Path: utzoo!utgpu!watmath!clyde!att!alberta!ubc-cs!van-bc!tessera
From: lphillips@lpami.wimsey.bc.ca (Larry Phillips)
Newsgroups: comp.sys.amiga
Subject: Re: FFS with 512 byte buffers
Message-ID: <2188@van-bc.UUCP>
Date: 29 Jan 89 09:22:06 GMT
Sender: tessera@van-bc.UUCP
Lines: 57

In <2144@vu-vlsi.Villanova.EDU>, cheung@vu-vlsi.Villanova.EDU (Wilson Cheung) writes:
>	I've been hearing people getting satisfactory results with FFS using
>a 32K setting for MaxTransfer.

The best performance is had on FFS by using no MaxTransfer entry at all.
MaxTransfer is a limiting number, and no entry for it allows the file system to
ask for as much data as it wants at a time. MaxTransfer is only required if the
driver cannot handle unlimited size data requests.

>Unfortunately the driver for my hard disk will only permit a MaxTranser of 512K bytes.

That is unfortunate. Are you sure about the 512 bytes figure? I have seen a
number of figures for different products, but none as low as this. What
driver/controller are you running?

>As far as I can tell FFS with this setting seemed just a slow as AmigaDos
>format.  Would this limitation on MaxTransfer be the cause of an FFS that
>offers no speed improvement?  or is it something else.

Absolutely. FFS gains most of its speed improvement by allowing the file system
to request data from contiguous sectors in large chunks. Without MaxTransfer
limitations, the data portion of a large file (say 400K) might be loaded in
anywhere from 1 read request and up, depending on the physical location of the
sectors containing it. With MaxTransfer set to 512 bytes, it will take the same
number of requests to load as it would have under the old file system (800
requests). By the way, the maxTransfer value in the mountlist specifies BYTES,
not blocks.

>	Anyway why exactly would a driver have problems if MaxTransfer was
>set too high?

A controller is usually limited by the number of blocks it can accept as a
length parameter. The driver software, if it asks for more than this from the
hardware, will either get an error, or will transfer a number of byte somewhat
less than asked for; <bytes requested> modulo <maximum hardware allows>.

A driver, in order to make best use of the FFS, must accept any size transfer
request, and break it down into chunks that the hardware can handle, do them,
and only then must it tell the requesting program that the transfer has
completed. This method will make the best of the hardware speed, while making
the process transparent to the file system.

The only recommendation I can think of is to wait for an upgrade to the driver,
or if that will not happen, change to another controller. Runninf FFS at a
maxTransfer of 512 bytes offers only marginal improvement in speed and drive
data capacity.

-larry

--
Frisbeetarianism: The belief that when you die, your soul goes up on
                  the roof and gets stuck.
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|   //   Larry Phillips                                                |
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|        COMPUSERVE: 76703,4322                                        |
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