[aklug] Re: lvm and raid?

On Fri, 15 May 2009, Lee wrote:

> I'm noodling around with raid and lvm (Virtualbox is wonderful!).
>
> Googling shows a lot of folks are setting up two sets of software raid, one for a boot
> partition and one for 'everything else', for example:
>
> /dev/md0 /boot /dev/sda1 /dev/sdb1
> /dev/md1 /dev/sda2 /dev/sdb2
>
>
> then they'll make an lvm group out of /dev/md1 (I understand that grub doesn't grok lvm,
> hence the mdraid for /dev/md0).
>
> but lvm can be set up to do it's own mirroring, so I'm puzzled as to why so many are
> using mdraid to mirror underneath lvm ?
>
> Anyone have any knowledge or insight into this?

Because the mirroring capabilities of LVM is far too brittle. At some point
(like it has in ZFS) LVM & RAID layers will merge, but until then we're
stuck doing things the old way. Hardware level redundancy is better suited
for pure RAID.

A lesser reason (since the advent of initramfs) is that LVM still requires
userland tools to initialize, whereas MD's RAID in the kernel can
autoconfigure *if* it's compiled in statically and you've marked your
partitions as RAID partions (0xfd).

         --Arthur Corliss
           Live Free or Die
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Received on Fri May 15 14:53:11 2009