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qcow2: implement lazy refcounts

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Lazy refcounts is a performance optimization for qcow2 that postpones
refcount metadata updates and instead marks the image dirty.  In the
case of crash or power failure the image will be left in a dirty state
and repaired next time it is opened.

Reducing metadata I/O is important for cache=writethrough and
cache=directsync because these modes guarantee that data is on disk
after each write (hence we cannot take advantage of caching updates in
RAM).  Refcount metadata is not needed for guest->file block address
translation and therefore does not need to be on-disk at the time of
write completion - this is the motivation behind the lazy refcount
optimization.

The lazy refcount optimization must be enabled at image creation time:

  qemu-img create -f qcow2 -o compat=1.1,lazy_refcounts=on a.qcow2 10G
  qemu-system-x86_64 -drive if=virtio,file=a.qcow2,cache=writethrough

Update qemu-iotests 031 and 036 since the extension header size changes
when we add feature bit table entries.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2012-07-27 09:05:22 +01:00 committed by Kevin Wolf
parent 91cf8a35e7
commit bfe8043e92
6 changed files with 107 additions and 24 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
block/qcow2-cluster.c
View File

@ -662,7 +662,10 @@ int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
qcow2_cache_depends_on_flush(s->l2_table_cache);
}
qcow2_cache_set_dependency(bs, s->l2_table_cache, s->refcount_block_cache);
if (qcow2_need_accurate_refcounts(s)) {
qcow2_cache_set_dependency(bs, s->l2_table_cache,
s->refcount_block_cache);
}
ret = get_cluster_table(bs, m->offset, &l2_table, &l2_index);
if (ret < 0) {
goto err;

73
block/qcow2.c
View File

@ -214,6 +214,41 @@ static void report_unsupported_feature(BlockDriverState *bs,
}
}
/*
* Sets the dirty bit and flushes afterwards if necessary.
*
* The incompatible_features bit is only set if the image file header was
* updated successfully. Therefore it is not required to check the return
* value of this function.
*/
static int qcow2_mark_dirty(BlockDriverState *bs)
{
BDRVQcowState *s = bs->opaque;
uint64_t val;
int ret;
assert(s->qcow_version >= 3);
if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
return 0; /* already dirty */
}
val = cpu_to_be64(s->incompatible_features | QCOW2_INCOMPAT_DIRTY);
ret = bdrv_pwrite(bs->file, offsetof(QCowHeader, incompatible_features),
&val, sizeof(val));
if (ret < 0) {
return ret;
}
ret = bdrv_flush(bs->file);
if (ret < 0) {
return ret;
}
/* Only treat image as dirty if the header was updated successfully */
s->incompatible_features |= QCOW2_INCOMPAT_DIRTY;
return 0;
}
/*
* Clears the dirty bit and flushes before if necessary. Only call this
* function when there are no pending requests, it does not guard against
@ -752,6 +787,11 @@ static coroutine_fn int qcow2_co_writev(BlockDriverState *bs,
goto fail;
}
if (l2meta.nb_clusters > 0 &&
(s->compatible_features & QCOW2_COMPAT_LAZY_REFCOUNTS)) {
qcow2_mark_dirty(bs);
}
cluster_offset = l2meta.cluster_offset;
assert((cluster_offset & 511) == 0);
@ -994,6 +1034,11 @@ int qcow2_update_header(BlockDriverState *bs)
.bit = QCOW2_INCOMPAT_DIRTY_BITNR,
.name = "dirty bit",
},
{
.type = QCOW2_FEAT_TYPE_COMPATIBLE,
.bit = QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR,
.name = "lazy refcounts",
},
};
ret = header_ext_add(buf, QCOW2_EXT_MAGIC_FEATURE_TABLE,
@ -1176,6 +1221,11 @@ static int qcow2_create2(const char *filename, int64_t total_size,
header.crypt_method = cpu_to_be32(QCOW_CRYPT_NONE);
}
if (flags & BLOCK_FLAG_LAZY_REFCOUNTS) {
header.compatible_features |=
cpu_to_be64(QCOW2_COMPAT_LAZY_REFCOUNTS);
}
ret = bdrv_pwrite(bs, 0, &header, sizeof(header));
if (ret < 0) {
goto out;
@ -1289,6 +1339,8 @@ static int qcow2_create(const char *filename, QEMUOptionParameter *options)
options->value.s);
return -EINVAL;
}
} else if (!strcmp(options->name, BLOCK_OPT_LAZY_REFCOUNTS)) {
flags |= options->value.n ? BLOCK_FLAG_LAZY_REFCOUNTS : 0;
}
options++;
}
@ -1299,6 +1351,12 @@ static int qcow2_create(const char *filename, QEMUOptionParameter *options)
return -EINVAL;
}
if (version < 3 && (flags & BLOCK_FLAG_LAZY_REFCOUNTS)) {
fprintf(stderr, "Lazy refcounts only supported with compatibility "
"level 1.1 and above (use compat=1.1 or greater)\n");
return -EINVAL;
}
return qcow2_create2(filename, sectors, backing_file, backing_fmt, flags,
cluster_size, prealloc, options, version);
}
@ -1485,10 +1543,12 @@ static coroutine_fn int qcow2_co_flush_to_os(BlockDriverState *bs)
return ret;
}
ret = qcow2_cache_flush(bs, s->refcount_block_cache);
if (ret < 0) {
qemu_co_mutex_unlock(&s->lock);
return ret;
if (qcow2_need_accurate_refcounts(s)) {
ret = qcow2_cache_flush(bs, s->refcount_block_cache);
if (ret < 0) {
qemu_co_mutex_unlock(&s->lock);
return ret;
}
}
qemu_co_mutex_unlock(&s->lock);
@ -1603,6 +1663,11 @@ static QEMUOptionParameter qcow2_create_options[] = {
.type = OPT_STRING,
.help = "Preallocation mode (allowed values: off, metadata)"
},
{
.name = BLOCK_OPT_LAZY_REFCOUNTS,
.type = OPT_FLAG,
.help = "Postpone refcount updates",
},
{ NULL }
};

