Comments on: The Transition to Ext4

By: Aigars Mahinovs

Aigars Mahinovs — Tue, 01 Dec 2009 22:23:55 +0000

Yes, where Xen pretends that /dev/data/production_root and /dev/data/production_swap are actually partition 1 and 2 on a virtual hard drive, for KVM you need to have an actual drive (like a /dev/data/production LVM LV) with a partition table, partitions and a boot loader.

If your systems can boot on hardware, they should be able to boot on KVM. I mean, even Windows boots on a KVM. You don’t even really need those hardware bits in the processor nowadays on the host. You will loose some speed if you don’t have the VT extension in the CPU and use an older kernel that does not understand virtio and virtnet devices, but for most situations that is less of a problem and the benefits from simpler control are overwhelming (especially, if migrating from some old bare metal hardware).

By: Chris Smart

Chris Smart — Tue, 01 Dec 2009 00:19:25 +0000

Hmm.. can one send magic keys to a virtualised machine? You’re gonna need that to sync your filesystem when something goes wrong. If Squeeze (mid next year?) will come with ext4, then I hope that they either have a very recent kernel or backport fixes.

-c

By: etbe

etbe — Mon, 30 Nov 2009 21:45:55 +0000

Anon: Yes, there is simply no way around it for me. Ext4 is what I need to do.

Aigars: When you say that an image of a drive is needed, I presume you mean that the Xen trick of making several LVM volumes look like partitions on one larger disk isn’t supported. Even if I was using that Xen feature it wouldn’t be a big deal, I could turn it off.

The RHEL5 Xen server I run is not going to be changed any time soon.

I run a bunch of Debian i686 Xen servers that can’t run KVM. I run one Debian Xen server that could potentially run KVM. But it seems best to run Xen on all my virtual servers instead of using Xen on all but one.

James: That’s interesting, it seems to have appeared in version 2.6.18-138.el5. If they had done that earlier it would have saved me some pain.

Anon: Do you have a reference to exactly what uninit_bg and flex_bg do?

By: Anonymous

Anonymous — Mon, 30 Nov 2009 17:50:31 +0000

(Followup to my first comment: uninit_bg and flex_bg provide the fsck benefit, not just uninit_bg; both of those represent backward-incompatible features if you turn them on.)

By: James

James — Mon, 30 Nov 2009 17:48:00 +0000

The RHEL 5.4 kernel has XFS support. Userspace tools aren’t in the standard RHEL yet though (I don’t know why).

Several of the XFS developers now work for Redhat as well. And xfs_repair is much faster than the ext4 version in my experiences.

By: Aigars Mahinovs

Aigars Mahinovs — Mon, 30 Nov 2009 17:44:34 +0000

I’ve migrated a bunch of vhosts on my server from Xen to KVM with libvirt. The main problem was that it requires you to have an image of a hard drive and not just of a partition, so I had to learn how to use kpartx, but otherwise it was much easier – all native kernels, fresh software, easier to see memory usage and manage the hosts remotely (virt-manager is awesome), no need to have guest kernel images in the host, …. And it is pretty easy to turn on virtualisation-aware hard drive and network access to get enough of a performance to not worry about it much.