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Re: [Xen-users] Questions on qcow, qcow2 versus LVM

To: matt@xxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Questions on qcow, qcow2 versus LVM
From: Grant McWilliams <grantmasterflash@xxxxxxxxx>
Date: Wed, 23 Dec 2009 08:56:06 -0800
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On Mon, Dec 21, 2009 at 4:54 AM, Matthew Law <matt@xxxxxxxxxxxxxxxxxx> wrote:
I've been using lvm under centos to create the backing store for domUs and
although the performance seems acceptable it has some shortcomings.  The
biggest of which is the LVM bug which prevents me from removing an lv (it
says it is still mounted and it definitely isnt).  I thought this was just
a centos bug but it appears to be evident in debian and ubuntu too and I
really can't afford a reboot of the dom0 everytime I want to remove a
logical volume that was once accessed but now isn't!

What are people's experiences with using qcow or qcow2 images over LVM
volumes?  The reason I chose LVM was for it's ease of management and
relative ease with which you can shrink and enlarge a domU filesystem.
Can you do this with qcow?  How does it perform on a dom0 with many
running domUs (ie. dozens) ?



I'm going to chime in on the Qcow2 portion of your question - it doesn't work. Depending on which version of Xen you have you will have different bugs and even if you can get some form of Qcow to work it doesn't support a backing store so it's basically worthless. I'm considering moving 40 VMs to KVM just because I ran into a thousand brick walls with Qcow and Xen. I want to use Xen but at the end of the day I just need to get my work done.

If you're looking to do COW you can use dmsetup to and snapshots to layer read-only disks with write-only disks but you'll just run into the same LV bug that you've been battling. It's sad that we don't have real COW support in a hypervisor as powerful as Xen. Apparently it's just not that important.

I wanted to have an image in ramdisk and set up a snapshot of that as read only with the writes going to disk for performance reasons but if you try to boot a DomU off an image in ram it will screw up your machine and only a reboot will fix it. You can't even start ANY other domains until the reboot. Or at least I couldn't find any.

Grant McWilliams

Some people, when confronted with a problem, think "I know, I'll use Windows."
Now they have two problems.
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