Re: RAID system max throughput

• DENNIS WILLIAMS <DWILLIAMS_at_LIFETOUCH.COM>

> 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
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Author: Steven Lembark
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