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Re: Do Disk Heads Stay in the Begining or Middle of a disk when idle ?

From: Nuno Souto <>
Date: 3 Jul 2002 20:10:42 -0700
Message-ID: <> (Bass Chorng) wrote in message news:<>...

> Does anybody know which is right ? Or does it depend on the disk manufacturer
> ( I really doubt that though ) ?

Neither is completely right. Or rather: they are both absolutely right! It is highly dependent on the disk controller firmware and how it quiesces a disk drive when there is no directed traffic.

Some storage access devices (like the Shark) will attempt to behave "intelligently" by taking ad-hoc decisions such as "hover heads in middle of disk if not busy".

Others will hover in the middle of the most active partition of the disk. Which is NOT necessarily the middle of the disk.

Others yet will just move the heads to the disk's outer diameter, a small outer rim on the lower disk platter periphery that provides the head movement mechanism with a "synch" signal.

That is how SCSI disks keep track of where the head is vis-a-vis sector 0, cylinder 0: by using the info recorded on the lower platter.

The variations are endless. Steve's advice is probably based on the state of SCSI disk in single controller manufacturing as he looked into the issue.

IBM's advice is obviously based on some internal optimisation which they have added to the Shark storage system. The important difference here is that IBM's Shark is NOT a disk. It is a storage access system, (or in English: a disk farm). It has its own intelligence and mini-OS.

It WILL behave in a different manner than the typical Adaptec SCSI controller + Seagate Barracuda combo you get at the local PC supplier. And so on.

So, caveat emptor:

If you have a vendor that provides that sort of detail info then follow the advice of the vendor.

If your vendor does NOT provide this info, then do NOT follow IBM's advice just because they say so, or you'll be landing into trouble.

And if you have no vendor but you just buy disks and controllers as needed, then follow Steve's advice: it's intended for people who know enough details about storage technology to figure these things out (or even bother about them).

Nuno Souto Received on Wed Jul 03 2002 - 22:10:42 CDT

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