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Date: Sat, 30 Aug 2003 17:17:17 +1200
From: Mark Kirkwood <markir@paradise.net.nz>
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Subject: IO To IDE Blocking In 5.1
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I am running Freebsd 5.1 on a Dell 410 with 2xPII and 2xIDE drives (each 
on its own channel).

Drive #1 as 1 filesystem  mounted on /, drive #2 has 1 filesystem, 
mounted on /data1
(both have softupdates enabled)

i) start a process doing some io on drive #1
ii) start iostat 5

iostat blocks until the process is completed. If I start iostat *first*, 
then it does not block, and
I can see the io activity generated by the process when it starts.
If I try this test on drive #2, then neither case blocks.

I am running the SMP kernel, but can reproduce this using GENERIC.

Any ideas ?

P.s : This came to my attention whilst experimenting with the smp 
kernel, I was able to get nice 2 processer scaling with 2 running 
processes *except* if they both needed to do IO (even if they were doing 
it on different drives).

Note : Have previously run 5.1 on a HP VE6, and dont recall encountering 
this issue.

regards

Mark