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Re: Tuning RMAN backup and recovery

From: Mark Brinsmead <>
Date: Sun, 2 Dec 2007 17:55:46 -0700
Message-ID: <>

Thank you, Ranko.

Of course, I was simplifying for the purpose of explanation.

As it happens, the platters all spin synchronously, and -- as you point out yourself -- the heads all move synchronously. That there are multiples of each is actually completely irrelevant with respect to understanding basic disk drive physics, or the nature of "I/O contention". In terms of "geometry", the disk effectively still *logically *has one head and one platter. The actual number of heads and platters generally affects only one thing: transfer rate. And transfer rate itself is almost meaningless in the context of single-block random reads -- at least as long as (average-seek-time + rotational-latency) exceeds single block transfer time by orders of magnitude.

When I referred to disks with "multiple heads" no longer existing, I was actually referring to "multiple heads *per platter*". Such things *used* to exist (N independent heads per platter increases random I/Os per second by a factor of *at least* N, and with a good scheduling algorithm, much better than that) but I have not seen such a device in a very long time.

The references you have linked to do look quite good. I have not read them in detail, but these do look like good resources for somebody who wants to better understand how disk drives work. And a good basic understanding of disk drive behaviour is very important indeed to a DBA trying to tune I/O performance.

On Dec 2, 2007 7:03 AM, Ranko Mosic <> wrote:

> <It has a spinning platter, and a (one!) moving arm.>
> Disk has more than one platter.
> Each platter has two heads, one on the top of the platter and one on
> the bottom, so a hard disk with three platters (normally) has six
> surfaces and six total heads
> This means that when the actuator moves, all of the heads move
> together in a synchronized fashion. Heads cannot be individually sent
> to different track numbers
> On 11/28/07, Mark Brinsmead <> wrote:
> ...
> >
> > Don, to better understand the issue of I/O contention, remember how a
> > disk works. It has a spinning platter, and a (one!) moving arm. (Yes,
> > multi-actuator disk drives exist. But I haven't actually seen one for
> > decades; they are too costly to manufacture and most IT managers are so
> > fixated on metrics like $/GB that foolish matters like "throughput" and
> > response time are irrelevant. At least until well after the purchasing
> > decision has been made.)
> >
> > To read a given block of data, the disk drive need to move the arm to
> the
> > right track, and then wait for the required data to rotate beneath the
> head.
> > Sometimes it is a large movement ("seek"), and sometimes it is small.
> > Sometimes, the required data comes up right under the head, others you
> need
> > to wait for a full rotation.
> >
> > Simple first-year college math (which I confess to having mostly
> > forgotten nearly 20 years ago) shows pretty easily that for random I/Os,
> you
> > will need -- on average -- to move the arm half way across the disk, and
> > spin the platter for one half of a rotation. Once the required
> datablock is
> > beneath the read head, the data is read; usually in much less than a
> > millisecond.
> ... <>
> --
> Regards,
> Ranko Mosic
> Consultant Senior Oracle DBA
> B. Eng, Oracle 10g, 9i Certified Database Professional
> Phone: 416-450-2785
> email:

-- Mark Brinsmead
  Senior DBA,
  The Pythian Group

Received on Sun Dec 02 2007 - 18:55:46 CST

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