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RAIDframe level 5 write performance(was: tstile lockup)

There seems to be a fundamental problem with writing to a level 5 RAIDframe set,
at least to the block device.

I've created five small wedges in the spared-out region of my 3TB SAS discs.
In case it matters, they are connected to an mpt(4) controller.

Then I configured a 5-component, 32-SpSU, level 5 RAID set.
Unless I've gone mad, 32 sectors per SU equal 16k per SU, i.e. 64k per stripe.

Writing to that RAID's block device (raid2d) in 64k blocks gives me a dazzling
troughput of 2.4MB/s and a dd mostly waiting in vnode.
Writing to the raw device (rraid2d) gives 240MB/s, i.e. is two orders of
magnitude faster.
Reading is at 22MB/s for the block device and 315MB/s for the raw device.
Writing 16k chunks to the block dev drops to 530kB/s.

To make sure there's nothing wrong with my discs, I the configured a two-
component level 1 array with 128 SpSU (again giving a 64k stripe size, I hope).
With that, I write 57MB/s to the block and 85MB/s to the raw device.
Reading is 21MB/s for the block dev and 87MB/s for the raw dev.

So there's nothing wrong with my discs, I think; there's something wrong with
level 5 RAIDframe. My impression is that NetBSD 4 didn't have that issue but
I can't test because the discs on that machine are too large.

How can I analyze/debug this? Performance is so insanely bad I cannot possibly
put this file server into production.

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