Rsync signatures and incremental tape backup

Richard Patterson
Fri, 25 Feb 2005 00:02:26 -0000

Hello,

Andrew Trigdell's original rsync paper (actually
thesis) discussed the idea of using rsync to make
incremental tape backups based not on whole files but
rather parts of files. Sadly, this functionality is
not actually present in the rsync program. I'd like to
explore adding this ability.

There are basically three things that need to be done
to enable efficient (partial-file) incremental tape
backups, namely:
1) Generation of rsync signatures from existing files.
2) Generation of a binary patch from existing files +
an rsync signature.
3) Application of a binary patch to existing files.

Rsync already has the facility to generate and apply
binary patches -- namely, batch-mode operation. Thus
all that remains to be added is read and write support
for signatures.

It seems to me the best way is through two new
options: --read-signature and --write-signature.
--read-signature would operate similarly to
--compare-dest. --write-signature would generate
signature files from the destination.

Finally, one would need to be able to run rsync
without an actual destination parameter when used with
--write-batch. Thus we would have the following
combinations:
For a full backup:
--write-batch --write-signature
For a differential backup:
--write-batch --read-signature --write-sigature
For a leaf incremental backup:
--write-batch --read-signature
For a restore, one would specify --read-batch with no
source.

I'm willing to implement this, but wanted to get some
feedback first. Comments?

--Ian Turner



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