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AU size and stripe size tips

Oracle Database Tips by Donald BurlesonDecember 14, 2015

Question:  I see the ASM "allocation_unit_size" columns in the v$asm_diskgroup table, but I don't understand the relationship between the allocation unit (AU) size and the stripe size.  Is the stripe size the same as the AU size.  If so, what is the difference between the find-grained stripe size and the coarse-grained stripe size?

Answer:  The "stripe size" is the number of "contiguous data blocks" on each physical disk and from this quote, they appear to be the same thing.  Note that stripe size = stripe depth.

Oracle notes that it is the ASM allocation Unit determines the unit of ASM storage (i.e. size of file extent , also called as stripe depth) and the stripe depth defaults to 1M.

When Oracle ASM stripes data, Oracle separates the data file into stripes and deals then evenly, out across each physical disk spindle.

"The stripes are equal in size to the effective AU. The coarse-grained stripe size is always equal to the AU size.

The fine-grained stripe size always equals 128 KB; this provides lower I/O latency for small I/O operations such as redo log writes."

It appears that coarse-grained striping and fine-grained striping are done automatically at ASM file allocation time:

"Coarse-grained striping provides load balancing for disk groups while fine-grained striping reduces latency for certain file types by spreading the load more widely.

To stripe data, Oracle ASM separates files into stripes and spreads data evenly across all of the disks in a disk group.

- The coarse-grained stripe size is always equal to the AU size (not the data extent size).

- The fine-grained stripe size always equals 128 KB in any configuration; this provides lower I/O latency for small I/O operations such as redo log writes."


- "Starting in version 11gR2 Grid Infrastructure, Oracle changed the default striping method for ONLINELOG from fine to coarse. . . . The performance of redo writes are much better with fine-grained striping, 7% for a busy system and write latency improvements around 12%. " - EMC


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