ASM Allocation Unit size and storage waste

[Dennis Bielefield]

We have an HP-UX environment with an XP24000 storage array on which we have configured Oracle 11g ASM storage. We have implemented 8 MB Allocation Unit (AU_SIZE) for the data and flash diskgroups. On our flash diskgroup we have noticed severe storage waste for flash logs where, for example, 64 MB would be allocated, but less than 8 MB used out of a flash log. We're wondering if using a smaller AU_SIZE for flash will help (1 MB or less) and are interested in what other implementations use for their AU_SIZE. Attached are some figures.

The bottom line is for flash logs, out of 4.77 TB allocated, only 3.01 TB is actually in use, wasting 1.71 TB which we could really use...

What are other implementations using for their AU_SIZE? Has anyone explored using an AU_SIZE smaller than the default 1 MB. I've seen recommendations for a 4 MB size for some systems. Thanks.

[TwoProc]

This isn't an answer, only maybe a helpful resource to help in your thoughts:

Pretty good summary article on ASM and AU_SIZE:

http://www.remote-dba.net/t_11g_new_asm_performance.htm

>>> note the last paragraph at the end re: redo logs, which in my mind, somewhat parallel flash back areas, at least in the cut/slice utility of them, much more so than a regular db file:

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By using fine grained striping for redo log files, a lower latency is reached because smaller units get distributed across all disk members of an ASM disk group. Oracle uses AUs as stripes for files in ASM storage and spreads a file in an ASM disk group across all disks in a fail group. So now there is striping within a fail group and mirroring across the fail groups within a disk group. By using coarse striping for datafiles, Oracle achieves load balancing of read write operations across all disks within a disk group.
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above from "ASM Performance Enhancements in Oracle 11g" - Burleson

FWIW - Burleson always has good books on Oracle topics. Never seen a bad one, and I've purchased a number of them over the years.
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[Dennis Bielefield]

Thanks John for your reply. Yes, I had read this one already; agreed, it is a pretty good write-up, while not helping with this particular issue. Anyone else have any insights?

[Dennis Bielefield]

Hi TwoProc, after researching a bit more, I am going to look a bit into the idea of changing the stripe type on the Flash Logs from fine to coarse, as well as the AU_SIZE on the FLASH diskgroup from 8 MB to 1 MB. I think the two of these will prevent the total overallocation of space to the small flash logs.

[TwoProc]

Well, did it work?