[linux-lvm] Q: LVM over RAID, or plain disks? A:"Yes" = best of both worlds?

[hansbkk at gmail.com]

On Tue, Nov 30, 2010 at 2:14 PM, Nataraj <incoming-centos at rjl.com> wrote:
>> TopRAID1's LVM is likely to be running over a RAID6 set , so I'm not
>> depending on the TopRAID mirroring for reliability, just using it for
>> the above volume cloning.
>
> Your raid 1 backups won't mirror any snapshots of your LV's unless you
> specifically setup mirroring of the snapshots after they exist.

Ah, getting clearer to me, I was thinking I'd be mirroring the LV
itself, but you're right, taking a snapshot and mirroring that is a
much better idea.

So here's a summary of steps, please confirm:
  - create a snapshot of a given volume
  - create a new RAID1 mdN between that and a physical partition (blank?)
  - let that get sync'd up
  - break the RAID (fail the partition?), remove the drive
  - delete the snapshot

The below is less clear to me, especially if the above is correct:

>> If so, would it be possible/better for the host In normal operations
>> to mount the underlying LV directly rather than the degraded top-level
>> RAID1?
>
> No, you want to have mdadm assemble the raid volume, even if in degraded
> mode with only one drive and then access the LV on top of the md device.
>  Even if you were able to mount the LV and bypass raid, that would be
> pointless because you would not update the bitmap and superblock and the
> integrety of the raid set would be lost.

During normal operations - when I'm not in the process of taking a
RAID-backup of my LV snapshot - it seems to me that the "Top-RAID" mdN
doesn't even exist right? It's set up to mirror between a snapshot and
a regular partition, both of which don't even exist during these
normal operations.

Therefore during normal operations the host *is* mounting the LV
directly, not via the "Top-RAID"mdN.

I wasn't talking about accessing the "Bottom-RAID" which creates the
underlying PV - this is transparent to LVM anyway, right?

Thanks again for your help (or in advance to anyone else who would
like to chime in)