Re: [9fans] venti arena

["Russ Cox" <rsc@plan9.bell-labs.com>]

I've never understood all the inconsistencies you
just pointed out, so I dug around for a little while.

The numbers given by fmtarenas are the real ones.

The index uses ``virtual'' addresses which get mapped
into (arena-name, offset) pairs.  The base for the virtual
addresses is 1M.  If you had multiple arena partitions 
you'd see that the index entries for the arenas in that
disk start where the previous disk left off.  These virtual
addresses are the numbers that fmtindex prints.

I think checkarenas is just buggy.  There are head and tail
blocks around each arena's data section, and checkarenas
is printing

	[datastart, datastart+datasize+2*blocksize)

instead of

	[datastart-blocksize, datastart+datasize+blocksize)

That is, checkarenas thinks datastart has the blocksize subtracted
from it already, but that's not the case.  The http://server/index
listing uses the same code, and is thus similarly off.
This is why the backup script I posted subtracts one block size
before using the offset in the listing.  (Though that was just
trial and error on my part at the time.)

Russ