Came across an odd problem last week. We had an LT03 tape connected to a T2000 via a
qlogic card. We could only write about 50MB/s to the thing, should have been getting
around 2Gb/s (bit, not byte). Everything was connected at 2Gb/s. Odd. Dtracing showed
that the biowait() was taking a really long time. Seems the qlc driver was taking its
time writing. Reading from the tape was fine, no worries there, could get 250MB/s there.
Then late Friday evening (GMT time) one of the guys who works in storage and knows about
the qlogic driver pointed out this setting in the qlc.conf (for s10updates mind):
He asked that I try with this set to 2048.
The throughput went from 50MB/s to 250MB/s just like that :)
So if you've got qlogic fibre to tape and want faster throughput try this out.
It was a slow tape, but now no longer!