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author | Stefan Hajnoczi <stefanha@linux.vnet.ibm.com> | 2012-07-27 09:05:22 +0100 |
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committer | Kevin Wolf <kwolf@redhat.com> | 2012-08-06 22:39:14 +0200 |
commit | bfe8043e9214d2fc6572cc72b5f2218308747acd (patch) | |
tree | 40d56873d87524c6b4b062f43851e44ededcae9c /block/qcow2.h | |
parent | 91cf8a35e777cdbdc3bf0d39c319ac9f76b7a296 (diff) | |
download | qemu-bfe8043e9214d2fc6572cc72b5f2218308747acd.zip qemu-bfe8043e9214d2fc6572cc72b5f2218308747acd.tar.gz qemu-bfe8043e9214d2fc6572cc72b5f2218308747acd.tar.bz2 |
qcow2: implement lazy refcounts
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>
Diffstat (limited to 'block/qcow2.h')
-rw-r--r-- | block/qcow2.h | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/block/qcow2.h b/block/qcow2.h index b5fefc0..b4eb654 100644 --- a/block/qcow2.h +++ b/block/qcow2.h @@ -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 |