Re: [9fans] bug in disk/format

[rsc@plan9.bell-labs.com]

I can't reproduce your problem.

Was this the first 9fat ever present on the disk?
Was there an earlier 9fat that you had mounted
before running the format and the remount?

The only thing I can think of is that you had run
9fat: earlier, so dossrv had the FATs in its
buffer cache.  When you reformatted, dossrv kept
using the old FATs, hence the breakage of
files on sector boundaries.

I found a different problem, though, which is
fixed on sources.  The amount of space required by
the FATs depends on the number of bits per FAT
entry, which depends on the number of FAT entries,
which depends on the amount of disk space left
over after subtracting out the space used by the FATs.

Oh, and if you somehow divine the number of bits
per FAT, the amount of space required by the FATs
still depends on the number of FAT entries still
depends on the number of clusters, which depends
on the amount of disk space left over after
subtracting out the space used by the FATs.

Before we had an approximation that didn't work
when you were close to the dividing line between
12-bit and 16-bit FAT entries (having an 8MB 9fat
would put you close enough to cause problems).
Now we just guess until we find a fixed point.

This isn't your problem, though -- this bug
makes reads fail much earlier than 55000 bytes
into the file.

Russ