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

[Phil Turmel]

Hi Hans, (?)

On 11/30/2010 02:34 AM, hansbkk at gmail.com wrote:
> 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.

I think you are making this overly complex, insisting on a RAID1 operation to backup from on filer to the other.  Consider having each disk on filer #2 configured as a single LVM PV/VG, so it can stand alone in a rotation.  The try the alternate below.

> So here's a summary of steps, please confirm:
>   - create a snapshot of a given volume

Here's where you are over-complicating things:
>   - 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

As an alternate, with simpler recovery semantics:
  Create matching LV on non-RAID PV/VG on filer #2
  dd + netcat + dd or other technique to dup the snapshot on filer #1 to filer #2

>   - delete the snapshot

Now, you have a single disk in your backup set that can be mounted on either filer, and either copied back into service, or in an emergency, used directly (live) in filer #1.

This approach also gives you the *option* to implement the backup transfer with file system conversions, compression, free space removal, or any other administrative adjustments you need.  A RAID mirror can only duplicate the raw block device.

HTH,

Phil