Ocfs2 users - Jul 2010 - partition offset/alignment on SAN devices.

Hey folks,

I've been getting requests from the users that they want the OCFS2
filesystems aligned to 64k boundaries, however, when I began the
project, the dummy of me, created the OCFS2 filesystem on top of the
whole disk device (no partition) for example, one of the SAN devices
would show up as /dev/dm-10, I created the filesystem using the
following command:

mkfs.ocfs2 -L SOMELABEL /dev/dm-10


is this supported, am I safe or did I just shoot myself in the foot,
mouth & brain?


thank you,
Enrique.


-- 
Enrique Sanchez Vela
------------------------------------------

On Thu, Jul 08, 2010 at 01:18:30PM -0400, Enrique Sanchez
wrote:
> I've been getting requests from the users that they want the OCFS2
> filesystems aligned to 64k boundaries, however, when I began the
> project, the dummy of me, created the OCFS2 filesystem on top of the
> whole disk device (no partition) for example, one of the SAN devices
> would show up as /dev/dm-10, I created the filesystem using the
> following command:
> 
> mkfs.ocfs2 -L SOMELABEL /dev/dm-10
> 
> is this supported, am I safe or did I just shoot myself in the foot,
> mouth & brain?
	It will certainly work.  The big danger is that you might fdisk
the drive without thinking, stomping on the ocfs2 volume.

Joel

-- 

"Copy from one, it's plagiarism; copy from two, it's
research."
        - Wilson Mizner

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127

On 08/07/10 18:18, Enrique Sanchez wrote:
> Hey folks,
> 
> I've been getting requests from the users that they want the OCFS2
> filesystems aligned to 64k boundaries, however, when I began the
> project, the dummy of me, created the OCFS2 filesystem on top of the
> whole disk device (no partition) for example, one of the SAN devices
> would show up as /dev/dm-10, I created the filesystem using the
> following command:
> 
> mkfs.ocfs2 -L SOMELABEL /dev/dm-10
> 
> 
> is this supported, am I safe or did I just shoot myself in the foot,
> mouth & brain?
> 
> 
> thank you,
> Enrique.
> 
> 
It's not a dumb thing to do... It'll probably be the most I/O optimal
solution...

Assuming your SAN does something sensible and optimally fits new LUNs start
sectors to raid-stripe
sizes sympathetically, you should be fine.

Apologies if you already know this bit...

This 64k (32/128k whatever) issue is usually only a problem if you've used a
fdisk to create a
partition to put your data on. For hysterical reasons the first partition is
created an arbitrary
amount of kb into the disk. This almost never lines up with the LUN raid-stripe.
Then the I/O going
from app <> FS <> LUN <> disks overlaps block sizes - causing
unnecessary I/O through the chain and
sub-optimal performance.

If you must use Dos partition tables - you can offset the first partition by 64k
- that should give
you your alignment back.

Or you could use LVM if you want server-side storage management - LVM seems to
create Logical
Volumes at sensible boundaries. But if you want that last bit of performance,
LVM is an overhead in
itself...

James M

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