* Anthony Liguori <anthony@xxxxxxxxxxxxx> [2010-03-17 10:55:47]: > On 03/17/2010 10:14 AM, Chris Webb wrote: > >Anthony Liguori<anthony@xxxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
