Re: superblock backups, ASM vs OCFS2
- From: "Alex Gorbachev" <ag@xxxxxxxxxxxx>
- To: jeremy.schneider@xxxxxxxxxxxxxx
- Date: Sun, 16 Dec 2007 09:53:42 -0500
Jeremy,
Oracle normally skips the first block of datafiles on raw devices to
avoid intervention with LVM headers. I wish they do that for ASM but
in ASM that space if used by ASM itself for similar purpose as in LVM.
On the other hand, ASM disks should be created on top of partitions
(in Linux terminology) but not the whole disks themselves. In fact,
ASMLib (if you happen to use it) requires that - it won't mark ASM
disk unless it's a partition. This would save your disk in case when
the header of the LUN/physical disk is corrupted. Though, partition
table may be hosed but that's recoverable and you can easilly back it
up with "dd" - it's static.
Cheers,
Alex
On Dec 13, 2007 11:50 AM, Jeremy Schneider
<jeremy.schneider@xxxxxxxxxxxxxx> wrote:
> 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
>
--
Alex Gorbachev, Oracle DBA Brewer, The Pythian Group
http://www.pythian.com/blogs/author/alex http://www.oracloid.com
BAAG party - www.BattleAgainstAnyGuess.com
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