On a powerful (by my standards) machine with 16GB RAM and an
7200 RPM HDD marketed for "enterprise" use, indexing a 8.1G (in
git) LKML snapshot from Sep 2019 did not finish after 7 days
with the default number (3) of Xapian shards (`--jobs=4') and
`--batch-size=10m'.
Indexing starts off fast, but progressively get slower as
contents of the inbox (including Xapian + SQLite DBs) could no
longer be cached by the kernel. Once the on-disk size
increased, HDD seek contention between the Xapian shard workers
slowed the process down to a crawl.
With a single shard, it still took around 3.5 days to index on
the HDD. That's not good, but it's far better than not
finishing after 7 days. So allow unfortunate HDD users to
easily specify a single shard on public-inbox-init.
For reference, a freshly TRIM-ed low-end TLC SSD on the SATA II
bus on the same machine indexes that same snapshot of LKML in
~7 hours with 3 shards and the same 10m batch size. In the past,
a higher-end consumer grade MLC SSDs on similar hardware indexed
a similarly sized-data set in ~4 hours.