From: pcg@aber.ac.uk (Piercarlo Grandi) Subject: Re: SCSI Performance (Yet Again) Date: Sun, 22 Aug 1993 20:02:01 GMT
>>> On Sat, 21 Aug 1993 16:47:58 GMT, root@hip-hop.suvl.ca.us (Remco
>>> Treffkorn) said:
Remco> Listen, the point here is simply that some people are wondering if the
Remco> SCSI performance is all the way up there *or* if it could be improved.
Oh yes, several.
Remco> Natually they are comparing notes. The problem is only that nobody
Remco> can agree on the correct way to measure performance. Iozone is certainly
Remco> not perfect, but gives you a relative measure if compiled correctly.
Yes, it gives you a relative measure, but perhaps it is not of disk
performance.
Actually one would want to have an idea of how good each of these
components is *in isolation*;
1) the generic SCSI code
2) the HA specific SCSI code
3) the disk SCSI code
4) the filesystem code
5) the cache handling code.
Also, it is rather useful to observe not only how long (elapsed) the
operations take, but also the system CPU time taken.
My *impression* is that the SCSI drivers are rather OK (e.g. by
comparison with SVR4, which has some hideously inefficient drivers), in
that they have relatively little delay and consume relatively little
CPU, and that the biggest problems are, in order of decreasing
importance:
a) the filesystems don't do read/write clustering.
b) advising for expected access patterns should be
possible/automatic.
c) the cache code should be eliminated (mmap'ing everywhere).
d) the allocation clustering algorithm could be improved.
Point c) is being done by Linus Torvalds; but it can have a severely
negative performance impact, as Sun discovered a couple years ago,
unless it is done after b).
Doign a) and d) would not be too difficult, and would eliminate the need
for blocks and fragments (a poor idea anyhow).