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Size of the bitmap directory in bytes. It is the cumulative
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size of all (nb_bitmaps) bitmap directory entries.
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16 - 23: bitmap_directory_offset
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Offset into the image file at which the bitmap directory
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starts. Must be aligned to a cluster boundary.
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== Full disk encryption header pointer ==
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The full disk encryption header must be present if, and only if, the
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'crypt_method' header requires metadata. Currently this is only true
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of the 'LUKS' crypt method. The header extension must be absent for
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other methods.
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This header provides the offset at which the crypt method can store
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its additional data, as well as the length of such data.
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Byte 0 - 7: Offset into the image file at which the encryption
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header starts in bytes. Must be aligned to a cluster
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boundary.
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Byte 8 - 15: Length of the written encryption header in bytes.
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Note actual space allocated in the qcow2 file may
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be larger than this value, since it will be rounded
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to the nearest multiple of the cluster size. Any
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unused bytes in the allocated space will be initialized
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to 0.
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For the LUKS crypt method, the encryption header works as follows.
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The first 592 bytes of the header clusters will contain the LUKS
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partition header. This is then followed by the key material data areas.
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The size of the key material data areas is determined by the number of
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stripes in the key slot and key size. Refer to the LUKS format
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specification ('docs/on-disk-format.pdf' in the cryptsetup source
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package) for details of the LUKS partition header format.
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In the LUKS partition header, the "payload-offset" field will be
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calculated as normal for the LUKS spec. ie the size of the LUKS
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header, plus key material regions, plus padding, relative to the
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start of the LUKS header. This offset value is not required to be
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qcow2 cluster aligned. Its value is currently never used in the
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context of qcow2, since the qcow2 file format itself defines where
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the real payload offset is, but none the less a valid payload offset
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should always be present.
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In the LUKS key slots header, the "key-material-offset" is relative
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to the start of the LUKS header clusters in the qcow2 container,
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not the start of the qcow2 file.
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Logically the layout looks like
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+-----------------------------+
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| QCow2 header |
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| QCow2 header extension X |
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| QCow2 header extension FDE |
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| QCow2 header extension ... |
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| QCow2 header extension Z |
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+-----------------------------+
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| ....other QCow2 tables.... |
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. .
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. .
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+-----------------------------+
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| +-------------------------+ |
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| | LUKS partition header | |
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| +-------------------------+ |
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| | LUKS key material 1 | |
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| +-------------------------+ |
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| | LUKS key material 2 | |
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| +-------------------------+ |
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| | LUKS key material ... | |
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| +-------------------------+ |
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| | LUKS key material 8 | |
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| +-------------------------+ |
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+-----------------------------+
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| QCow2 cluster payload |
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. .
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. .
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. .
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+-----------------------------+
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== Data encryption ==
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When an encryption method is requested in the header, the image payload
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data must be encrypted/decrypted on every write/read. The image headers
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and metadata are never encrypted.
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The algorithms used for encryption vary depending on the method
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- AES:
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The AES cipher, in CBC mode, with 256 bit keys.
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Initialization vectors generated using plain64 method, with
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the virtual disk sector as the input tweak.
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This format is no longer supported in QEMU system emulators, due
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to a number of design flaws affecting its security. It is only
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supported in the command line tools for the sake of back compatibility
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and data liberation.
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- LUKS:
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The algorithms are specified in the LUKS header.
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Initialization vectors generated using the method specified
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in the LUKS header, with the physical disk sector as the
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input tweak.
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== Host cluster management ==
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qcow2 manages the allocation of host clusters by maintaining a reference count
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for each host cluster. A refcount of 0 means that the cluster is free, 1 means
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that it is used, and >= 2 means that it is used and any write access must
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perform a COW (copy on write) operation.
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The refcounts are managed in a two-level table. The first level is called
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refcount table and has a variable size (which is stored in the header). The
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refcount table can cover multiple clusters, however it needs to be contiguous
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in the image file.
