Silly me... the '--checksum' is only for 'Full' so that explains the
difference between 'incrementals' and 'fulls'... along with presumably
why my case wasn't caught by an incremental.
I still don't fully understand the comment referencing V3 and replacing
--checksum with --ignore-times.
Is the point that v3 compared both full file and block
checksums while in v4 --checksum only compares full file checksums?
And so v3 is more conservative since there might be checksum
collisions of 2 non-identical files at the file-checksum level that
would be unmasked by checksum differences at the block level?
(presumably a very rare event -- presumably < 2^128 since the hash
itself is 128 bits and the times and size are also checked)
"" wrote at about 23:54:14 -0400 on Sunday, June 7, 2020:
> Can someone clarify how --checksum works in v4?
> And specifically, when could it get 'fooled' thinking 2 files are
> identical when they really aren't...
>
> According to config.pl:
>
> The --checksum argument causes the client to send full-file
> checksum for every file (meaning the client reads every file and
> computes the checksum, which is sent with the file list). On the
> server, rsync_bpc will skip any files that have a matching
> full-file checksum, and size, mtime and number of hardlinks. Any
> file that has different attributes will be updating using the block
> rsync algorithm.
>
> In V3, full backups applied the block rsync algorithm to every
> file, which is a lot slower but a bit more conservative. To get
> that behavior, replace --checksum with --ignore-times.
>
>
> While according to the 'rsync' man pages:
> -c, --checksum
> This changes the way rsync checks if the files have been changed
> and are in need of a transfer. Without this option, rsync uses a
> "quick check" that (by default) checks if each file’s size and time
> of last modification match between the sender and receiver. This
> option changes this to compare a 128-bit checksum for each file
> that has a matching size. Generating the checksums means that both
> sides will expend a lot of disk I/O reading all the data in the
> files in the transfer (and this is prior to any reading that will
> be done to transfer changed files), so this can slow things down
> significantly.
>
>
> Note by default:
> $Conf{RsyncFullArgsExtra} = ['--checksum'];
>
> So in v4:
> - Do incrementals and fulls differ in how/when checksums are used?
> - For each case, what situations would cause BackupPC to be fooled?
> - Specifically, I don't understand the comment of replacing --checksum
> with --ignore-times since the rsync definition of --checksum
> says that it deosn't look at times but a 128-bit file checksum.
>
> The reason I ask is that I recompiled a debian package (happens to be
> libbackuppc-xs-perl) to pull in the latest version 0.60. But I forgot
> to change the date in the Changelog. When installing the package, the
> file dates were the same even though the content and file md5sums for
> some files had changed.
>
> Specifically,
> /usr/lib/x86_64-linux-gnu/perl5/5.26/auto/BackupPC/XS/XS.so
> had the same size (and date due to my mistake) but a different file
> md5sum.
>
> And an incremental backup didn't detect this difference...
>
>
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