This gives better page cache utilization for Xapian indexing on
slow storage by improving locality for random I/O activity on
the Xapian DB.
Instead of doing a single-pass to index both SQLite and Xapian;
this indexes them separately. The first pass is identical to
indexlevel=basic: it indexes both over.sqlite3 and msgmap.sqlite3.
Subsequent passes only operate on a single Xapian shard for
documents belonging to that shard. Given enough shards, each
individual shard can be made small enough to fit into the kernel
page cache and avoid HDD seeks for read activity.
Doing rough tests with a busy system with a 7200 RPM HDD with ext4,
full indexing of LKML (9 epochs) goes from ~80 hours (-j0) to
~30 hours (-j8) with 16GB RAM with 7 shards configured and fsync(2)
disabled (--no-sync) and `--batch-size=10m'.