[linux-lvm] Mirrored LV

[malahal at us.ibm.com]

Koen Vermeer [koen at vermeer.tv] wrote:
> Hi,
> 
> I was thinking of using a mirrored LV as a easier to use alternative to
> a RAID1 PV (with matching VG and LV). I tried to find the information I
> need on how this works and how to set it up, but so far, I failed.
> 
> First, am I right in trying to use LVM in this way? I have two disks,
> and I just want to mirror some LVs.

You can by using corelog (aka, log in memory), but read on...

> Second, I am confused by the apparent need to have this log stored on
> another device. Actually, this doesn't seem true in two ways: There's
> the option of having the log in memory and then the documentation only
> says that the log is 'usually on a separate device'. Does that mean that
> using a mirrored LV makes no sense on a system with two disks? Or should
> I just keep the log in memory in this case? Or can I use another PV on
> one of the disks? Should this be mirrored as well? What happens if I
> loose that data?

The log records what regions of your master device are in sync with your
other mirror device. It is used to synchronize the failed regions from
the master to your other device when your 'other device' comes back to
life. This should work whether your log device is in memory or on a PV
by itself as long as you don't de-activate or restart your machine. The
log data on a PV can be useful for reboots.

-Malahal.