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Alignment size?

To: xfs@xxxxxxxxxxx
Subject: Alignment size?
From: Michael Tokarev <mjt@xxxxxxxxxx>
Date: Fri, 13 Aug 2010 02:10:39 +0400
Openpgp: id=804465C5
Organization: Telecom Service, JSC
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9.1.10) Gecko/20100619 Icedove/3.0.5
Hello.

I used XFS for a long time on many different
servers, and it works well.  But now I encountered
an.. unexpected problem.

The question is: on one of our servers, XFS requires
different alignment size for O_DIRECT operations than
on others.  Usually it's 512 bytes, but on this server
it is 4096 - both min_io and alignment (this is from
XFS_IOC_DIOINFO ioctl).

I'm not sure what the reason for this is.
On this server, the underlying block device is raid5
(linux sw raid), but we had other machines with raid5
which didn't have that alignment requiriments.

The problem with that is that Oracle db, which we use
with XFS alot, refuses to work on this machine, or,
rather, XFS refuses to process I/O in 512-byte chunks
from oracle (control files and redolog files).

I know it is a frequent combination which is used in
production in many places, and is used here alot too,
but I haven't seen anyone mentioning this issue we
have now, with "larger than usual" alignment size
requiriments.

Is there a way to remedy this somehow, without
reformatting whole 600+ gb?

Thank you!

/mjt

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