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[Qemu-devel] [PATCH v2 5/6] qcow2: drop flush in update_cluster_refcount
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PATCH v2 5/6] qcow2: drop flush in update_cluster_refcount() |
Date: |
Mon, 4 Mar 2013 15:02:34 +0100 |
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 <address@hidden>
---
block/qcow2-refcount.c | 2 --
1 file changed, 2 deletions(-)
diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 3d29d30..92519ea 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -532,8 +532,6 @@ static int update_cluster_refcount(BlockDriverState *bs,
return ret;
}
- bdrv_flush(bs->file);
-
return get_refcount(bs, cluster_index);
}
--
1.8.1.4
- [Qemu-devel] [PATCH v2 0/6] qcow2: cache flush fixes and performance improvements, Stefan Hajnoczi, 2013/03/04
- [Qemu-devel] [PATCH v2 1/6] qcow2: flush refcount cache correctly in alloc_refcount_block(), Stefan Hajnoczi, 2013/03/04
- [Qemu-devel] [PATCH v2 2/6] qcow2: flush refcount cache correctly in qcow2_write_snapshots(), Stefan Hajnoczi, 2013/03/04
- [Qemu-devel] [PATCH v2 3/6] qcow2: set L2 cache dependency in qcow2_alloc_bytes(), Stefan Hajnoczi, 2013/03/04
- [Qemu-devel] [PATCH v2 5/6] qcow2: drop flush in update_cluster_refcount(),
Stefan Hajnoczi <=
- [Qemu-devel] [PATCH v2 6/6] qcow2: drop unnecessary flush in qcow2_update_snapshot_refcount(), Stefan Hajnoczi, 2013/03/04
- [Qemu-devel] [PATCH v2 4/6] qcow2: flush in qcow2_update_snapshot_refcount(), Stefan Hajnoczi, 2013/03/04
- Re: [Qemu-devel] [PATCH v2 0/6] qcow2: cache flush fixes and performance improvements, Kevin Wolf, 2013/03/04