From mboxrd@z Thu Jan  1 00:00:00 1970
From: jmk@plan9.bell-labs.com
To: 9fans@cse.psu.edu
Subject: Re: [9fans] undocumented fs config options?
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Date: Wed,  9 May 2001 00:10:52 -0400
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On Tue May  8 11:20:27 EDT 2001, anothy@cosym.net wrote:
> in reading through /sys/src/fs/worms and the file
> server source, there seem to be two undocumented
> device types: r and x.
> 
> x is Devswab - i believe this bit-swaps whatever's
> on the physical disk, presumably for moving a file
> system between different-endian servers. correct?

yes, 'x' does the non-trivial swap between big and little
endian filesystem formats. this allowed us to move our old
sony jukeboxes from a big-endian mips magnum to a little-endian
pc. since then we've copied the data off the sonys and
turned them off.

> 
> i'm not clear on r; it seems related to be the same
> as w, but for optical disks (described in
> port/portdat.h as "scsi video drive"), or l with no
> label (l is described as "scsi video drive
> (labeled)"). in the examples, r, like l, is only
> used within a j device to describe the unmountable
> platters. it seems either r or l is required in the
> device3 section of a j definition; fsconfig(8)
> makes no mention of that.

my understanding is that 'r' and 'l' only differ in that
with 'l' one block is reserved on the platter for a label.

> 
> can l or r be used on regular disks? what's the
> difference between r and w devices?
> -α.

i believe 'r' and 'l' devices can only be used as the third
part of a 'j' device. there is no individual 'init' routine for
'r' and 'l' devices, it's part of the 'j' initialisation.

that's not to say it might be possible to concoct a filesystem
specification with 'r' or 'l' devices not associated with a
jukebox that would pass the configuration parsing, but it wouldn't
work.