Comments on: Allocation Unit and Extents In ASM

By: Amit

Amit — Mon, 24 Nov 2008 05:14:03 +0000

Yes, stripe size will remain as 128k unless you change the _asm_stripesize. Also as mentioned earlier, it would make sense to change the datafile template to fine instead of coarse. You can use following command to do the same for datafile template

ALTER DISKGROUP DGNAME ALTER TEMPLATE  datafile ATTRIBUTES (FINE);

By: Patrick Huynh

Patrick Huynh — Mon, 24 Nov 2008 05:04:41 +0000

Hi Amit,

Thanks for the reply. I just have 1 more question.
As you mentioned above “The ASM coarse striping is always equal to the disk group AU size, but fine striping size always remains 128KB in any configuration.”

If i have 64M AU, does that mean for the fine stripe, the AU will stripe across all available disks in the diskgroup with the size 0f 128K?

Thanks,
Patrick.

By: Amit

Amit — Fri, 21 Nov 2008 04:00:01 +0000

Patrick,

You need to set _asm_stripesize=1048576 parameter in spfile/pfile to change the stripe size. As mentioned these settings will be valid for new diskgroups and not existing diskgroups. You will also be required to change the templates for datafiles to incorporate this new stripe size definiton.

Please note that you cannot change the setting for existing diskgroups.

By: Patrick Huynh

Patrick Huynh — Thu, 20 Nov 2008 22:29:07 +0000

How do i change the stripe size?

By: Amit

Amit — Thu, 20 Nov 2008 07:10:50 +0000

As mentioned earlier , benefit of increased Au size is that it reduces the memory required for ASM Metadata.
I have seen database configuration using 16 Mb Allocation unit with 1 Mb stripe size . I believe you can use AU size of 64 Mb with stripe size of 1-2 M as you seem to be working with data warehouse and is likely to grow to larger size.

By: Patrick Huynh

Patrick Huynh — Thu, 20 Nov 2008 06:37:42 +0000

Hi,

I have a database which is about 25Tb. Is it better to pick the largest AU size (64) for my disk group?

Thanks,
Patrick.

By: jason arneil

jason arneil — Mon, 06 Oct 2008 10:21:52 +0000

I would say one of the benefits of both larger AU size and indeed variable sized extents was to reduce the amount of SGA in the RDBMS instance that is utilised for keeping the extent maps!

jason.