A restore read one pack at a time over one SFTP connection, which over a long
line is bound by the round trip and by what one TCP connection gets: about
2-3 MiB/s from a Hetzner Storage Box to Canada, with PHP mostly waiting.
Repository::restore() now first lists the ranges of packs the restore needs,
in order (ReadAheadPlan), and the location reads them ahead into unlinked
files in a spool folder in the target (IReadsAhead). Over SFTP,
SftpRangeFetcher does that on four connections of its own, 16 requests in
flight each, taking the answers of whichever connection has one. A range that
fails, a damaged copy, or a blob that moved since the plan is read the usual
way, which reports what is wrong. Other locations restore as before.
On a test line with 120 ms RTT and 0.2% loss, a 1.6 GiB restore went from
5.1 to 17.8 MiB/s, all files identical.
dillard