Message-ID: <611ad3510712130850j4575aea3k4d43481b27699908@mail.gmail.com>
Date: Thu, 13 Dec 2007 10:50:52 -0600
From: "Jeremy Schneider" <jeremy.schneider@ardentperf.com>
To: Oracle-L <oracle-l@freelists.org>
Subject: superblock backups, ASM vs OCFS2
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Just wondering, does anyone know much about "superblock" backups in ASM vs
OCFS2?

I ran into an interesting case a month or so back where someone had
accidentally tried to initialize their ASM disks with linux LVM...  and
written the LVM headers to the disk.  It was just a few bytes at the very
top of the disk - but it was enough to totally hose ASM.  Which started me
thinking, "if this was a filesystem then I'd have a backup superblock that I
could recover".  Who knows - maybe ASM has a backup of its header block -
but it's all proprietary and if there's a tool that will recover an ASM
header then it's probably buried at Oracle support somewhere.

Looks like OCFS2 includes superblock backups since this patchset:
http://lkml.org/lkml/2007/1/22/148

Not sure if ckfs will recover them but since it's open source it'd be
trivial to put together a utility that would recover a superblock.

This seems to me to be a great reason to choose OCFS2 over ASM.  Recovering
a backup superblock is MUCH faster than recreating the entire volume and
restoring data from backup!!!  I don't even know if you could use dd to try
to backup your ASM disk headers - since it's proprietary I don't know what's
in those blocks.

Anyone have any thoughts on this?  Is there something I'm missing here?

Jeremy


-- 
Jeremy Schneider
Chicago, IL
http://www.ardentperf.com/category/technical

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Just wondering, does anyone know much about &quot;superblock&quot; backups in ASM vs OCFS2?<br><br>I ran into an interesting case a month or so back where someone had accidentally tried to initialize their ASM disks with linux LVM...&nbsp; and written the LVM headers to the disk.&nbsp; It was just a few bytes at the very top of the disk - but it was enough to totally hose ASM.&nbsp; Which started me thinking, &quot;if this was a filesystem then I&#39;d have a backup superblock that I could recover&quot;.&nbsp; Who knows - maybe ASM has a backup of its header block - but it&#39;s all proprietary and if there&#39;s a tool that will recover an ASM header then it&#39;s probably buried at Oracle support somewhere.
<br><br>Looks like OCFS2 includes superblock backups since this patchset:<br><a href="http://lkml.org/lkml/2007/1/22/148">http://lkml.org/lkml/2007/1/22/148</a><br><br>Not sure if ckfs will recover them but since it&#39;s open source it&#39;d be trivial to put together a utility that would recover a superblock.
<br><br>This seems to me to be a great reason to choose OCFS2 over ASM.&nbsp; Recovering a backup superblock is MUCH faster than recreating the entire volume and restoring data from backup!!!&nbsp; I don&#39;t even know if you could use dd to try to backup your ASM disk headers - since it&#39;s proprietary I don&#39;t know what&#39;s in those blocks.
<br clear="all"><br>Anyone have any thoughts on this?&nbsp; Is there something I&#39;m missing here?<br><br>Jeremy<br><br><br>-- <br>Jeremy Schneider<br>Chicago, IL<br><a href="http://www.ardentperf.com/category/technical">http://www.ardentperf.com/category/technical
</a><br>

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