Re: diff compression

Hello,

I've been diagnosing why I got differential dar backup almost the same size
as full after updating dar from 2.3 to 2.4 (yes I know...). I've come up
with some discussion about dar&hard links on cifs share, which was my setup
as well (http://sourceforge.net/p/dar/mailman/message/26577785).

Now (this is a bit off-topic, but bear with me for a bit more), I figured it
out the problem is because cifs share will produce files with hard link
count = 0, which was apparently fixed in later kernels (see samba bug report
here: https://bugzilla.samba.org/show_bug.cgi?id=9346, which makes me
wonder, should there be some way to handle this?). As hard link count
changed after I have upgraded entire system from Centos6 to Centos7
(relevant kernel commit:
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/fs/cif
s?id=74d290da476f672ad756634d12aa707375d3564d), dar produced a differential
backup of almost the same size as full backup and is probably not related to
dar version change...

Back on topic...suppose a copy of a directory (that's included in daily
backup) is made. Old directory probably has files that have already been
stored in some previous full backup...A worrying admin might run 'hardlink'
over both old&new directories to save both server space and backup size. But
- every inode is now changed in the old directory as well because hard link
count has increased. Dar will store every file in a new data structure that
stores hard links (if a differential backup is made). No backup space is
actually saved (1 copy of file data is stored in this new diff backup in
either case, even if hardlink was not run)...

Will delta compression take this into account? Inode type has changed from
normal file to hard linked file, but data is the same - so ideally, it
should only store a very small delta? Have you considered in the algorithm
how delta is stored if inode changes type?

Regards,
gulikoza



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