Back in the days when you had to low level format your hard disk every now and then, and reading floppies was a time critical task that was almost too much for the cpu to handle, and hardware track buffers weren't an integral part of your disk drive, it was interesting to set the sector interleave on your disks.

The interleave factor set the physical geometry order of the sectors on the disk. (Example: an 8 sector track) An interleave factor of '1' would put physically neighboring sectors in consecutive order (0 1 2 3 4 5 6 7) where an interleave of 2 would put one sector in between (0 4 1 5 2 6 3 7) or three (0 3 6 1 4 7 2 5).

The idea was that the typical read/write pattern would be to access the sectors in consecutive order; but if your machine was too slow, the logical next sector would pass under the head before it was ready to do something with it, and then you'd have to wait for the entire rest of the disk to spin by to get the sector you wanted, giving a worst case of one revolution per sector, which would be horrible.

So, you interleave the sectors, so that sectors you don't (yet) care about are going under the head while the computer is busy calculating the checksum for the next sector (or whatever), and thus, reduce the number of revolutions required to read the sectors.

You might have wanted to low level format your hard drive after a cpu upgrade so to re-optimize the interleave. You might want to reformat a floppy with a different interleave to use it for an atypical purpose.

Of course, these days, this is nearly a totally obsolete concept. It was much more relevant when the low level disk bit banging was implemented in software. These days, interleave is typically set in stone and optimized for the disk controller attached to the drive, which usually includes track buffers that can read in the entire track in one pass anyway, so sequential sectors on modern disks are most probably contiguous. Also, most Real operating systems implement read ahead, which tries to predict the next needed sector, and buffer it.

Other semi-obsolete (and more advanced) disk optimization concepts include head optimization and spindle optimization, which were implemented in early versions of BSD UNIX. Today's closest equivalent would include stuff like the elevator algorithm, which is still relevant because it is a big picture kind of thing.

This was brought to you by the Save Our Archaic Technical Terms Society.

In`ter*leave" (?), v. t. [imp. & p. p. Interleaved (?); p. pr. & vb. n. Interleaving.] [Pref. inter- + leaf.]

To insert a leaf or leaves in; to bind with blank leaves inserted between the others; as, to interleave a book.


© Webster 1913.

Log in or registerto write something here or to contact authors.