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

["Matthew Law" <matt@xxxxxxxxxxxxxxxxxx>]

On Wed, December 23, 2009 4:56 pm, Grant McWilliams wrote:
> 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.

What does kvm buy you? Does it have a working cow backing store?

I did some more tests last night and it isn't just the swap LVs that are
affected -its all of them. So I can create, grow or shrink an LV but not
remove it without a reboot.  That is a huge limiter and has really screwed
me up.

My next plan is to use opensolaris or nexenta and export zvols over iSCSI.
 That brings with it a bunch of unknowns along with extra cost and
complexity.  It should offer better snapshots than LVM, thin provisioning,
deduplication and better data protection, however, so that's why I'm going
to give it a try before I am forced to abandon Xen entirely.

Matt


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