Re: superblock backups, ASM vs OCFS2
- From: "Jeremy Schneider" <jeremy.schneider@xxxxxxxxxxxxxx>
- To: "Ghassan Salem" <salem.ghassan@xxxxxxxxx>
- Date: Thu, 13 Dec 2007 17:38:01 -0600
FYI, from the traffic on the ocfs-users list it sounds like the disk headers
contain information that isn't captured by md_backup. But I'm going to hold
off on saying anything else until I've tried it out...
On 12/13/07, Ghassan Salem <salem.ghassan@xxxxxxxxx> wrote:
>
> Jeremy,
> asmcmd, in 11.1.0.6 already has a 'backup' command (and a restore). Is it
> enough? I haven't tried it yet to see what it can recover.
>
> rgds
>
> On Dec 13, 2007 8:03 PM, Jeremy Schneider <jeremy.schneider@xxxxxxxxxxxxxx>
> wrote:
>
> > OCFS2 and ASM both support async and directIO (note 279069.1)**.
> > Performance-wise, I'm pretty sure that there's not a big difference one way
> > or the other for day-to-day operation. There may be some minor differences
> > when extending files or creating new ones but I don't think that they're
> > anything to fuss over.
> >
> > Since there's no major difference in performance-related capabilities,
> > for me the choice would probably hinge on other factors.
> >
> > - OCFS2 is open-source. (+1 for power-users!)
> > - OCFS2 already does superblock backups. (+1 for recoverability.)
> > - ASM is integrated with Database Console/Grid Control. (+1 for
> > manageability.)
> > - ASM does cluster-aware volume management. (+1 for flexibility.)
> >
> > I do think that the last one is a strong point in favor of ASM. Volume
> > management really can simplify things. And with ASM you get volume
> > management and still have a single point of contact for any bugs you
> > discover - no finger-pointing. Also, I don't know of any open-source LVM
> > that I'd really want to use to do any kind of redundancy - so if you need
> > software mirroring on linux then I think that ASM is really the best
> > solution for you. And if you want ease-of-management then ASM is better in
> > that area too.
> >
> > Now you could get volume management and stay open-source with OCFS2 by
> > using cLVM. However I don't think that cLVM currently supports mirroring or
> > parity. IBM's EVMS also has cluster functionality and is GPL - but from the
> > release history and mailing lists it seems like noone's maintaining it
> > anymore. And I think you can buy ServiceGuard for Linux and get
> > cluster-aware volume management there too. There might be more commercial
> > solutions that I'm not aware of.
> >
> > Not to mention GFS, which I'd also really like to spend more time
> > learning someday. GFS has its own feature called "pools" which also provide
> > volume management though I think that RedHat is moving toward cLVM for
> > cluster volume management with GFS. You can run your Oracle RAC database on
> > GFS (note 329530.1) but if there are any filesystem-related bugs then
> > Oracle will tell you to call RedHat for the fix (note 423207.1).
> >
> > Anyway, just some random musings... I've always liked being able to dig
> > into the source code of whatever's running on my systems so I'm kindof keen
> > on OCFS2... but it does appear that the general push is toward ASM these
> > days.
> >
> > DB+RAC+ASM/OCFS+OEL+Oracle VM... is there anything Oracle doesn't do
> > these days? Next year I'm anticipating Oracle Open Office... after all
> > it's the final frontier...
> >
> > ========
> > Addendum: while I was writing this email there were a few responses on
> > the ocfs-users list (I cross-posted).
> >
> > Definitive answers:
> > OCFS: "Backup superblock has been available with ocfs2-tools 1.2.3 (Mar
> > 2007)."
> > http://oss.oracle.com/projects/ocfs2/dist/documentation/ocfs2_faq.html#BACKUPSB
> >
> >
> > ASM: "In 11.1.0.7 (and beyond) we will have a backup of the disk header
> > (the
> > first 4k). Prior to that, we have been able in most circumstances to
> > reconstruct the disk header using KFED. KFED will still be the tool to
> > restore the disk header going forward, it will just be a simpler, more
> > reliable procedure."
> >
> > So it looks like OCFS backs up your superblock as long as you got it
> > after March. And it is often possible to reconstruct an ASM disk header and
> > backups will be made in 11.1.0.7 and beyond. And if you ever wondered
> > what that KFED tool was for...
> > ========
> >
> >
> > On 12/13/07, Finn Jorgensen <finn.oracledba@xxxxxxxxx> wrote:
> > >
> > > Jeremy,
> > >
> > > I don't know the answer to your question but this reasoning :
> > >
> > > >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!!!
> > >
> > > is like saying you should use MySQL instead of Oracle because if you
> > > lose one file you can just restore that one file and carry on. No
> > > downtime,
> > > no recovery. Once human error enters the picture all bets are off. What if
> > > your "someone" had decided to dd a bunch of data into the device your
> > > OCFS2
> > > filesystem is built on? Your superblock backup would have been obsolete.
> > > What if the performance requirements for your database aren't satisfied by
> > > OCFS2?
> > >
> > > Your question is valid and interesting I just don't see it as a reason
> > > to choose one option over the other.
> > >
> > > Finn
> > >
> > >
> > > On 12/13/07, 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
> > > >
> > >
> > >
> >
> >
> > --
> > Jeremy Schneider
> > Chicago, IL
> > http://www.ardentperf.com/category/technical
> >
>
>
--
Jeremy Schneider
Chicago, IL
http://www.ardentperf.com/category/technical
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