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It contains pointers to the second level structures which are called refcount
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blocks and are exactly one cluster in size.
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Given a offset into the image file, the refcount of its cluster can be obtained
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as follows:
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refcount_block_entries = (cluster_size * 8 / refcount_bits)
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refcount_block_index = (offset / cluster_size) % refcount_block_entries
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refcount_table_index = (offset / cluster_size) / refcount_block_entries
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refcount_block = load_cluster(refcount_table[refcount_table_index]);
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return refcount_block[refcount_block_index];
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Refcount table entry:
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Bit 0 - 8: Reserved (set to 0)
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9 - 63: Bits 9-63 of the offset into the image file at which the
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refcount block starts. Must be aligned to a cluster
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boundary.
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If this is 0, the corresponding refcount block has not yet
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been allocated. All refcounts managed by this refcount block
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are 0.
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Refcount block entry (x = refcount_bits - 1):
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Bit 0 - x: Reference count of the cluster. If refcount_bits implies a
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sub-byte width, note that bit 0 means the least significant
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bit in this context.
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== Cluster mapping ==
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Just as for refcounts, qcow2 uses a two-level structure for the mapping of
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guest clusters to host clusters. They are called L1 and L2 table.
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The L1 table has a variable size (stored in the header) and may use multiple
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clusters, however it must be contiguous in the image file. L2 tables are
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exactly one cluster in size.
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Given a offset into the virtual disk, the offset into the image file can be
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obtained as follows:
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l2_entries = (cluster_size / sizeof(uint64_t))
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l2_index = (offset / cluster_size) % l2_entries
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l1_index = (offset / cluster_size) / l2_entries
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l2_table = load_cluster(l1_table[l1_index]);
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cluster_offset = l2_table[l2_index];
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return cluster_offset + (offset % cluster_size)
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L1 table entry:
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Bit 0 - 8: Reserved (set to 0)
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9 - 55: Bits 9-55 of the offset into the image file at which the L2
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table starts. Must be aligned to a cluster boundary. If the
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offset is 0, the L2 table and all clusters described by this
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L2 table are unallocated.
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56 - 62: Reserved (set to 0)
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63: 0 for an L2 table that is unused or requires COW, 1 if its
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refcount is exactly one. This information is only accurate
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in the active L1 table.
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L2 table entry:
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Bit 0 - 61: Cluster descriptor
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62: 0 for standard clusters
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1 for compressed clusters
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63: 0 for a cluster that is unused or requires COW, 1 if its
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refcount is exactly one. This information is only accurate
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in L2 tables that are reachable from the active L1
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table.
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Standard Cluster Descriptor:
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Bit 0: If set to 1, the cluster reads as all zeros. The host
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cluster offset can be used to describe a preallocation,
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but it won't be used for reading data from this cluster,
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nor is data read from the backing file if the cluster is
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unallocated.
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With version 2, this is always 0.
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1 - 8: Reserved (set to 0)
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9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a
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cluster boundary. If the offset is 0, the cluster is
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unallocated.
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56 - 61: Reserved (set to 0)
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Compressed Clusters Descriptor (x = 62 - (cluster_bits - 8)):
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Bit 0 - x-1: Host cluster offset. This is usually _not_ aligned to a
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cluster or sector boundary!
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x - 61: Number of additional 512-byte sectors used for the
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compressed data, beyond the sector containing the offset
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in the previous field. Some of these sectors may reside
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in the next contiguous host cluster.
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Note that the compressed data does not necessarily occupy
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all of the bytes in the final sector; rather, decompression
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stops when it has produced a cluster of data.
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Another compressed cluster may map to the tail of the final
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sector used by this compressed cluster.
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If a cluster is unallocated, read requests shall read the data from the backing
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file (except if bit 0 in the Standard Cluster Descriptor is set). If there is
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no backing file or the backing file is smaller than the image, they shall read
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zeros for all parts that are not covered by the backing file.
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== Snapshots ==
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qcow2 supports internal snapshots. Their basic principle of operation is to
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switch the active L1 table, so that a different set of host clusters are
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exposed to the guest.
