Oracle FAQ Your Portal to the Oracle Knowledge Grid

Home -> Community -> Mailing Lists -> Oracle-L -> Re: RAC in NAS

Re: RAC in NAS

From: Mark Brinsmead <>
Date: Thu, 27 Jul 2006 23:39:29 -0400
Message-ID: <>

Agreed. You can live without AIO.

You'd be *amazed* at the push-back I got from various sources, though, when I suggested turning
on I/O slaves. After all, that's "obsolete", isn't it? ;-) (Not!)

But it *sucks* when you don't *know* you're living without it. I recently saw this happen to a client
who had migrated from DAS to NFS (without doing enough homework). The first hint that AIO was
not in use manifested months later with the deployment of a new application and the sudden appearance
of huge numbers of block_buffer waits, incomplete checkpoints, and all manner of other bad things
resulting from the (Synchronous) DBWR being unable to write fast enough.

Sadly, unless (and until) you know exactly *where* to look, it is almost impossible to determine whether
or not the database is actually *performing* Asynch I/O. (As most here undoubtedly realise, looking
for DISK_AYNCH_IO=TRUE does *not* do the job!)

(Almost) just for chuckles, I opened an SR with Oracle support, asking questions like "how can
I test whether my DB is doing Asynch I/O on Linux?" and "knowing that Asynch I/O is unsupported,
what are the risks of doing so anyway?". After almost two weeks, the questions are unanswered,
even though I was able to answer them myself with less than an hour of surfing Metalink and Google.

For the record: "cat /proc/slabinfo", and "there is no risk -- the Linux kernel will stop you" respectively
appear to be the answers to these questions. And hat's off (again) to Werner Puschitz for his excellent
website -- the information I found there seems to be a good deal better than Metalink...

Anyway, this is a large part of where the homework comes in. Those who fail to do it probably won't
even suspect that AIO is not being used until *long* after they start using NFS. Of course, that might
beg the question: if it has no observable affect, is it *really* a problem? But I'll leave that one to the
philosophers to ponder. I *know* I won't bother to ask OSS. ;-)

On 7/26/06, Mladen Gogala <> wrote:
> [...]
> Asynch I/O is not that important. Oracle can emulate it using I/O slaves.
> Granted,
> it's not as good as the real thing, but you will not sufer much, either.
> Direct I/O
> is much more important and it is supported. FC5 is the sign of things to
> come. It
> does support full NFS4 version, with cient caching and async I/O included.
> EL5 is
> likely to have those features, minus bugs, discovered by free beta
> testers, like
> me.
> --
> Mladen Gogala

-- Mark Brinsmead
   Staff DBA,
   The Pythian Group

Received on Thu Jul 27 2006 - 22:39:29 CDT

Original text of this message