LVM + RAID follow up

[warlord at MIT.EDU (Derek Atkins)]

Quoting dsr at tao.merseine.nu:

>> Man, this is just confusing..
>>
>> I would think that after a while you'd have a LOT of MDn devices..
>> and eventually you wind up in this similar situation where you're
>> breaking down your drives into many "small" partitions.
>
> After a while, you note that you have n 2TB drives and it's time
> to realign everything. You do a backup, just in case. Then you
> tell the RAID system to remove a drive. That gets repartitioned
> into a tiny part and a huge part. When you have enough of those,
> you tell the RAID that it's a decayed RAID. Create new LVM
> partitions. Copy over the old data, LVM partition by LVM
> partition. Remove the remaining drives from the old RAID,
> repartition them and restore them as part of the new RAID. Let
> it rebuild.

That sounds very complicated.  One would think that someone would
have written tools to accomplish these tasks.  I can't be the first
person to think about this problem but not want an infinitely increasing
number of MDn RAID arrays.

>> Do you?  If I assume that the RAID5 is all one partition, then if the
>> partition size increases I can just run resize2fs to increase the size
>> of the file system.  The only question is the RAID5 parts, will it
>> be able to build a filesystem out of a 3x400GB + 3x800GB partition?
>
> Each partition for RAID must be the same size.

Odd..  According to a modern mdadm man page (and a great LVM-RAID 
resource [0])
each partition does NOT have to be the same size -- but the RAID will only
use the size of the smallest partition.   But it means that if you intend
to replace ALL your drives at the same time then you can do that and then
grow the RAID array to use the new space, and then resize the filesystem.

I think this page mostly answers the questions I have.  :)

> -dsr-

[0]: http://www.gagme.com/greg/linux/raid-lvm.php
-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available


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