[TriLUG] Debian, LVM, and RAID
thehead at patshead.com
Sun Jan 29 13:33:40 EST 2006
Kevin Otte wrote:
>> How did you mirror the partition sizes?
> By hand with fdisk.
Darn, my late night brain thought it was on to something :p
> The way I see it, the RAID information is stored in a "superblock"
> outside the partition, and the LVM PV information is stored in the first
> few sectors of the partition. Thus when pvscan ran across all the
> partitions, it saw the same UUID in two different partitions. This is
> also why I was able to fail back to using the single disk after the fiasco.
It cannot be correct that the RAID info is stored outside of the
partition. Both md and lvm operate on simple block devices. If you
want to go crazy you can put an md on top of an lv, on top of an md, on
top of an lv, on top of a... In other words, the information for each
has to be stored on the partition (the lowest level block device).
> It turns out there is a setting in /etc/lvm/lvm.conf to control whether
> or not LVM ignores partitions with an MD superblock. The default is to
> ignore them (which seems sane to me), but Debian for some reason as told
> it NOT to ignore them.
My Ubuntu machine has the same setting (md_component_detection = 0). I
have LVM2 running on top of both RAID 0 and RAID 5 md devices. LVM
doesn't notice the signatures on the underlying partitions.
Is it possible that things are slightly different for RAID 1? It would
be convenient if a RAID 1 md were intentionally set up in such a way
that you could use it as a non-md in a pinch. I might have to look into
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