Without really tuning anything, after a bit of time, the time needed to do history lookups drops to microseconds, and as long as a `sync' isn't needed, innd doesn't get stuck. Theoretically, a sync, where you are in fact seeking rather wildly over the disk to update these files, happens once a day at expiry. Depending on the speed of the drive (and I haven't optimized this at all, using a single drive for OS, logs, history, and part of spool, with a second drive for the rest of the spool, far from an ideal setup), this seems to mean only a few minutes of downtime. Actually building the new .index and .hash files goes quite a bit faster, like by a factor of 3 to 4, so clearly the update of these files during the `sync' could stand improved sorting.