Re: Are large RAID stripe sizes useful with FreeBSD?

Ivan Voras wrote:
Most of new hardware RAID controllers offer stripe sizes of 128K, 256K
and some also have 512K and 1M stripes. In the simplest case of RAID0 of
two drives, knowing that the data is striped across the drives and that
FreeBSD issues IO request of at most 64K, is it useful to set stripe
sizes to anything larger than 32K? I suppose something like TCQ would
help the situation but does anyone know how is this situation usually
handled on the RAID controllers?

Large I/O sizes and large stripe sizes only benefit benchmarks and a
narrow class of real-world applications. Large stripes have the
potential to actually hurt RAID-5 performance since they make it
much harder for the card to a full stripe replacement instead of a
read-modify-xor-write.

I hate to be all preachy and linux-like and tell you want you need or
don't need, but in all honesty, large i/o's and stripes usually
don't help typical filesystem-based mail/squid/mysql/apache server
apps. I do have proof-of-concept patches to allow larger I/O's for
selected controllers on 64-bit FreeBSD platforms, and I intend to clean
up and commit those patches in the next few weeks (no, I'm not ready for
nor looking for testers at this time, sorry).

Scott
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