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Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_D
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT) |
Date: |
Wed, 21 May 2008 01:04:47 +0200 |
Le mardi 20 mai 2008 à 23:36 +0100, Jamie Lokier a écrit :
> Anthony Liguori wrote:
> > >This patch implements wrappers for raw_pread and raw_pwrite which align
> > >all file accesses and make qcow(2) work with cache=off. This method
> > >might not be the most performant one (compared to fixing qcow, qcow2 and
> > >everything else that might be using unaligned accesses), but unaligned
> > >accesses don't happen that frequently and with this patch really all
> > >image accesses should be covered."
>
> It's a useful patch.
>
> One little esoteric consequence you might want to document.
>
> This occurs when a guest is running something like a database or
> journalling filesystem, and is reliant on the host disk's integrity
> properties.
>
> One property of disks is that if you overwrite a sector and the're
> power loss, when read later that sector might be corrupt. Even if the
> new data is the same as the old data with only some bytes changed,
> some of the _unchanged_ bytes may be corrupt by this.
>
> When it writes to sector-aligned offset, there is a possibility that
> the guest is depending on power failure not causing corruption of
> neighouring sectors. This is typical with some kinds of journalling.
>
> 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.
Sector unaligned guest offset are converted to sector aligned offset.
Sector aligned guest offset are written as is.
And as we use O_DIRECT and aligned offset and data count I think we
increase disk integrity (regarding the case without O_DIRECT and without
aligned access...), so we should document the case without O_DIRECT use,
not the case with O_DIRECT use...
Regards,
Laurent
--
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"The best way to predict the future is to invent it."
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- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), (continued)
- Re: [Qemu-devel] Re: [PATCH][v2] Align file accesses with cache=off (O_DIRECT), Andrea Arcangeli, 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), Andrea Arcangeli, 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), Andrea Arcangeli, 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), Andrea Arcangeli, 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), Avi Kivity, 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),
Laurent Vivier <=
- 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), 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), Andreas Färber, 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), Jamie Lokier, 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), Laurent Vivier, 2008/05/23