Re: ASM on SAN

From: Greg Rahn <>
Date: Mon, 22 Feb 2010 15:20:55 -0800
Message-ID: <>

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 Mon Feb 22 2010 - 17:20:55 CST

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