On Wed, 22 Jun 2011 10:56:07 +0200 Kevin Wolf <kw...@redhat.com> wrote:
> Am 21.06.2011 19:29, schrieb Luiz Capitulino: > > I'm getting the following times when doing a savevm and delvm in current > > HEAD eb47d7c5d (time in minutes, each time corresponds to a savevm/delvm > > run): > > > > savevm: 5:28m, 11:00m, 11:10m > > delvm: 4:30m, 4:40m, > 15m > > > > Now, trying with qemu 0.13.0 I get: > > > > savevm: < 1:00m, 4:00m, 4:34m > > delvm: few seconds for all cases > > > > Yes, you read it correctly, I tried with 0.13.0 because 0.14.0 also has the > > bug. This is the pattern I see when I run strace against HEAD while running > > the savevm command: > > > > pwrite(7, > > "\0\1\0\1\0\1\0\1\0\2\0\2\0\2\0\2\0\2\0\2\0\2\0\2\0\2\0\2\0\2\0\2"..., > > 65536, 196608) = 65536 > > fdatasync(7) = 0 > > > > Ie. a fdatasync() follows every single pwrite(). Something similar also > > happens > > with delvm. I don't see this pattern with 0.13.0. > > > > The good news is that I've tracked it down and Mr. git bisect says that: > > > > 29c1a7301af752de6721e031d31faa48887204bd is the first bad commit > > commit 29c1a7301af752de6721e031d31faa48887204bd > > Author: Kevin Wolf <kw...@redhat.com> > > Date: Mon Jan 10 17:17:28 2011 +0100 > > > > qcow2: Use QcowCache > > > > Use the new functions of qcow2-cache.c for everything that works on > > refcount > > block and L2 tables. > > > > Signed-off-by: Kevin Wolf <kw...@redhat.com> > > > > :040000 040000 83e364185d37845bb27f1dccd1249d14cc7a9a1e > > 0c91964a52b5869333d4fb2cb0fa83104151359e M block > > Let me guess... You're using cache=writethrough? I'm using default options. To do the tests I started qemu with: # qemu -hda disks/test2.img -enable-kvm -m 1G -monitor stdio > > Previously, qcow2_update_snapshot_refcount() had implemented its own > kind of writeback cache that would be used even with cache=writethrough. > Now we're using the generic Qcow2Cache, which implements a write-through > behaviour for cache=writethrough. > > We could fix this by temporarily switching the cache to writeback mode. > > Kevin >
