[Bug 12570] Problems with --checksum --existing

samba-bugs at samba.org samba-bugs at samba.org
Sun Nov 3 12:37:48 UTC 2019


--- Comment #2 from Haravikk <samba at haravikk.com> ---
I was about to post on basically the same issue, but found this; I use rsync to
do a lot of incremental backups where ZFS or similar isn't an option (not that
common, but still comes up now and then). To guarantee correctness I like to
run a periodic consistency check with --checksum to be certain that none of the
files have changed at rest on the receiver, just like how I scrub a ZFS pool
from time to time.

Problem is that rsync's --checksum mode is insanely slow when done for a large
amount of files, much slower than it should be, even allowing for a slow sender
or receiver.

I had always assumed that rsync at each end just set about gathering metadata
in the background, while communicating, "I have X with checksum Y" -> "I don't,
send it" or such, but this doesn't appear to be the case with --checksum, as it
can take hours before anything even *begins* sending, let alone the actual time
to finish.

It seems a lot like the incremental file list behaviour of modern rsync is
being disabled when --checksum mode is enabled, but is there any good reason
why that should be the case?

I can't think of any reason why it should be different, as a checksum
ultimately is just a value to be compared, just like a file-size and/or
timestamp, it just takes a bit longer to generate each one.

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