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Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_D
From: |
Paul Brook |
Subject: |
Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT) |
Date: |
Wed, 21 May 2008 02:05:36 +0100 |
User-agent: |
KMail/1.9.9 |
On Wednesday 21 May 2008, Anthony Liguori wrote:
> Paul Brook wrote:
> >> When sector-aligned guest offsets are converted to sector-unaligned
> >> writes (e.g. due to qcow2 etc.), that property is no longer satisfied,
> >> and power failure of the host disk can cause more damage than the
> >> guest is designed to be resistant to.
> >
> > Seems like the easiest solution would be to have qcow always align its
> > writes. We don't do on the fly compression, so it should be fairly easy
> > to make this happen with minimal overhead.
>
> That's not sufficient. O_DIRECT imposes not only offset alignment
> requirements but also requirements on the buffer being read to. Most of
> the code in QEMU does not properly align the read/write buffers.
In that case you need both. For correct operation the qcow layer needs to
ensure that all file offsets are block aligned (amongst other things, I
wouldn't be surprised if there are more subtle problems with metadata
updates).
The memory buffer alignment can occur wherever is most convenient, that's
trivially atomic w.r.t. unexpected interruptions.
Paul
- [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), (continued)
- [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Anthony Liguori, 2008/05/20
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Anthony Liguori, 2008/05/20
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Jamie Lokier, 2008/05/20
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Anthony Liguori, 2008/05/20
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Kevin Wolf, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT),
Paul Brook <=
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Anthony Liguori, 2008/05/20
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Kevin Wolf, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Jamie Lokier, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Avi Kivity, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Jamie Lokier, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Anthony Liguori, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Avi Kivity, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Anthony Liguori, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Avi Kivity, 2008/05/21
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Jamie Lokier, 2008/05/21