On Fri Jun 19, 2009 at 01:15:41PM +0400, Michael Tokarev wrote:
> Wil Reichert wrote:
>> When using LVM on top of RAID 5, is it still worthwhile to pass RAID
>> stripe information to the filesystem on creation? Or do the PE's in
>> LVM blur the specific stripe sizes & I'd want to use some multiple of
>> those instead?
>
> It's a very good question, especially in context of RAID5.
>
> Yes it is still a good idea to pass that info because it is still a
> RAID5 which requires proper treatment wrt unaligned writes and keeping
> redundancy.
>
> But the thing is that RAID5 and LVM are not good to each other UNLESS
> RAID5 consists of 3, 5 or 9 (or 17 etc) drives -- i.e. 2^N+1, so that
> there's 2^N data drives.
>
> This is because LVM can only have blocksize as a power of two and in
> order to be useful that blocksize should be a multiple of RAID5 data
> row size (stripe size etc).
>
> This is only possible when RAID5 has 2^N data drives or 2^N+1 total
> drives. The same is for RAID4, and for RAID6 it's 2^N+2 since RAID6
> has 2 parity drives.
>
> But if you can't match LVM blocksize and RAID strip size, there's
> *almost* no point at telling raid parameters to the filesystem: no
> matter how hard you'll try, LVM will make the whole thing non-optimal.
>
> Ok, depending on the number of drives, *some* logical volumes will
> be properly aligned, but definitely not all of them. For example
> on a 4-drive RAID5 array, only every 3rd volume will be ok, --
> provided the volumes are allocated in full one after another,
> without holes and fragmentation.
>
> When everything is properly aligned, it's still worth the effort
> IMHO to tell the filesystem about true raid properties.
>
You'll also need to get the LVM header padding right, otherwise the
filesystem won't start on a stripe boundary and the alignment will all
go wrong. I've no idea of the syntax but I recall seeing it discussed
on here several times. A quick search throws up:
http://www.issociate.de/board/goto/1859627/stride_/_stripe_alignment_on_LVM_?.html
Cheers,
Robin
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