using skiplist for quotas.db

[Kjetil Torgrim Homme]

we're running Cyrus 2.2.12 on a server with many virtual domains and
users.  currently, we're using quotalegacy, but running quota(8)
requires reading many directories (1736 to be exact) to enumerate the
quota roots (keys) and opening the quota file for each of more than 100k
users.

usually, this completes in 1-2 minutes, but during high I/O load on the
system (backup), it can take 15 minutes.

so I wonder, will skiplist be a better choice?  obviously running
quota(8) will be a very cheap operation, but I'm worried about
contention on the quota database during delivery etc.  (these users are
for the most part not actively using the system -- they get less than
one message per day each(!) -- so I probably worry too much.)

also, will the skiplist file get fragmented?  if so, is there a tool to
defragment it?
-- 
Kjetil T.