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Re: IO rate for U320 SCSI 15K RPM Hard Drive

From: Mark Brinsmead <>
Date: Sat, 14 Oct 2006 14:51:15 -0600
Message-ID: <>


In fact, it is not uncommon -- when price is no object -- to use the outer 10% or less or the spindle(s). I base this remark mostly on things like TPC benchmark full disclosure reports, where the discerning (and maybe even
not-so-discerning) reader will note that the disk space provisioned for the database is 10x (or more) than what
is actually used. You can be certain that the benchmarkers are not buying all of those disks and leaving 90%
of them untouched...

The advantages of using only the "outer X%" of a disk are pretty well known. If you choose X low enough
(usually doesn't need to be much lower than 10, I would think) you should be able to drive your "average"
seek times amazingly low. And it doesn't exactly hurt the throughput for (large) sequential I/Os, either.

Sadly, my personal experience differs considerably from Alex's; I have very rarely been able to convince
management that a RAID-5 stripe of 10x500GB ATA disks might not be ideal for high demand databases,
or that the corporate ERP databases perhaps should not share physical spindles with the e-mail system.
Selling managers who are fixated on metrics like dollars-per-gigabyte (or dollars-per-terabyte now) on such
concepts can be very difficult. ;-)

In defence of the dollars-fixated managers, though, this sort of consideration is probably only relevant for the largest and most demanding of environments. With suffiently large block cache in the database (for reads)
and NV cache in the disk array (for writes), it is not too terribly difficult to engineer a (modest) database that very rarely performs physical I/O... Well, except at backup time. Or reporting. or...

Don't get me wrong -- I am a firm believer in BAARF, and I really do think building a 4TB database on 6 disks
is almost always insane. But I am also starting to encounter more and more databases where the memory
(available to be) used for block buffers ranges from 60% to 200% (!) of the total database size. (Yes, I
have encountered servers with 8GB of RAM and 2GB databases...)

On 10/13/06, Alex Gorbachev <> wrote:
> At one company I worked for we divided the disk into 4 quarters and
> used only outside two or one for most demanding databases. In extream
> cases we could go as low as 12 or less percent of disk surface.
> ...

-- Mark Brinsmead
   Senior DBA,
   The Pythian Group

Received on Sat Oct 14 2006 - 15:51:15 CDT

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