Sector Interleave

[Johnny Billquist]

On 2015-12-01 02:06, Jerry Weiss wrote:
> On Nov 30, 2015, at 6:35 PM, Paul Koning <paulkoning at comcast.net> wrote:
>>
>>
>>> 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 "modern"?
>>> (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.
>>
>> 	paul
>>
>
> 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 EK-0TU58-UG-001_TU58_DECtape_II_Users_Guide_Oct78.pdf).

I wasn't aware that it did any block interleaving. But yes, file 
placement was extremely important. Are you sure it interleaved blocks?

> 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.
>
> Jerry
>
> *Yes - I skipped over DECtape.   I’ve leave that one to the many experts on the list.

DECtape never did interleaving that I know of. But there too, file 
placement was very important. I don't think it would even work if you 
interleaved the blocks, as the tape driver normally figured out which 
was to spool depending on the current block at the head, and which block 
you wanted to get to.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
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