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When creating a snapshot, the L1 table should be copied and the refcount of all
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L2 tables and clusters reachable from this L1 table must be increased, so that
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a write causes a COW and isn't visible in other snapshots.
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When loading a snapshot, bit 63 of all entries in the new active L1 table and
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all L2 tables referenced by it must be reconstructed from the refcount table
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as it doesn't need to be accurate in inactive L1 tables.
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A directory of all snapshots is stored in the snapshot table, a contiguous area
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in the image file, whose starting offset and length are given by the header
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fields snapshots_offset and nb_snapshots. The entries of the snapshot table
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have variable length, depending on the length of ID, name and extra data.
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Snapshot table entry:
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Byte 0 - 7: Offset into the image file at which the L1 table for the
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snapshot starts. Must be aligned to a cluster boundary.
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8 - 11: Number of entries in the L1 table of the snapshots
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12 - 13: Length of the unique ID string describing the snapshot
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14 - 15: Length of the name of the snapshot
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16 - 19: Time at which the snapshot was taken in seconds since the
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Epoch
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20 - 23: Subsecond part of the time at which the snapshot was taken
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in nanoseconds
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24 - 31: Time that the guest was running until the snapshot was
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taken in nanoseconds
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32 - 35: Size of the VM state in bytes. 0 if no VM state is saved.
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If there is VM state, it starts at the first cluster
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described by first L1 table entry that doesn't describe a
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regular guest cluster (i.e. VM state is stored like guest
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disk content, except that it is stored at offsets that are
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larger than the virtual disk presented to the guest)
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36 - 39: Size of extra data in the table entry (used for future
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extensions of the format)
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variable: Extra data for future extensions. Unknown fields must be
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ignored. Currently defined are (offset relative to snapshot
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table entry):
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Byte 40 - 47: Size of the VM state in bytes. 0 if no VM
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state is saved. If this field is present,
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the 32-bit value in bytes 32-35 is ignored.
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Byte 48 - 55: Virtual disk size of the snapshot in bytes
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Version 3 images must include extra data at least up to
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byte 55.
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variable: Unique ID string for the snapshot (not null terminated)
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variable: Name of the snapshot (not null terminated)
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variable: Padding to round up the snapshot table entry size to the
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next multiple of 8.
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== Bitmaps ==
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As mentioned above, the bitmaps extension provides the ability to store bitmaps
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related to a virtual disk. This section describes how these bitmaps are stored.
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All stored bitmaps are related to the virtual disk stored in the same image, so
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each bitmap size is equal to the virtual disk size.
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Each bit of the bitmap is responsible for strictly defined range of the virtual
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disk. For bit number bit_nr the corresponding range (in bytes) will be:
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[bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1]
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Granularity is a property of the concrete bitmap, see below.
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=== Bitmap directory ===
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Each bitmap saved in the image is described in a bitmap directory entry. The
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bitmap directory is a contiguous area in the image file, whose starting offset
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and length are given by the header extension fields bitmap_directory_offset and
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bitmap_directory_size. The entries of the bitmap directory have variable
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length, depending on the lengths of the bitmap name and extra data.
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Structure of a bitmap directory entry:
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Byte 0 - 7: bitmap_table_offset
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Offset into the image file at which the bitmap table
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(described below) for the bitmap starts. Must be aligned to
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a cluster boundary.
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8 - 11: bitmap_table_size
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Number of entries in the bitmap table of the bitmap.
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12 - 15: flags
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Bit
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0: in_use
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The bitmap was not saved correctly and may be
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inconsistent.
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1: auto
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The bitmap must reflect all changes of the virtual
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disk by any application that would write to this qcow2
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file (including writes, snapshot switching, etc.). The
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type of this bitmap must be 'dirty tracking bitmap'.
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2: extra_data_compatible
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This flags is meaningful when the extra data is
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unknown to the software (currently any extra data is
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unknown to Qemu).
