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 root@192.168.1.150:/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 root@192.168.1.150 'rm /dev/shm/sample.data' $ rsync -avz --progress -e 'ssh -o "ProxyCommand nc %h %p"' ./sample.data root@192.168.1.150:/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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