rsync - Aug 2022 - rsync slowness

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?

I've definitely not seen that.  If you can produce a working example
and tar it up for us to look at, that might be interesting/useful.

Just to check, though: you do not have --checksum/-c on, right?

On Sat, Aug 06, 2022 at 05:54:12PM +0000, Gregory Heytings via rsync
wrote:
> 
> 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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