From: Luca Canali <>
Date: Tue, 23 Feb 2010 10:02:13 +0100
Message-ID: <>

BTW, MOS Note 759429.1 recommends "Disable ASM Variable sized extents if using a 4MB AU size and files will exceed 80G", there is also a short explanation in the note. While this seems to apply to, I wonder if it is also the case for 11.2 and for "non-exadata" DBs too.


-----Original Message-----
From: [] On Behalf Of Greg Rahn Sent: 23 February 2010 00:21
To: Martin Bach
Cc:; oracle-l
Subject: Re: ASM on SAN

The default in 11g for AU_SIZE is still 1MB, and with Exadata it is set to 4MB. The reason for this is that it places 4x as much data on a single ASM Disk (also called a Grid Disk with Exadata, which is really a partition of a physical disk) which results in more sequential read throughput. In terms of sequential scan throughput, there is only modest gains after 4MB.

As mentioned by some of the other responses AU_SIZE in 11g can be set to 1, 2, 4, 8, 16, 32, and 64 MB and it is also variable in size. It is AU_SIZE for the first 20k extents, then 8*AU_SIZE for the next 20k extents, then 64*AU_SIZE for the next 20k, etc. ( see )
One of the reasons for this was to reduce the metadata sizes as the number of extents get larger.

On Mon, Feb 22, 2010 at 8:17 AM, Martin Bach <> wrote:
> I remember a MOS document "Note 810484.1 "Recommendation for 4MB ASM
> Allocation Unit (AU) size for Oracle 11g" which seems to have
> disappeared when I tried to look it up is recommending au_size > 1MB. I
> found reference to it under the exadata best practices but I wondered
> when you'd set the AU_SIZE to a non-default value.

Greg Rahn

Received on Tue Feb 23 2010 - 03:02:13 CST

Original text of this message