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qcow2: Gather clusters in a looping loop

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Instead of just checking once in exactly this order if there are
dependendies, non-COW clusters and new allocation, this starts looping
around these. This way we can, for example, gather non-COW clusters after
new allocations as long as the host cluster offsets stay contiguous.

Once handle_dependencies() is extended so that COW areas of in-flight
allocations can be overwritten, this allows to continue with gathering
other clusters (we wouldn't be able to do that without this change
because we would have missed a possible second dependency in one of the
next clusters).

This means that in the typical sequential write case, we can combine the
COW overwrite of one cluster with the allocation of the next cluster as
soon as something like Delayed COW gets actually implemented. It is only
by avoiding splitting requests this way that Delayed COW actually starts
improving performance noticably.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Kevin Wolf 2013-03-27 11:43:49 +01:00 • committed by Stefan Hajnoczi
parent 2c3b32d256
commit ecdd5333ab
2 changed files with 44 additions and 32 deletions
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2
tests/qemu-iotests/044.out
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@ -1,6 +1,6 @@
No errors were found on the image.
7292415/33554432 = 21.73% allocated, 0.00% fragmented, 0.00% compressed clusters
Image end offset: 4296447488
Image end offset: 4296448000
.
----------------------------------------------------------------------
Ran 1 tests