On Nov 30, 2015, at 4:45 PM, Fred Cisin <cisin
at xenosoft.com> wrote:
> Oversimplified remedial tutorial:
> Ideally, the system reads a sector, does what it has to do with the content, and goes
back for the next one, and can read every sector of the track in a single revolution.
From: "Paul Koning" <paulkoning at comcast.net>
Your writeup was aimed at floppy disks, but
interleave may also appear on hard drives. I don't remember it in reasonably modern
systems, but it shows up on CDC 6000 systems.
On Mon, 30 Nov 2015, Mike Stein wrote:
----- Reply ----- Definitely an issue with IBM
PC/XTs and clones; I recall testing every new combination of HD and controller for most
efficient interleave before I delivered to the client.
1) Are there any examples newer than PC/XT 5160?
Although, obviously, completely hidden from the user, is it still used on anything
(Should ALL verbs be changed to past tense?)
2) Is it used on anything besides spinning rust?
Not that I know of.
I remember using interleave on SAN systems with (S)ATA drives, back around 2002-2004 or
so when ATA and/or SATA did not yet support command queueing. So you could only issue one
command per drive, then in the interrupt handler you'd have to handle the completion
and issue the next. It turns out you could not do that without interleave, or something
analogous. For example, you can leave the sector addressing unchanged but break transfers
up into sectors, and issue them in interleaved order. Similarly, when sorting commands
offered by applications, you can order them in this manner for the subset of commands for
a given track.
3) Besides all of my examples being floppy, what
else should be changed/corrected in what I wrote?
The only thing I would change is to mention that this is/was found on hard drives also.
The TU58 was a block addressable using a cassette tape drive famously(?) called DECtape
II. File placement on the two different linear tracks was a necessary art, especially
if you were booting RT11 regularly. This helped it to stream or not rewind in sensitive
places. The 1:2 interleave was ?built-in? to the block formatting (see
I used a late model device, pulling data from clinical diagnostic computers without too
many challenges. However, compared to reliability of DECtape*, DECtape iI was not in the
same class IMHO.
*Yes - I skipped over DECtape. I?ve leave that one to the many experts on the list.