261 lines
10 KiB
Plaintext
261 lines
10 KiB
Plaintext
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== 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 (only valid value is 2)
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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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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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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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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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The remaining space between the end of the header extension area and the end of
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the first cluster can be used for other data. Usually, the backing file name is
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stored there.
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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 / sizeof(uint16_t))
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refcount_block_index = (offset / cluster_size) % refcount_table_entries
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refcount_table_index = (offset / cluster_size) / refcount_table_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:
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Bit 0 - 15: Reference count of the cluster
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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 (for normal clusters):
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Bit 0 - 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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62: 0 (this cluster is not compressed)
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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 the active L1
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table.
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L2 table entry (for compressed clusters; x = 62 - (cluster_size - 8)):
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Bit 0 - x: Host cluster offset. This is usually _not_ aligned to a
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cluster boundary!
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x+1 - 61: Compressed size of the images in sectors of 512 bytes
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62: 1 (this cluster is compressed using zlib)
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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 the active L1
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table.
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If a cluster is unallocated, read requests shall read the data from the backing
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file. If there is no backing file or the backing file is smaller than the image,
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they shall read 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 form 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. Must be ignored.
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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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