On Mon, Mar 15, 2010 at 06:43:06PM -0500, Anthony Liguori wrote: > I knew someone would do this... > > 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. > > cache=writethrough provides a much stronger data guarantee. Even in the > event of a host failure, data integrity will be preserved.
Actually cache=writeback is as safe as any normal host is with a volatile disk cache, except that in this case the disk cache is actually a lot larger. With a properly implemented filesystem this will never cause corruption. You will lose recent updates after the last sync/fsync/etc up to the size of the cache, but filesystem metadata should never be corrupted, and data that has been forced to disk using fsync/O_SYNC should never be lost either. If it is that's a bug somewhere in the stack, but in my powerfail testing we never did so using xfs or ext3/4 after I fixed up the fsync code in the latter two. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
