Messages in this thread | | | Date | Wed, 17 Mar 2010 10:55:47 -0500 | From | Anthony Liguori <> | Subject | Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter |
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On 03/17/2010 10:14 AM, Chris Webb wrote: > Anthony Liguori<anthony@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.
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.
Regards,
Anthony Liguori
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