Re: Tuning RMAN backup and recovery

  • From: "Mark Brinsmead" <pythianbrinsmead@xxxxxxxxx>
  • To: "Ranko Mosic" <ranko.mosic@xxxxxxxxx>
  • Date: Sun, 2 Dec 2007 17:55:46 -0700

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

On Dec 2, 2007 7:03 AM, Ranko Mosic <ranko.mosic@xxxxxxxxx> 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 <pythianbrinsmead@xxxxxxxxx> 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: mosicr@xxxxxxxxxx

-- Mark Brinsmead
  Senior DBA,
  The Pythian Group

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