SCSI drives with read-ahead caching

[ingr!b11!gibson at uunet.uu.net]

In article <1990Aug13.011415.6197 at rice.edu> you write:

>I have an SS1 with a Fujitsu M2263S-512 (655Mb) drive attached; the drive
>supports read-ahead caching, which I have enabled. What I would like to
>know is whether it is worth adjusting file systems on the drive with
>tunefs to take advantage of this cache?
>
>Has anybody any experience with caching drives and tuning? 

It has been our experience that you can get excellent (sequential) read
performance by turning on read-ahead and setting rotational delay (via
tunfs) to 0ms. Sadly, the write performance goes way down. I believe that
with a new driver, called by some a "low level scatterer-gatherer" the
write performance would return to normal.

With SCSI II drives becoming available now the read-ahead parameters are
getting very confusing. - number and/or size of segments - read/write
retention priority - max/min prefetch - cross cylinder boundary enable -
etc