Xref: utzoo comp.sys.amiga:61082 comp.sys.amiga.tech:13104
Path: utzoo!attcan!uunet!mcsun!unido!mpirbn!p554mve
From: p554mve@mpirbn.mpifr-bonn.mpg.de (Michael van Elst)
Newsgroups: comp.sys.amiga,comp.sys.amiga.tech
Subject: Re: Tiny Tiger hard disk
Keywords: hard disk tiny tiger maxtransfer FFS
Message-ID: <1078@mpirbn.mpifr-bonn.mpg.de>
Date: 2 Jul 90 22:27:15 GMT
References: <sam.645470800@s.ms.uky.edu> <1227@metaphor.Metaphor.COM> <1990Jul2.074747.19497@metro.ucc.su.OZ.AU>
Reply-To: p554mve@mpirbn.UUCP (Michael van Elst)
Distribution: comp.sys.amiga,comp.sys.amiga.tech
Organization: Max-Planck-Institut fuer Radioastronomie, Bonn
Lines: 37

In article <1990Jul2.074747.19497@metro.ucc.su.OZ.AU> matth@extro.ucc.su.OZ.AU (Matthew Hannigan) writes:

>Could someone in the know tell us exactly what Maxtransfer is for?
>The 1.3 book is not particularly enlightening. Is it size or speed
>fr'instance?

Standard device drivers are not required to handle read and write operations
with an arbitrary sized buffer. F.e. the trackdisk.device supports multiples
of sectors only. Some drivers limit the maximum size of a single operation.

With the old file system there wasn't a problem. It started only single sector
operations for only part of the sector is used for data. The fast file system
uses the complete sector for data and therefore can transfer multiple sectors
from or into a larger buffer.

In order to not hit the maximum transfer size limit of the driver, you can
tell the fast file system a specific limit. This is the maxtransfer value.
Common values are 0, 1, 65536 and 131072 for device drivers with no limit,
operations limited to single sectors or with a 8 bit sector counter (signed
and unsigned).

Standard hard disk drivers are 'dumb' and will perform only the operations
that are explicitly requested. So if you start smaller reads (or writes)
these are single actions that will require a complete disk revolution
for contigous sector accesses. The more operations are done the more
revolutions of the media are needed.

One could provide 'smarter' drivers that are using a caching scheme
to provide a better average performance (and a slightly degraded peak
performance). Future versions of the file system could handle this better
inside the file system but todays file system does not.

-- 
Michael van Elst
UUCP:     universe!local-cluster!milky-way!sol!earth!uunet!unido!mpirbn!p554mve
Internet: p554mve@mpirbn.mpifr-bonn.mpg.de
                                "A potential Snark may lurk in every tree."