Path: utzoo!mnetor!uunet!portal!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: Turbodos and the ideal disk cache
Message-ID: <1042@atari.UUCP>
Date: 20 Apr 88 18:34:07 GMT
References: <8804192256.AA06919@orville.nas.nasa.gov>
Organization: Atari Corp., Sunnyvale CA
Lines: 44

From article <8804192256.AA06919@orville.nas.nasa.gov>, 
by rowley@orville.nas.nasa.GOV (Karl Rowley):

> How much a delayed write cache would really buy on the ST is a good 
> question.  In a lot of cases it may not buy much over a write-through 
> cache.

But there is a significant place where it buys a lot -- copying lots of
files to a disk. 

Think about it: GEMDOS is constantly reading the directory of the disk
looking for an empty slot, creating a file in that slot (zero length),
reading the FAT looking for space, writing the file's data (can't help
that), then writing the FAT and the new file size to the directory. 

For the next file, the cycle repeats.

If you could save the writes to the FAT and directory until all the
files are copied, you would save quite a bit.  Remember that the FAT and
root directory are at the beginning of the disk, so the head is seeking
back and forth all this time.  With cached reads AND DELAYED WRITES you
save the seek time as well. 

Unfortunately, you don't know when the whole operation is complete. 
That's why it's dangerous.  It gets much worse with removable media, of
course. 

Apple's idea is to leave eject buttons off their drives.  GEMDOS's idea
(and MS-DOS's) is to keep the disk as up-to-date as possible, and to
keep inconsistencies to a minimum time window.  (Orphaned clusters and
data written before the FAT aren't inconsistencies, they're lost data. 
FAT written before data or directory entry written before FAT is an
inconsistency.)

If we could retrain people to pop disks only when the light goes out, we
would have a 5-second write-delay cache with no hazard, but that kind of
retraining is tough: after all, if you power up with no disk in the
drive, the light stays on FOREVER (on 1040 and Mega with a bootable hard
drive).  Not a good foundation for retraining.  On the other hand, that
might be a start...

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt