For production environment I guess I'll switch to md+lvm for mirroring
so that I'll be able to safely snapshot the mirrored fs. I'll keep
playing with devmapper mirror combined with snapshot (without lvm) and
report if it misbehaves. So far it hasn't, but I only did some limited
experiments. I hope some future version of lvm will add support for
this combination.
Yoav
Jonathan Brassow wrote:
No, there isn't a reason LVM prevents it, other than we haven't gotten
to it yet. That being said... because we haven't gotten to it yet
also means there probably hasn't been a whole lot of testing of such a
configuration. Therefore, it's difficult to quantify the safety.
brassow
On Oct 28, 2008, at 7:32 PM, Yoav wrote:
I know how to do it through device-mapper. I'm just worried that
maybe there's a reason why lvm prevents it. Are you aware of such
reason, or is it safe?
Jonathan Brassow wrote:
It is possible to do all the things you are asking through
device-mapper... but there is no easy way to do it. (and LVM does
not yet have the capabilities to manage this yet either.)
brassow
On Oct 27, 2008, at 9:28 PM, Yoav wrote:
According to lvcreate, a snapshot of a mirrored volume cannot be
created ("Snapshots and mirrors may not yet be mixed"). However,
devmapper does not seem to have the same limitation when used
directly (not through lvm).
Is there a current reason for lvcreate to enforce this rather harsh
limitation?
What are the risks I'm taking by manually creating a dm-snapshot
from a dm-mirror device? (of course I'm also replacing the
dm-mirror device with a snapshot-origin of the actual dm-mirror
device).
If there is an actual reason for preventing this, what is the
suggested way to consistently backup a mirror volume? Is there a
way to temporarily take one of the mirror "legs" (mimage devices)
offline in order to back it up, and resync it later?
Yoav
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