request: add TCP buffer options to rsync CLI?

Lawrence D. Dunn ldunn at
Tue Nov 1 18:03:39 GMT 2005

Dear rsync folks,
   I'd like to request/suggest that cli options to set TCP send/receive buffers
   be added to rsync client-side.

   I'm aware that a daemon's config-file can set socket options for 
the server side
   (e.g. SO_SNDBUF, SO_RCVBUF).  That is useful.
   But when trying to get high-throughput rsync over
   long paths (i.e. large bandwidth*delay product), since the client-side cannot
   also set TCP buffers, throughput will be limited.
   There are some OS's that are starting to do receive-side buffer-autotuning
   (latest Linux, and probably Vista).
   But for the rest, I think the most straightforward way to enable 
high throughput
   would be to also let the client-side make TCP buffer requests.

   Request -in-a-nutshell: something like --tcp_sndbuf  and --tcp_rcvbuf options
   that result in the same setsockopt calls as in the rsync socket.c code
   available to rsyncd.conf.

   If I've totally missed something, and such functionality is already
   available, my apologies (but I'd appreciate a pointer!)

More detail:
   I was helping resolve a throughput issue between a research network
   in France (Renater), and FermiLab in the US (Tridge- Dan Yokum says "hi!").
   FermiLab distributes data from the Sloane Digital Sky Survey (SDSS)
   using rsync.  (cf., and
   for the rsync reference).
   Renater was using rsync to pull large amounts of data from FermiLab 
across a fast,
   long link, and was getting poor throughput (~20mbits/sec).

   The core issues turned out to be
   1. Too-small TCP send-buffer on the FermiLab server side
   (which Dan Y. repaired via rsyncd.conf, and which was later assisted
   by installing a recent Linux that allowed send-buffer-autotuning),
   2. Too-small TCP receive-buffer on the client-side.
   I couldn't see how to enlarge TCP buffers from the rsync client-side.
   By using a web100-enabled client machine (,
   we managed to upsize the TCP receive buffers (and went from 20Mbps 
to ~400Mbps).
   This by grabbing the running rsync process with a web100 tool,
   and manually changing the buffers on-the-fly.

   But the process would be a *lot* simpler if the rsync tool could request
   TCP buffer allocation from the rsync command line.
   The sys-admins would still have to configure large max_tcp_buffers,
   but at least the clients could then request large-buffers when needed.
   Hopefully someday "all" OS's will incorporate TCP receive-buffer-autotuning.
   But in the mean time, I think this would be a significant, 
practical addition.

Related side-material:
1. For  '-e ssh' users, i.e. people that want to get high throughput from
   rsync, and encrypt their material, folks should be aware that there
   is currently a fixed-limit on SSL-layer buffers.  So we get TCP buffers
   sized correctly, only to be limited by small-windowing-behavior of 
an "upper layer" (SSL).
   Chris Rapier (PSC) has addressed this, and is working to get his 
fix incorporated
    into the SSL libs.  In the mean time, patch is available via:

2. I happen to use rsyncX at home(backups), to handle Apple's HFS+ metadata;
      I appreciate all your work on rsync over the years!

Thanks for considering this,
Larry Dunn
ldunn at
(Manager, Advanced Architecture, Cisco)

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