Path: utzoo!mnetor!uunet!mcvax!nikhefh!gert
From: gert@nikhefh.hep.nl (Gert Poletiek)
Newsgroups: comp.sys.atari.st
Subject: The DANGERS of TURBO-DOS
Message-ID: <451@nikhefh.hep.nl>
Date: 14 Apr 88 07:31:11 GMT
References: <8804131634.AA17900@ucbvax.Berkeley.EDU>
Reply-To: gert@nikhefh.hep.nl (Gert Poletiek)
Organization: Nikhef-H, Amsterdam (the Netherlands).
Lines: 44
Summary: Bad disk caching in TurboDos/GemDos

In article <8804131634.AA17900@ucbvax.Berkeley.EDU> WALDI@DHDIHEP1.BITNET writes:
>
>Hello! I tried TURBODOS (downloaded from UH-INFO). Really impressing
>speed-up, BUT: It spoilt a disk. Apparently it did not recognize
>a disk change, and copied the old disk's FAT to the changed new disk.
>The disks have different serial numbers, and I did never have this problem
>with TOS-GEMDOS, although I'm aware it might happen under certain circum-
>stances there. Has anyone similar experiences with TURBODOS?
>----------------- Roland Waldi

It becomes more and more clear that TURBO-DOS is not a disk caching program
but a real file system. I it is very likely that the French guys that did
this TurboDos took a disassembly of the original TOS/GemDos as a guidance.

If all of this is true, then TurboDos is as vulnerable to disk change events
as the original GemDos. However, DISK CHANGE EVENTS ARE HANDLED BY THE BIOS,
and the old GemDos has problems with these too. BUT, the old GemDos does
practically do no disk caching whatsoever (you can't call two disk sectors a
cache, can you?). There is a provision in GemDos for a disk cache mechanism,
although in the old GemDos this does not work correctly (i.e., the cached
blocks are invalidated every once in a while causing the cache to do noo harm
to your disk, it just eats memory).

If TurboDos has fixed these problems it may be even more dangerous than the
original GemDos. GemDos' provision for disk caching is NOT A WRITE THROUGH
CACHE, but like the UNIX buffer cache A DELAYED WRITE CACHE. This is very
dangerous if you do not have a system call like 'sync' on UNIX. What happens
when say 20 blocks are in the buffer cache when you decide to eject the
floppy by hardware (your thumb)?? You'll get the problems described by Ronald
Waldi.

Solution: IFFF TurboDos works a bit like our local hack on GemDos (called
JamDos) then you might avoid the described problem by quitting from the
current application (too bad if that's the DeskTop, there's no way out of
that), since the original GemDos would then flush all cached buffers to the
the disks.


Gert Poletiek

NIKHEF-H, Dutch National Institute for Nuclear and High Energy Physics
          Kruislaan 409, P.O.Box 41882, 1009 DB Amsterdam, The Netherlands
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