* Anthony Liguori <anth...@codemonkey.ws> [2010-03-17 10:55:47]:
> On 03/17/2010 10:14 AM, Chris Webb wrote:
> >Anthony Liguori<anth...@codemonkey.ws> writes:
> >
> >>This really gets down to your definition of "safe" behaviour. As it
> >>stands, if you suffer a power outage, it may lead to guest
> >>corruption.
> >>
> >>While we are correct in advertising a write-cache, write-caches are
> >>volatile and should a drive lose power, it could lead to data
> >>corruption. Enterprise disks tend to have battery backed write
> >>caches to prevent this.
> >>
> >>In the set up you're emulating, the host is acting as a giant write
> >>cache. Should your host fail, you can get data corruption.
> >Hi Anthony. I suspected my post might spark an interesting discussion!
> >
> >Before considering anything like this, we did quite a bit of testing with
> >OSes in qemu-kvm guests running filesystem-intensive work, using an ipmitool
> >power off to kill the host. I didn't manage to corrupt any ext3, ext4 or
> >NTFS filesystems despite these efforts.
> >
> >Is your claim here that:-
> >
> > (a) qemu doesn't emulate a disk write cache correctly; or
> >
> > (b) operating systems are inherently unsafe running on top of a disk with
> > a write-cache; or
> >
> > (c) installations that are already broken and lose data with a physical
> > drive with a write-cache can lose much more in this case because the
> > write cache is much bigger?
>
> This is the closest to the most accurate.
>
> It basically boils down to this: most enterprises use a disks with
> battery backed write caches. Having the host act as a giant write
> cache means that you can lose data.
>
Dirty limits can help control how much we lose, but also affect how
much we write out.
> I agree that a well behaved file system will not become corrupt, but
> my contention is that for many types of applications, data lose ==
> corruption and not all file systems are well behaved. And it's
> certainly valid to argue about whether common filesystems are
> "broken" but from a purely pragmatic perspective, this is going to
> be the case.
>
I think it is a trade-off for end users to decide on. cache=writeback
does provide performance benefits, but can cause data loss.
--
Three Cheers,
Balbir
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