Re: [9fans] replica

[forsyth@caldo.demon.co.uk]

>>Then you have to spend time downloading the
>>remote file when most of the time you're going to throw it away
>>because the local file _has_ been locally modified.
>>I could have included sha1 hashes in the replica(8) metadata.
>>Oh well.  I expected this not to happen very often,
>>and so far have not been proved wrong.

the advantage of keeping a hash (eg, md5 or sha1) is that it distinguishes
contents of files, which usually is what really matters,
whereas modification times and file length by themselves
aren't really adequate (in general, in my experience).  Qid.path and Qid.vers
might do i suppose, but are unique to Plan 9, and even they can change
when the contents of a file hasn't actually changed.  the hash makes it easier
to track versions historically (eg, if i have a file with a given hash code,
i can check distribution packages or logs to see whether that file was
ever actually distributed as it stands, and if so, when).  similarly,
if i've got an external replica, i can also use
the hash values to relate it to the contents of a (worm) file system dump.
there is a risk with hashing of false hits, but perhaps venti suggests
that people can be relaxed about that if the hash is peculiar enough.

in short, i thought the use of hash values in wrap was a good idea
and was surprised it wasn't part of replica!