Re: Matching storage stripe size with ASM allocation unit?
Date: Tue, 27 May 2008 09:27:20 -0700 (PDT)
Thanks, Alex and Gopal. We use external storage as most shops do. I was looking for an explanation why for best performance the storage team needs to use a stripe size of 1M for Oracle ASM, assuming they're extremely collaborative and 1M is technically feasible. Nitin and others' ASM book has one paragraph on p.76 that sounds relevant:
"the best stripe size [contiguous space on 1 disk, not "stripe width" in their terminology which means stripe size times number of striped disks] for reading is 1MB. However, few storage arrays support that large a stripe. For efficient writing, it is desirable to have a single ASM extent span all the disks in the RAID 5 set; for example, with four data and one parity, a stripe size of 256K would mean that a full 1MB ASM extent write would hit all disks, and no extra reads would be required for calculating parity. Note that for this to work, the correct alignment is critical."
I thought the ASM allocation unit was only meaningful during file or ASM disk creation or resizing. It looks like it does play a role in regular I/O operations. Also, the book implies that if storage array supports, 1M stripe size (i.e. 4MB stripe width in case of 4+1 RAID 5) is better than 256K stripe size; we fall back to 256K only because "few storage arrays support" 1M stripe size. The 1M stripe size implied here is exactly what the Oracle consultant told me.
Yong HuangReceived on Tue May 27 2008 - 11:27:20 CDT