I finally take the time to report an rsync slowness pattern that I've been
seeing for years.
Assuming:
a directory with many (> 20K) files, for example a maildir, on the sending
side, and
a partial copy of that directory on the receiving side, with "enough"
missing missing (say 5K),
then the receiving side will do the following: it will take about one
minute to start transferring the missing files, and will apparently hang
about one minute every 200 files or so. After a few (about 10 or 20)
iterations, the receiving side apparently hangs.
On the receiving side rsync is using 100% CPU, on the sending side not
more than a few percents. In case it matters, both sides are using ext4
filesystems, and Debian GNU/Linux.
I tried --msgs2stderr -M--msgs2stderr, but that does not print any error
message, I also tried to disable compression, to use --whole-file, to use
--no-inc-recursive, ..., to no avail.
Any hints?
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