readahead

[Andries.Brouwer@cwi.nl]

[readahead.c has badly readable comments, on a standard
80-column display: many lines have a size just slightly
over 80 chars]

In the good old days we had tunable readahead.
Very good, especially for special purposes.

I recall the days where I tried to get something off
a bad SCSI disk, and the kernel would die in the retries
trying to read a bad block, while the data I needed was
not in the block but just before. Set readahead to zero
and all was fine.

Yesterday evening I was playing with my sddr09 driver,
reading SmartMedia cards, and found to my dismay that
the kernel wants to do a 128 block readahead.
Not only is that bad on a slow medium, one is waiting
a noticeable time for unwanted data, but it is worse
that setting the readahead no longer works.

[Indeed, it is very desirable to be able to set readahead
to zero. It is also desirable to be able to set it to a
small value. Today on 2.5.8 both are impossible, readahead.c
insists on a minimum readahead of 16 sectors.]

Andries