Dear rsync devs,
I recently concluded a bug hunt to trace why my rsync-ing to an SBC was
much slower than the corresponding iperf3-reported speeds. To give a
concise summary of the situation, in slow wifi links using SSH with
ProxyCommand tremendously speeds up things:
$ dd if=/dev/urandom bs=1M count=50 of=sample.data
50+0 records in
50+0 records out
52428800 bytes (52 MB, 50 MiB) copied, 0.24135 s, 217 MB/s
$ rsync -avz --progress ./sample.data [email protected]:/dev/shm/
sending incremental file list
sample.data
52,428,800 100% 575.23kB/s 0:01:28 (xfr#1, to-chk=0/1)
sent 52,458,757 bytes received 35 bytes 543,614.42 bytes/sec
total size is 52,428,800 speedup is 1.00
$ ssh [email protected] 'rm /dev/shm/sample.data'
$ rsync -avz --progress -e 'ssh -o "ProxyCommand nc %h %p"'
./sample.data [email protected]:/dev/shm/
sending incremental file list
sample.data
52,428,800 100% 3.40MB/s 0:00:14 (xfr#1, to-chk=0/1)
sent 52,458,757 bytes received 35 bytes 2,690,194.46 bytes/sec
total size is 52,428,800 speedup is 1.00
If it isn't clear - the speed of the upload went from 543 kbytes/sec to
2690 kbytes/sec.
If you want to see why - and how I traced this down - you can read my
complete report in the UNIX StackExchange forum. To avoid being classified
as a spammer, I won't include a link - search for question 434825 in the
search bar. The executive summary is that SSH disables Nagle's algorithm by
default - and in slow links this can cause tremendous impact (as you see
above).
Hope this helps people that backup over slow links (wi-fi or otherwise).
Cheers,
Thanassis Tsiodras, Dr.-Ing.
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