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Re: [libvirt] PATCH: Disable QEMU drive caching



On Wed, Oct 08, 2008 at 12:03:33PM +0100, Daniel P. Berrange wrote:
> QEMU defaults to allowing the host OS to cache all disk I/O. THis has a
> couple of problems
> 
>  - It is a waste of memory because the guest already caches I/O ops
>  - It is unsafe on host OS crash - all unflushed guest I/O will be
>    lost, and there's no ordering guarentees, so metadata updates could
>    be flushe to disk, while the journal updates were not. Say goodbye
>    to your filesystem.
>  - It makes benchmarking more or less impossible / worthless because
>    what the benchmark things are disk writes just sit around in memory
>    so guest disk performance appears to exceed host diskperformance.
> 
> This patch disables caching on all QEMU guests. NB, Xen has long done this
> for both PV & HVM guests - QEMU only gained this ability when -drive was
> introduced, and sadly kept the default to unsafe cache=on settings.

  Right !

  I think for integrity reason we should revert that default at the
libvirt level and swicth caching to off. I would not be against a
way to reactivate it optionally, assuming we have a clean way to express
it at the XML level (I don't think we have currently maybe an
optional cache="on|off" attribute could be added to device/disk/target)
because in some circumstances like cache of read-only devices available
to multiple domain, it can make sense to keep caching on the host os.

  So I'm fine with the patch going in as-is, but maybe we need
one patch on top to reenable the cache in an explicit case by case
basis.

Daniel

P.S.: can you try to get patches with -p to get the contextual function
      without them it's harder to review exactly where things goes,
      especially when there is a line number shift due to other pending
      patches, thanks !

-- 
Daniel Veillard      | libxml Gnome XML XSLT toolkit  http://xmlsoft.org/
daniel veillard com  | Rpmfind RPM search engine http://rpmfind.net/
http://veillard.com/ | virtualization library  http://libvirt.org/


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