[Ocfs2-users] partition offset/alignment on SAN devices.

[Luis Freitas]

Thomas and James,

   Usually the partition is aligned to the Cylinder boundary. In the case of a 
LUN, the cylinder bondary might have no sense at all? Funny, I never tought 
about this before.

  There are some tools that can overwrite a few sectors on the start of the disk 
(lilo, grub, DOS fdisk), and expect the disk to have a partition table, where 
these sectors are not used for this reason. 


   I don't know if OCFS2 has provisions for leaving this space unused.

Regards,
Luis



________________________________
From: "Thomas.Zimolong at bmi.bund.de" <Thomas.Zimolong at bmi.bund.de>
To: james.masson at tradefair.com; ocfs2-users at oss.oracle.com
Sent: Fri, July 9, 2010 11:24:38 AM
Subject: Re: [Ocfs2-users] partition offset/alignment on SAN devices.

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

Hi folks,

we had that issue too with our CX4-480 and the guys from EMC telling us not
to forget the alignment.

So we first wondered how to automate this, because we'd have had to align the
partitions on quite a few LUNs (more than 40).
Since you'd have to use the expert options of fdisk, we did'nt find a quick
solution with sfdisk, though with some further investigation there will
surely be one.

But in the end this was not necessary anyway. We specifically requested that
at EMC and they confirmed, that alignment is only a point when using
partitions at all. We don’t use partitions, so no todo.

IMHO: Concerning the risk of destroying whatever is on the LUN: If someone
thinks the LUN is unused, just because he/she sees no partition on it, isn't
that more of "a lack of diligence"?
We have a similar situation with ASM-devices: Even if you have a partition on
it, it's not mountable (as there's some ASM data inside it and no FS). So
you'd always have to check more than that to be sure, that the device is
unused.

Maybe you can clarify that with the vendor of your shared disks.

Greetings,
Thomas

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