Re: ASM versus Filesystems

  • From: Thomas Roach <troach@xxxxxxxxx>
  • To: Jon.Crisler@xxxxxxx
  • Date: Fri, 5 Mar 2010 21:51:14 -0500


I would like to add a couple of things. You can still do your backups to an
ASM diskgroup. In our case, we backup to FRA and then backup our FRA to
Tape. When we restore from tape, it will restore directly to the DB from
tape if the backup is not in FRA.

What we do is we keep at least 1 full backup in FRA, plus all incrementals
for 1 week, and keep anything over that on tape.

Also, you can have disk snapshot facilities, but you need to put your
database in backup mode (and test your backups). The reason you need to do
this is because database operations are still happening and blocks are being
written to disk. This is so that blocks that are touched for the first time
during the backup window are written to the redo stream since a block can be
corrupted while it is being backed up (Oracle is in the middle of a write
operation and only half the new data gets written during a snapshot of that
block). The benefit RMAN has is that it can check a block for corruption
where snapshots are unaware of what is and isn't a valid oracle block, it
just backs up the data.

ASM is very powerful and scalable. I was able to add luns and remove luns
(rebalancing them into ASM) without having to take my database down. Also,
only use ASM with block devices (scsi, iscsi etc...) and not on top of a
filesystem or NFS as the extra layers don't gain you anything. If you do use
disk based snapshots then if you add luns/remove luns, make sure you update
what you snapshot. If you miss just one disk, you are in deep trouble.

I also bought the Oracle Press Book by Nitin Vengurlekar (spelling) which is
a very good book.

Good Luck!


On Fri, Mar 5, 2010 at 9:09 PM, Crisler, Jon <Jon.Crisler@xxxxxxx> wrote:

>  The only bad thing about ASM is the learning curve; honestly I think it
> was fear of the unknown that kept me away from ASM for so long.  Once you
> jump in and get comfortable with it you too will wonder why you stayed away.
> If you do backups to disk that are later picked up by a tape program, you
> will still need a filesystem to hold the backup, or if you use some sort of
> disk snapshot facility it may not support ASM, but those are the only
> drawbacks I am aware of.   It does require one to get more familier with
> RMAN.   Compared with OCFS2 or other clustering filesystems it seems to be
> quite reliable, and if you are using it with RAC then you save a bundle on
> license cost for other clustering filesystem software (like GFS, Veritas
> etc.).
> Jon - aka “Capt Aubrey”
> *From:* oracle-l-bounce@xxxxxxxxxxxxx [mailto:
> oracle-l-bounce@xxxxxxxxxxxxx] *On Behalf Of *patrick obrien
> *Sent:* Friday, March 05, 2010 5:45 PM
> *To:* Oracle L
> *Subject:* ASM versus Filesystems
> Oracle Admins,
> I've been an AIX Admin for years, I'm a junior Oracle DBA and I apologize
> if the ASM Topic has come up lately. As an AIX Admin, using filesystems
> seems the best option for me.
> Reading up on Oracle's ASM technology, it looks like this could be a great
> option, primarily for performance reasons. Oracle would then own more real
> estate, so it can use its tools to better tune the entire system. Its almost
> too good to be true.
> But what are the caveats?
> AIX Filesystems offer me control on filesystems/directory sizes, increased
> performance and systems control. Filesystems are nice when managing backups
> too. With the advent of the NAS/SAN, maybe I can just hand it all over to
> Oracle.
> Any body not like ASM out there?
> Thank you,
> Patrick.

Thomas Roach

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