sym53c8xx2: incredible sloth after parity error / SCSI bus reset

[Nix <nix@esperi.org.uk>]

About once a year I get a SCSI parity error on one of my systems (the
only one with SCSI). I presume the cabling is substandard, but given my
coordination deficits and the rarity of the errors I'd do far more
damage replacing it than leaving it be.

I had one of these today.

The system (2.6.23.9) spotted the error, and seemingly recovered:

Dec  1 12:53:40 loki warning: kernel: sym0: SCSI parity error detected: SCR1=132 DBC=50000000 SBCL=0
Dec  1 12:53:40 loki warning: kernel: sym0:0: ERROR (81:0) (8-0-0) (10/9d/0) @ (mem c2800048:ffffffff).
Dec  1 12:53:40 loki warning: kernel: sym0: regdump: da 00 00 9d 47 10 00 0e 00 08 80 00 80 00 0f 0a d0 58 3c 01 02 ff ff ff.
Dec  1 12:53:40 loki warning: kernel: sym0: SCSI BUS reset detected.
Dec  1 12:53:40 loki notice: kernel: sym0: SCSI BUS has been reset.

However, after that reset I/O to any device on that controller is
*incredibly* slow.

A monthly RAID check kicked off shortly afterwards and provided my first
clue. Load average >15, and:

md1 : active raid5 sda6[0] hdc5[3] sdb6[1]
      76807296 blocks super 1.2 level 5, 64k chunk, algorithm 2 [3/3] [UUU]
      [========>............]  check = 42.8% (16450780/38403648) finish=253.3min speed=1442K/sec

1442Kb/s is a bit less than I'd expect from a three-drive array with
disks capable of 40Mb/s easily.

/dev/sda:
 Timing buffered disk reads:    8 MB in  3.50 seconds =   2.29 MB/sec

A somewhat slower ATAPI disk on the same machine:

/dev/hdc:
 Timing buffered disk reads:  110 MB in  3.05 seconds =  36.08 MB/sec

So, um, what could cause this? Can I speed it up again other than by
rebooting (which I'm just about to do, but it is annoying).

-- 
`Some people don't think performance issues are "real bugs", and I think 
such people shouldn't be allowed to program.' --- Linus Torvalds
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