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Re: [Qemu-devel] qemu-img create: set "nocow" flag to solve performance
From: |
Paolo Bonzini |
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 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8 |
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
Re: [Qemu-devel] qemu-img create: set "nocow" flag to solve performance issue on btrfs, Chunyan Liu, 2013/09/26