Originally posted by: Wouter Liefting
Most LUNs on a SAN are built on top of a RAID-5 (or similar) volume, and RAID-5 stripes its blocks by default. So you don't have a lot of choice here. For best performance you may want to make sure that the LUN stripe size is identical to the AIX JFS2 block size.
Striping your JFS volume only makes sense if:
a. Your typical read or write is larger (a lot larger) than the block size - with just small reads and writes striping doesn't give you a lot of benefit anyway.
b. The LV that contains your JFS is spread across multiple PVs, and these PVs are LUNs that are part of different RAID sets in your SAN.
In short, you've got to view the whole stack of where your data is actually residing from the top (file) down (individual disks), and if striping increases the number of disks you can have working in parallel, it generally gives better performance. But only for large reads or writes which will actually put multiple disks to work.
Yes, and the queue depth is a tuneable parameter. Increasing it generally gives better performance, particularly if file access is not linear, but if you have lots of different LPARs all trying to access the same shared storage, each with a large queue depth size, it may lead to saturation of the storage box. In that case, reducing the queue depth may well give a better overall throughput. So with the queue depth, as with all tuneable parameters: Create a baseline first, change parameter, measure throughput against baseline, rinse, repeat.
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