Re: JFYI: ext4 bug triggerable by kvm

[Avi Kivity <avi@redhat.com>]

  On 08/17/2010 05:46 PM, Christoph Hellwig wrote:
> On Tue, Aug 17, 2010 at 09:44:49AM -0500, Anthony Liguori wrote:
>> I think the real issue is we're mixing host configuration with guest
>> visible state.
> The last time I proposed to decouple the two you and Avi were heavily
> opposed to it..

I wasn't that I can recall.

>> With O_SYNC, we're causing cache=writethrough to do writethrough
>> through two layers of the storage heirarchy.  I don't think that's
>> necessary or desirable though.
> It's absolutely nessecary if we tell the guest that we do not have
> a volatile write cache.  Which is the only good reason to use
> data=writethrough anyway - except for dealing with old guests that
> can't handle volatile writecache it's an absolutely stupid mode of
> operation.

I agree, but there's another case: tell the guest that we have a write 
cache, use O_DSYNC, but only flush the disk cache on guest flushes.

The reason for this is that if we don't use O_DSYNC the page cache can 
grow to huge proportions.  While this is allowed by the contract between 
virtual drive and guest, guest software and users won't expect a huge 
data loss on power fail, only a minor data loss from the last fraction 
of a second before the failure.

I believe this can be approximated by mounting the host filesystem with 
barrier=0?

-- 
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