WAL and --split-shards seems to help and avoid pathological
slowdowns, but btrfs is still miserable compared to ext4.
{over,msgmap}.sqlite3 (and Xapian) files can remain on a
different FS than git repos.
-* investigate SQLite WAL feasibility with CoW enabled on btrfs
-
-* investigate fragmentation effects on btrfs even without CoW
-
* adjustable commit interval for lei (done for -extindex + -index)
* SQLite VACUUM support on -compact