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xen-devel
Re: [Xen-devel] IO caching on xen?
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To: |
Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> |
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Subject: |
Re: [Xen-devel] IO caching on xen? |
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From: |
Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx> |
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Date: |
Wed, 16 Jul 2008 18:46:49 +0100 |
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Cc: |
marian klein <klein.marian@xxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "xen-discuss@xxxxxxxxxxxxxxx" <xen-discuss@xxxxxxxxxxxxxxx> |
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Wed, 16 Jul 2008 10:47:11 -0700 |
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Dan Magenheimer, le Tue 15 Jul 2008 12:32:53 -0600, a écrit :
> > marian klein, le Tue 15 Jul 2008 11:22:05 +0000, a écrit :
> > > I am worried that the xen (or any other VM) with zvol
> > device from ZFS
> > > (or any other file system.) may remove the benefits of ZFS,
> > > because the guest OS can write something to disk but the xen itself
> > > can do some buffering
> > > or lazy writing and it may delay to write to ZFS backend.
> >
> > Xen doesn't do any kind of buffering or lazy writing. It provides
> > barriers to the guest, to ensure any needed coherency.
>
> I'm not sure if this is related or not...
>
> I was party to some internal irc where it was believed
> to be the case that when a file-backed vbd using file:'...'
> is used, dom0's page cache may contain buffer pages that
> may be inconsistent with the disk
Ah, indeed, in that case the page cache is used, and the barriers may
not have proper effect.
> whereas a file-backed vbd using tap:aio:'...' does not suffer from the
> same problem.
blktap uses O_DIRECT to avoid buffering completely indeed.
Samuel
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