[tahoe-dev] Load balancing

Brian Warner warner at lothar.com
Mon Aug 31 23:46:38 UTC 2009

Marc Tooley wrote:

> The ability to actually delete shares, for those of us who are afraid
> to run the garbage collector because we're afraid to forget to renew
> leases, would be neat in support of features like this sort of
> re-balancing. (And perhaps for distant-future human-intervention space
> reclamation too.)

Hm, so basically it'd be less scary to delete a single share if you knew
that another copy existed elsewhere, like if you'd just done a checker
run, or if the protocol involved two servers talking to each other and
coordinating the move.

The ability to do this without fear would let you rebalance without
consuming more space. It wouldn't let you reclaim space for files you've
dereferenced. I suppose the relative sizes of these two potential
savings would depend upon whether you add servers or delete files more
quickly.. something that will be quite different for each user.

I suppose one idea to kick around would be define a stronger cap, to
distinguish between "read-and-delete-shares" caps and plain
"read-and-not-delete-shares" caps. This would be fairly easy to
implement if you used non-CHK keys, or if the stronger caps could have
an extra cryptovalue. Neither could be used to violate the
confidentiality or integrity guarantees of readcaps, but the stronger
one could be used to hurt availability (which, of course, cannot be
provided by cryptographic means, at least not with the same sort of
2**128 margins that we get out of AES/DSA).

I don't know one might make use of this, however: I don't know what
would prompt Alice to give Bob the read-and-not-delete-shares cap and
keep the read-and-delete-shares cap for herself, versus using a scheme
without -and-delete caps. Maybe she doesn't like Bob, and wants to make
him live in fear that she's going to delete the shared files that he's
linked to? And maybe she has an efficient way to keep track of all of
her own files, so she can delete with confidence, so she wants the
read-and-delete-shares caps to make the final deletion process easier?

Eh, lease-timers still feel like the most usable solution, even if both
you and I are afraid to actually turn on expiry :).


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