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I think it's easiest to explain with an example. So, every day at 10pm, I want to make a backup of my server to another server. I ddar or rsync the files to a

by mdasen 16y ago

I think it's easiest to explain with an example. So, every day at 10pm, I want to make a backup of my server to another server. I ddar or rsync the files to another server. Monday at 10pm, I do it; Tuesday at 10pm, I do it; Wednesday at 10pm, yep, did it. Now it's Thursday and for whatever reason, I have to restore the system to the state that it was on Monday. With rsync, changes have been overwritten. ddar and Tarsnap are useful because they allow you to have a full backup of each of those days, rather than just the latest. Likewise, they package it better than the old "do a full backup and then do incrementals on top of it" way so that you don't have to restore the original and then replay all the incrementals. Now, you could just store full backups each night, but that would take an incredible amount of space. ddar and Tarsnap allow you to have the convenience of full-backup while having the data de-duplicated so that it doesn't take up too much disk space. Rsync is more about saving bandwidth while transferring large amounts of files.

jonhohle 16y ago
Are you aware of the --link-dest option for rsync? It seems to do the same thing. I've been using this (via dirvish) for years for doing full nightly, deduped backups.
- cperciva 16y ago
  Hardlinks are only useful if you have files which are unmodified. Tarsnap (and I assume ddar) will take advantage when part of a file is unmodified.
- rlpb 16y ago
  rsync with --link-dest will store each version of a file separately, including parts that are the same. It also won't hard link files in different areas of the tree that are the same, storing them multiple times also. Nor will it relink files that disappear and reappear, or change and then change back. I wouldn't really call it deduped storage at all. The optimisation it does do might well be sufficient for your needs though.

by mdasen 16y ago

jonhohle 16y ago

Are you aware of the --link-dest option for rsync? It seems to do the same thing. I've been using this (via dirvish) for years for doing full nightly, deduped backups.

cperciva 16y ago
Hardlinks are only useful if you have files which are unmodified. Tarsnap (and I assume ddar) will take advantage when part of a file is unmodified.
rlpb 16y ago
rsync with --link-dest will store each version of a file separately, including parts that are the same. It also won't hard link files in different areas of the tree that are the same, storing them multiple times also. Nor will it relink files that disappear and reappear, or change and then change back. I wouldn't really call it deduped storage at all. The optimisation it does do might well be sufficient for your needs though.