Re: [gentoo-amd64] Re: Is my RAID performance bad possibly due to starting sector value?

[Rich Freeman <rich0@gentoo.org>]

On Sat, Jun 22, 2013 at 7:04 PM, Mark Knecht <markknecht@gmail.com> wrote:
>    I've been rereading everyone's posts as well as trying to collect
> my own thoughts. One question I have at this point, being that you and
> I seem to be the two non-RAID1 users (but not necessarily devotees) at
> this time, is what chunk size, stride & stripe width with you are
> using?

I'm using 512K chunks on the two RAID5s which are my LVM PVs:
md7 : active raid5 sdc3[0] sdd3[6] sde3[7] sda4[2] sdb4[5]
      971765760 blocks super 1.2 level 5, 512k chunk, algorithm 2 [5/5] [UUUUU]
      bitmap: 1/2 pages [4KB], 65536KB chunk

md6 : active raid5 sda3[0] sdd2[4] sdb3[3] sde2[5]
      2197687296 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] [UUUU]
      bitmap: 2/6 pages [8KB], 65536KB chunk

On top of this I have a few LVs with ext4 filesystems:
tune2fs -l /dev/vg1/root  | grep RAID
RAID stride:              128
RAID stripe width:        384
(this is root, bin, sbin, lib)

tune2fs -l /dev/vg1/data  | grep RAID
RAID stride:              19204
(this is just about everything else)

tune2fs -l /dev/vg1/video  | grep RAID
RAID stride:              11047
(this is mythtv video)

Those were all the defaults picked, and with the exception of root I
believe the array was quite different when the others were created.
I'm pretty confident that none of these are optimizes, and I'd be
shocked if any of them are aligned unless this is automated (including
across pvmoves, reshaping, and such).

That is part of why I'd like to move to btrfs - optimizing raid with
mdadm+lvm+mkfs.ext4 involves a lot of micromanagement as far as I'm
aware.  Docs are very spotty at best, and it isn't at all clear that
things get adjusted as needed when you actually take advantage of
things like pvmove or reshaping arrays.  I suspect that having btrfs
on bare metal will be more likely to result in something that keeps
itself in-tune.

Rich