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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
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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