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xen-users
Re: [Xen-users] Xen, LVM and snapshots
Ferenc Wagner schrieb:
(...)
I saw an old message from 2006 on this list, where it is said that
"Common understanding is that LVM2 snapshots are unstable".
I don't understand this statement.
Anything older than 2.6.18 (so, including 2.6.18) will use lots of
memory for a snapshot. This means, you won't be able to do more than a
handful of snapshots even on a system with lots of free memory (you may
expect trouble i.e. you have 512 MB RAM, only 30 MB used, and want to do
10-20 snapshots).
You have 400 MB memory left, did 30 snapshots, and it still works for
you? You may reconsider - when they fill, (kernel) memory usage will
rise, until your machine is unresponsive. As it's kernel memory being
used, oom-killer won't help much here; bigger swap won't help either,
because real RAM is needed.
To make the things worse, you will probably be unable to boot the
machine again. As the system starts, LVM volumes are detected, memory is
full again... You're out of luck, unable to even remove the snapshots in
order to boot properly, unless you have some more RAM sticks at hand to
fix the problem.
Coincidentally, 2.6.18 kernel is the one Xen is based on (at least
dom0), so if you want your Xen server to be a storage server at the same
time, you might run into problems as your system grows.
Some major distributions base on this kernel as well (i.e., Debian Etch).
A workaround might be:
- using bigger stripes with LVM (it should take less memory)
- use a newer kernel - as a rule of thumb - the newer, the better -
might be problematic with Xen dom0,
- have storage on a separate machine (i.e., iSCSI SAN) with a newer
kernel than your Xen dom0 machine
Newer kernels (2.6.20, maybe even 2.6.19, and later) are not affected by
this "phenomenon".
Another LVM disadvantage is that every snapshot means additional writes.
I.e. if you have a logical volume, and 4 snapshots, writing to the
origin will mean 4 more writes are needed. This is a serious scalability
problem.
--
Tomasz Chmielewski
http://wpkg.org
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