increasing the write block size for high latency

Adam Edgar aedgar at
Mon Jul 7 14:18:13 MDT 2014

I am trying to transfer a group of files in whole using rsync using the —files-from option across a network with high bandwidth but relatively high latency. When I log into the remote machine I see an rsync command running like thus:

rsync --server --sender -Re.sf -B16384 --files-from=- --from0 . /

Note that I have used the -B option to increase block size but that seems to just apply to the blocks used for checksumming instead of actual network I/O. I came to this conclusion by running strace on the PIDs on both remote and local hosts. On the server side I got a stream of 4K writes on fd 1 followed by a select on that same fd :

write(1, "\275\214\357\357\22y4\237\215Z\226\331\355Z!\340\373\227\340V~\20\35C\3\337_\27\257\236\321\204"..., 4092) = 4092
select(2, NULL, [1], [1], {60, 0})      = 1 (out [1], left {59, 999996})

I’d like to see if I can increase the write block size to cut down on the delay of each select to this unnamed pipe to sshd.  I’m not sure if that would help speed up the transfer as the issue may be below at the sshd level (which I can’t strace as a non-root user).  If anyone has experience with slowness at the ssh level please advice on which flags would allow me to speed up there. 

Any help will be much appreciated.

Adam S Edgar

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