-- DENNIS WILLIAMS <[EMAIL PROTECTED]>
> Whenever I discuss disk waits with my system administrator, I always get
> the reply that "the RAID system isn't anywhere near its rated
> throughput". Maybe I'm wrong, but I don't see any of the tuning books
> mentioning that as a relevant performance characteristic. However, I've
> never been able to move the discussion beyond this point. Can anyone
> straighten me out on this point or point me to a resource that might be
> applicable.
>
> Our system is Oracle 8.1.6, Compaq Tru64. We use hardware RAID-5 with a
> battery-backed RAM cache, and have about 3 RAID sets (plus some extra
> disks for redo logs, etc.), and performance is fine, but I'm always
> looking as to how we can improve Oracle performance. The application is
> our corporate ERP system.
Two things will zap you on device I/O: bandwidth or latency.
Most people look at bandwidth -- same basic numbers for both
networking and disks. Latency is basically the turnaround time.
If you screw up the setup of any I/O system then latency can
reduce performance to the point where bandwidth is irrelevant.
By analogy, you can put concrete tires on a Porsche and go
nowhere also.
The real measure of what's going on starts at the O/S level
looking at the frequency and duration of proc's in a device
wait state (a.k.a. "blocked for I/O") on the disks. If this
is minimal then forget it.
You can also end up with screwy results on large shared disk
systems due to competition. SAN's can get placed on overloaded
network segments; ERP's can easily get hot-spots from various
users colliding.
In general RAID5 with a stripe size == system I/O page will
perform rather nicely. If your system page sizes vary or
the raidset has an offball number of disks (e.g., 6 drives
for an 8K page) then you'll take a hit writing extra data
to maintain the RAID5 parity.
--
Steven Lembark 2930 W. Palmer
Workhorse Computing Chicago, IL 60647
+1 800 762 1582
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Author: Steven Lembark
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