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SubjectRe: disk head scheduling
On Fri, Mar 19, 1999 at 03:45:11PM -0500, Richard B. Johnson wrote:
> On Fri, 19 Mar 1999, Arvind Sankar wrote:
> > Yeah, but some manufacturers are good enough to put it in the tech notes.
> > Should the scheduling algo be put in as a device strategy function, with
> > fallback to the current elevator if the device doesnt have one? Then we could
> > implement two way elevator algos for those hard disks for which we can get
> > physical geometry info from the data sheets or somewhere.
> >
>
> The typical IDE drive has a single platter and two heads. Just
> like a floppy disk. The so-called geometry is specified only
> for compatibility with the real-mode BIOS (0x13) interface that
> expects heads/sectors/cylinders, like in the old ST-506 interface
> days.

IBM claims that my hard disk (Model IBM DHEA-38451) has 4 platters and 8 heads.
I assume these are physical, since there are either 15 or 16 logical heads in
CHS mode (settable via jumpers).

>
> Any attempt to optimize performance based upon this phony geometry will
> fail. The best you can do is attempt to keep read/write queues separate.

Who's talking about optimizing based on phony geometries? Did you read what
I wrote?

> In other words, if possible, queue a bunch of reads and do them all
> together, then queue a bunch of writes and queue them all together. This
> can cut down some time on the write-splice and other so-called rotational
> latencies.
>
> A write splice occurs when you need to write some new sectors between
> existing sectors, i.e., you are not going to write an entire track. This
> often requires that the disc make up to one complete rotation before a
> write can begin because the drive has to "learn" where the starting sector
> is by reading sectors. Note, all sectors on hard disks are "soft" which
> means that there is no index to tell the electronics when to turn on the
> write current. If writes get queued together, and you have multiple
> writes, there is a chance some writes will occur on the same track. This
> means that since the drive already knows where the sectors are on that
> track, it doesn't have to reread.

I don't get this. If this were true, then the average latency of a disk would
be the time for one complete rotation (half a rotation to find the start, and
half a rotation to actually get to the right place). But the average latency is
equal to _half_ the time for a complete rotation.

Bunching requests that are on the same track is good, since the drive
probably does some read-ahead and caching. But accesses that are far enough
apart that they can't be to the same track can be done in either order with
no significant difference in time.

>
> There is practically zero probability that optimization based upon phony
> geometry will accomplish anything because the real geometry boundaries are
> usually well hidden by the drive's sector buffer. All you do is waste

IBM is good enough to tell me that there are 8 zones on my platters, numbered
0 to 7 from the outside in. They also tell me the recording density at both
ends. From that we can work out a rough estimate of the number of sectors per
track and the number of tracks in a zone. If two requests are to different
tracks, then which is performed before the other is largely irrelevant, so I
can order my requests to minimize the total number of seeks.

> CPU cycles that could be used elsewhere. What happens is you slightly
> slow the overall operation of the entire machine without increasing the
> Disc performance at all. Wasted CPU cycles are never gotten back, you
> burned them up, making a beautiful I/O queue, then accomplish nothing
> for your effort.

Yes, but my machine is almost always waiting for disk I/O. The only time my
CPU utilization goes over 90% is when I'm running gcc. Usually my idle time is
what is over 90%.

The real argument against doing all this is that it will save at most one seek
per two requests, which is only 5% timewise. Probably not worth all the effort.

-- arvind

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