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Revision f9cb2860

IDf9cb2860bd12e223036b61a1f5d1444e57b7efce

Added by Stefan Hajnoczi about 11 years ago

qcow2: drop flush in update_cluster_refcount()

The update_cluster_refcount() function increments/decrements a cluster's
refcount and then returns the new refcount value.

There is no need to flush since both update_cluster_refcount() callers
already take care of this:

1. qcow2_alloc_bytes() calls update_cluster_refcount() when compressed
sectors will be appended to an existing cluster with enough free
space. qcow2_alloc_bytes() already flushes so there is no need to do
so in update_cluster_refcount().

2. qcow2_update_snapshot_refcount() sets a cache dependency on refcounts
if it needs to update L2 entries. It also flushes before completing.

Removing this flush significantly speeds up qcow2 snapshot creation:

$ qemu-img create -f qcow2 test.qcow2 -o size=50G,preallocation=metadata
  $ time qemu-img snapshot -c new test.qcow2

Time drops from more than 3 minutes to under 1 second.

Signed-off-by: Stefan Hajnoczi <>
Signed-off-by: Kevin Wolf <>

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