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From: | Chunyan Liu |
Subject: | Re: [Qemu-devel] qemu-img create: set "nocow" flag to solve performance issue on btrfs |
Date: | Thu, 26 Sep 2013 17:04:34 +0800 |
On Wed, Sep 25, 2013 at 02:38:36PM +0800, Chunyan Liu wrote:The full implications of the NOCOW attribute aren't clear to me. Does
> Btrfs has terrible performance when hosting VM images, even more when the
> guest in those VM are also using btrfs as file system.
> One way to mitigate this bad performance would be to turn off COW
> attributes on VM files (since having copy on write for this kind of data is
> not useful). We could improve qemu-img to ensure they flag newly created
> images as "nocow". For those who want to use Copy-on-write (for
> snapshotting, to share snapshots across VM, etc..) could be able to change
> this behaviour by 'chattr', either globally or per VM.
it really mean the file cannot be snapshotted?
Or is it purely a data
integrity issue where overwriting data in-place puts that data at risk
in case of hardware/power failure?
I think that would be fine. It's a ioctl(FS_IOC_SETFLAGS, FS_NOCOW_FL)
> I wonder could we add a patch to improve qemu-img create, to set 'nocow'
> flag by default on newly created images?
call so not even too btrfs-specific.
Stefan
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