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If it is set, the bitmap may be used as expected, extra
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data must be left as is.
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If it is not set, the bitmap must not be used, but
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both it and its extra data be left as is.
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Bits 3 - 31 are reserved and must be 0.
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16: type
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This field describes the sort of the bitmap.
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Values:
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1: Dirty tracking bitmap
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Values 0, 2 - 255 are reserved.
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17: granularity_bits
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Granularity bits. Valid values: 0 - 63.
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Note: Qemu currently supports only values 9 - 31.
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Granularity is calculated as
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granularity = 1 << granularity_bits
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A bitmap's granularity is how many bytes of the image
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accounts for one bit of the bitmap.
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18 - 19: name_size
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Size of the bitmap name. Must be non-zero.
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Note: Qemu currently doesn't support values greater than
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1023.
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20 - 23: extra_data_size
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Size of type-specific extra data.
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For now, as no extra data is defined, extra_data_size is
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reserved and should be zero. If it is non-zero the
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behavior is defined by extra_data_compatible flag.
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variable: extra_data
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Extra data for the bitmap, occupying extra_data_size bytes.
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Extra data must never contain references to clusters or in
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some other way allocate additional clusters.
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variable: name
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The name of the bitmap (not null terminated), occupying
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name_size bytes. Must be unique among all bitmap names
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within the bitmaps extension.
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variable: Padding to round up the bitmap directory entry size to the
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next multiple of 8. All bytes of the padding must be zero.
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=== Bitmap table ===
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Each bitmap is stored using a one-level structure (as opposed to two-level
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structures like for refcounts and guest clusters mapping) for the mapping of
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bitmap data to host clusters. This structure is called the bitmap table.
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Each bitmap table has a variable size (stored in the bitmap directory entry)
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and may use multiple clusters, however, it must be contiguous in the image
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file.
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Structure of a bitmap table entry:
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Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero.
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If bits 9 - 55 are zero:
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0: Cluster should be read as all zeros.
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1: Cluster should be read as all ones.
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1 - 8: Reserved and must be zero.
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9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to
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a cluster boundary. If the offset is 0, the cluster is
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unallocated; in that case, bit 0 determines how this
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cluster should be treated during reads.
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56 - 63: Reserved and must be zero.
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=== Bitmap data ===
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As noted above, bitmap data is stored in separate clusters, described by the
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bitmap table. Given an offset (in bytes) into the bitmap data, the offset into
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the image file can be obtained as follows:
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image_offset(bitmap_data_offset) =
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bitmap_table[bitmap_data_offset / cluster_size] +
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(bitmap_data_offset % cluster_size)
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This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see
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above).
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Given an offset byte_nr into the virtual disk and the bitmap's granularity, the
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bit offset into the image file to the corresponding bit of the bitmap can be
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calculated like this:
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bit_offset(byte_nr) =
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image_offset(byte_nr / granularity / 8) * 8 +
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(byte_nr / granularity) % 8
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If the size of the bitmap data is not a multiple of the cluster size then the
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last cluster of the bitmap data contains some unused tail bits. These bits must
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be zero.
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=== Dirty tracking bitmaps ===
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Bitmaps with 'type' field equal to one are dirty tracking bitmaps.
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When the virtual disk is in use dirty tracking bitmap may be 'enabled' or
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'disabled'. While the bitmap is 'enabled', all writes to the virtual disk
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should be reflected in the bitmap. A set bit in the bitmap means that the
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corresponding range of the virtual disk (see above) was written to while the
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bitmap was 'enabled'. An unset bit means that this range was not written to.
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The software doesn't have to sync the bitmap in the image file with its
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representation in RAM after each write. Flag 'in_use' should be set while the
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bitmap is not synced.
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In the image file the 'enabled' state is reflected by the 'auto' flag. If this
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flag is set, the software must consider the bitmap as 'enabled' and start
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tracking virtual disk changes to this bitmap from the first write to the
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virtual disk. If this flag is not set then the bitmap is disabled.
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