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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