[tahoe-dev] timestamps on mutable files

Zooko O'Whielacronx zookog at gmail.com
Mon Jan 4 22:37:30 UTC 2010


I'm not sure what to think about Jody's preference for mutable files
because of storage space concerns.  Tahoe-LAFS v1.5 *does* have
garbage collection -- it is just slow and it requires manual
operation.  So I'm not sure whether Jody is better or worse off by
using mutable files -- in terms of storage space.

Another performance issue with mutable files -- besides the RAM usage
and the download time earlier mentioned in this thread -- is that each
new mutable file requires its own RSA public/private key-pair, which
takes a lot of CPU cycles to generate.  Really a lot!  Like a billion
CPU cycles.  That's enough to actually inconvenience you if you make
new ones frequently enough, even if you have a modern super-powered
CPU running at 3 GHz.  (Future versions of the Tahoe-LAFS crypto cap
format will use a more efficient public key signature algorithm like
ECDSA so that you can create lots of mutable files without wasting
lots of CPU cycles.)

I guess these performance issues with mutable files need to be added
to the documentation in some appropriate place.

Could someone else please do that job?  I'm not sure off the top of my
head where such a warning would serve its purpose.  Please submit a
patch, even if it is just the output of "diff", attached to a ticket.

Here: I created #878.

Thanks!

Zooko

http://allmydata.org/trac/tahoe/ticket/878# warn users about the
performance issues of mutable files



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