files_external's SFTP read stream asks for the next 256 KiB only after
the previous answer has arrived, so reading costs one round trip per
request whatever the line speed: 4.9 MB/s at 47 ms to a Hetzner Storage
Box. SftpReadStream keeps up to eight READ requests in flight over the
same phpseclib connection (15 MB/s on that link), starting with one so
a small read does not fetch megabytes ahead.
A server that answers less than asked is handled by asking again from
where the answer ended; when more data follows, that size is used for
later reads over the same connection, never below the 32 KiB every
server must support. Without the needed connection methods, or when the
open fails, the storage's own stream is used as before.
Opening a repository lists index/, which holds one file per pack.
StorageBackend::list() checked every entry with is_dir(), and on a
remote storage each check is a round trip. Over SFTP to a Hetzner Storage
Box (33 ms) with a 60 GiB repository, occ backup:run (30 s steps) spent
nearly every step on these requests and uploaded 45 KiB/s; with this
change it uploads 5.8 MB/s. The cost grows with the number of packs.
index/, snapshots/, catalog/ and trash/info/ only ever hold objects:
list them with the new IBackend::listFiles(), one listing without
per-entry checks.
dillard