Re: [Xen-devel] Xen & Automated Disk Management for Domains

[Michael Vrable <mvrable@xxxxxxxxxxx>]

On Thu, Aug 05, 2004 at 11:26:09PM +0100, Ian Pratt wrote:
> AFAIK, LVM2 broke snapshot functionality, but that it'll be going
> back into 2.6.8 (which will likely be released in the next week
> or two).

Snapshots work with LVM2 on Linux 2.4, assuming all the device mapper
patches are applied to the kernel.  I just did a test with a non-Xen
Linux 2.4 kernel; I'll have to look into the problems I had with the Xen
kernel to see if I can figure out the cause of the trouble.

It's true that snapshot functionality wasn't merged into Linux 2.6 when
device mapper support was merged.  My understanding is that it is
possible to get this on current 2.6 systems with additional patches.
However, it looks like those patches currently pull in all of Linux
2.4.7-mm4, which makes the patch rather intrusive.  Linux 2.6.8 should
be better since it won't be necessary to apply any additional patches.

Also, on the subject the way snapshots are implemented in the device
mapper, I think I got confused yesterday.  I don't think any
modifications are necessary.

Say we start with a logical volume /dev/vg1/disk, then create an LVM
snapshot of vg1/disk called vg1/disk-snap.  We could mount vg1/disk-snap
as a read-only filesystem to store a consistent backup of vg1/disk; this
is the normal suggested use.  But LVM actually makes both resulting
devices writable, so we could equally well not touch vg1/disk and use
vg1/disk-snap as a writable, CoW block device.  (If we did accidentally
touch vg1/disk, LVM would keep those changes isolated from the
snapshots.)

While LVM won't allow snapshots of snapshots, multiple snapshots of the
same underlying (non-snapshot) device are allowed.  So we could start
with vg1/disk and create vg1/disk-copy1, vg1/disk-copy2, vg1/disk-copy3,
and so on, as CoW devices.

I think this meets all our needs for CoW devices...am I missing
something?  (Other than getting it to work in my XenoLinux kernel.)

--Michael Vrable


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