LVM snapshot

[Alexandre Oliva]

On Aug 13, 2004, "Bill Rugolsky Jr." <brugolsky at telemetry-investments.com> wrote:

> I encountered a "gotcha" when using snapshots (or DM mirror): if using an
> initrd, one needs to place those modules (dm-snapshot, dm-mirror)
> in the initrd, otherwise LVM2 won't activate the LVs when you reboot.

Ugh.

> Alexandre, where should this config info live?

Err...  I think mkinitrd should perhaps auto-detect the need for such
modules somehow.  Any ideas on how to do that?

Alternatively, we could just add them unconditionally if LVM is
enabled, which would presumably enable the system to reboot even if
major changes took place in the VG, that would require initrd to be
re-created otherwise.

Especially considering that pvmove requires mirroring support now, I
feel sympathetic to the idea of having dm-mirror unconditionally, such
that, if you happen to reboot in the middle of a pvmove, you don't end
up in big trouble.


Talking of dm-mirror, I haven't kept track of it; does anyone know how
it guarantees atomic writes to the replicas of an extent?  Does it
resync the whole device in case there's say loss of power or a crash
when the devices are out of sync, like raid 1, or are in-sync bits
maintained on a per-extent basis?  Or does it just punt at it?

-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}