rsync over Iridium modem, 240 bytes per second

[Michael Ashley]

Dear rsync gurus,

I'm using rsync to transfer large amounts (megabytes per day!) of
data over an Iridium modem link (240 bytes per second) from Antarctica.

One problem is that the Iridium link has a mean uptime of perhaps 30
minutes.

Implementing partial transfers is crucial, so I was using

rsync -av --partial --partial-dir=.rsync myfiles user at host:

The files are pre-compressed with bzip2.

Question 1: the documentation isn't clear about the interaction between
            the partial and partial-dir switches. And advice?

The problem with the above command is that the receiving rsync
processes seem to hang around for a long time, even after the link is
cut.

Question 2: is there a signal one can send to a receiving rsync to
            get it to write out its partial transfers to the
            partial-dir?

The next thing I tried was to add "--timeout 1000". This worked
reasonably well, except that IO buffering makes rsync think that
the network is dead even though the link is up, and data is trickling
out at 2400 baud.

So, I tried "--bwlimit=1". I really need "--bwlimit=0.24", but
rsync won't allow floating point there.

This still isn't very satisfactory, and I am still not maximising my
use of the link.

Question 3: what should I do?! Any other switches that are relevant
            to my situation? E.g., "--block-size" (what are the units
            here? the man page doesn't say).

Question 4: will "--partial" save every last byte that makes it
            through? Or does it truncate to the last "block" (which
	    might have taken 20 minutes to come through on a slow link).

I guess moving to an rsync daemon rather than ssh transport might be a
good idea.  I would like to be able to remove all network buffering
(not sure if this is possible, is the "--blocking-io" switch relevant
here?), and have rsync realise that the network is dead if nothing
comes through in, say, 60 seconds. Alternatively, I could use another
mechanism to determine the link status (an occasional ping?)  and then
send a SIG_SOMETHING to rsync to get it to clean up nicely and be
ready for the next connection.

I've asked a lot of questions here, but I think the answers might be
useful to other people in the same position.

Thank you in advance for your advice,
Regards, Michael