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SubjectRe: File read-ahead and Linux-1.3.85
  Date: 	Tue, 9 Apr 1996 23:22:42 +0000 (GMT)
From: Gerard Roudier <groudier@iplus.fr>

My patch was experimental but does the right work.
Yours seems to be good, but is catastrophic with drivers that can queue more
than one command at a time for a device.
I would be better that you let filemap.c unchanged.

You should see how well it didn't work on my system qiwth queue depths of 31.

Allowing more than 1 command per lun in a SCSI driver without support of
tagged queue is Silly, since command sorting algorithm of the
kernel is broken for nothing (save perhaps some micro-seconds/command).
This behaviour may have some bad effects on performances with production
systems.

Actually, I've found that 2 or 3 is optimal with tagged queuing turned off.
With only 1, the host adapter has to wait for the interrupt handler to queue
the next command, and host adapters with mailbox interfaces take a little
longer to start working on it than host adapters where the kernel is in more
direct control.

Leonard


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