From: Paolo Bonzini <pbonzini@redhat.com>
To: Chun Yan Liu <cyliu@suse.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] qemu-img create: set "nocow" flag to solve performance issue on btrfs
Date: Fri, 27 Sep 2013 11:02:03 +0200 [thread overview]
Message-ID: <5245498B.4060308@redhat.com> (raw)
In-Reply-To: <5245B92F020000660001DF93@soto.provo.novell.com>
Il 27/09/2013 10:58, Chun Yan Liu ha scritto:
>> If so, you could run QEMU with "cache=unsafe" and have
>> basically the same data safety guarantees as "cache=writeback" on
>> every other file system.
> "cache=unsafe" means it never calls fsync() ?
Yes. However, metadata writes are still done and ordered correctly with
respect to data writes.
> If so, it seems to be
> not proper to use this option to disk image of a running VM. In case
> someone has run the VM for a very long time and made many changes
> already on the disk image, but all those are in cache (?) Then due to
> a power off, all these changes are lost. Although the disk image is
> still in a consistent data status, but the data is maybe too old.
Perhaps for btrfs-cow a different strategy is better, with fsyncs
coalesced and issued by QEMU every N seconds.
Paolo
next prev parent reply other threads:[~2013-09-27 9:01 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-25 6:38 [Qemu-devel] qemu-img create: set "nocow" flag to solve performance issue on btrfs Chunyan Liu
2013-09-26 7:58 ` Stefan Hajnoczi
2013-09-26 8:54 ` Paolo Bonzini
2013-09-26 10:30 ` Chunyan Liu
2013-09-26 16:56 ` Paolo Bonzini
2013-09-27 8:58 ` Chun Yan Liu
2013-09-27 9:02 ` Paolo Bonzini [this message]
2013-09-26 9:04 ` Chunyan Liu
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