On (Thu) Jul 09 2009 [08:27:48], Robert Wimmer wrote: > Hi there, > > back in days before kernel 2.6.25/2.6.26 and KVM 70-77 KVM decided to > crash from time to time. That time we used XFS as filesystem (/ and /boot > where ext3/ext2). Since XFS worked so very well for us on physical > hosts the natural choise for our OSs in KVM of course was also XFS. > This was a bad idea because it caused some filesystem corruptions > on some KVMs when KVM crashed (without any message). > Somewhere I read that XFS in KVM should only be used with the > KVM parameter "cache=none". Since then this is now our default > for all KVMs (even with ext3). I thought by myself that KVM and an FS which > does heavy write caching like XFS is a bad choise so I decided that I can't > trust XFS inside a KVM anymore and so I switched all filesystems > in our KVMs to ext3. This was a good choice. No FS corruptions > anymore - well and no unplaned crashes of of KVM too ;-) > Since yesterday (no crash but FS corruptions)... > > I installed kernel 2.6.30-r2 in one of our guests. This was a not so > good idea. All hosts and guest running Gentoo. Host kernel is 2.6.29-r5 > and KVM is 84 (KVM 85 has issues with VNC display and 86 and > 87 not in portage currently). Using qow2 as KVM image format. > > I installed all the stuff we needed in the new KVM and a Postgres > database. But something was different. The database import was > suddenly fast as hell. I've never seen such good I/O throughput > in a KVM. Well after almost finished with the whole installation > process I noticed some strange ext3 messages in the "dmesg" > output. "Oh no... Not again problems with FS corruptions" I thought... > Well after a reboot of the KVM it was sure that the rootfs was > corrupted. /etc/hostname and some other files suddenly were > binary files :-( Lukely I was able to correct the problems with > fsck and get the files back from the backup. > > So what happend in 2.6.30? Ah... I remembered immediately that > the kernel developers decided to switch the default value of the > journaling mode (data=...) from "ordered" to "writeback". Well... > Now I know why the database import was so fast... But at what > price? I'm really curious what happens when the major distributions > roll out their distributions with this default option. Distributions will likely change the default. > So my question is: I'm the only one in the universe with this > FS problems? Am I completely wrong here? Is "data=ordered" > the recommended mode for ext3 in KVMs and even necessary > when KVM ist not crashing? This kind of stuff sometimes makes > live to so easy... ;-) Are you using virtio-block? In any case, not using a released version always has risks. Amit -- 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