13
block/qcow2.h
View File

@ -118,6 +118,14 @@ enum {
QCOW2_INCOMPAT_MASK = QCOW2_INCOMPAT_DIRTY,
};
/* Compatible feature bits */
enum {
QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR = 0,
QCOW2_COMPAT_LAZY_REFCOUNTS = 1 << QCOW2_COMPAT_LAZY_REFCOUNTS_BITNR,
QCOW2_COMPAT_FEAT_MASK = QCOW2_COMPAT_LAZY_REFCOUNTS,
};
typedef struct Qcow2Feature {
uint8_t type;
uint8_t bit;
@ -245,6 +253,11 @@ static inline int qcow2_get_cluster_type(uint64_t l2_entry)
}
}
/* Check whether refcounts are eager or lazy */
static inline bool qcow2_need_accurate_refcounts(BDRVQcowState *s)
{
return !(s->incompatible_features & QCOW2_INCOMPAT_DIRTY);
}
// FIXME Need qcow2_ prefix to global functions

26
block_int.h
View File

@ -31,8 +31,9 @@
#include "qemu-timer.h"
#include "qapi-types.h"
#define BLOCK_FLAG_ENCRYPT 1
#define BLOCK_FLAG_COMPAT6 4
#define BLOCK_FLAG_ENCRYPT 1
#define BLOCK_FLAG_COMPAT6 4
#define BLOCK_FLAG_LAZY_REFCOUNTS 8
#define BLOCK_IO_LIMIT_READ 0
#define BLOCK_IO_LIMIT_WRITE 1
@ -41,16 +42,17 @@
#define BLOCK_IO_SLICE_TIME 100000000
#define NANOSECONDS_PER_SECOND 1000000000.0
#define BLOCK_OPT_SIZE "size"
#define BLOCK_OPT_ENCRYPT "encryption"
#define BLOCK_OPT_COMPAT6 "compat6"
#define BLOCK_OPT_BACKING_FILE "backing_file"
#define BLOCK_OPT_BACKING_FMT "backing_fmt"
#define BLOCK_OPT_CLUSTER_SIZE "cluster_size"
#define BLOCK_OPT_TABLE_SIZE "table_size"
#define BLOCK_OPT_PREALLOC "preallocation"
#define BLOCK_OPT_SUBFMT "subformat"
#define BLOCK_OPT_COMPAT_LEVEL "compat"
#define BLOCK_OPT_SIZE "size"
#define BLOCK_OPT_ENCRYPT "encryption"
#define BLOCK_OPT_COMPAT6 "compat6"
#define BLOCK_OPT_BACKING_FILE "backing_file"
#define BLOCK_OPT_BACKING_FMT "backing_fmt"
#define BLOCK_OPT_CLUSTER_SIZE "cluster_size"
#define BLOCK_OPT_TABLE_SIZE "table_size"
#define BLOCK_OPT_PREALLOC "preallocation"
#define BLOCK_OPT_SUBFMT "subformat"
#define BLOCK_OPT_COMPAT_LEVEL "compat"
#define BLOCK_OPT_LAZY_REFCOUNTS "lazy_refcounts"
typedef struct BdrvTrackedRequest BdrvTrackedRequest;

12
tests/qemu-iotests/031.out
View File

@ -54,7 +54,7 @@ header_length 72
Header extension:
magic 0x6803f857
length 48
length 96
data <binary>
Header extension:
@ -68,7 +68,7 @@ No errors were found on the image.
magic 0x514649fb
version 2
backing_file_offset 0xc8
backing_file_offset 0xf8
backing_file_size 0x17
cluster_bits 16
size 67108864
@ -92,7 +92,7 @@ data 'host_device'
Header extension:
magic 0x6803f857
length 48
length 96
data <binary>
Header extension:
@ -155,7 +155,7 @@ header_length 104
Header extension:
magic 0x6803f857
length 48
length 96
data <binary>
Header extension:
@ -169,7 +169,7 @@ No errors were found on the image.
magic 0x514649fb
version 3
backing_file_offset 0xe8
backing_file_offset 0x118
backing_file_size 0x17
cluster_bits 16
size 67108864
@ -193,7 +193,7 @@ data 'host_device'
Header extension:
magic 0x6803f857
length 48
length 96
data <binary>
Header extension:

2
tests/qemu-iotests/036.out
View File

@ -46,7 +46,7 @@ header_length 104
Header extension:
magic 0x6803f857
length 48
length 96
data <binary>
*** done