On Mar 9, 2010, at 6:30 AM, Andy Feldman wrote: > On Tuesday 09 March 2010 07:55:38 datenritter wrote: >> By the way: This makes me worry. Dirvish does not automatically create >> "original" backups on a regular basis. So if you shred a file or lose it >> to a physical disc error, all those additional hardlinks won't help. > > You're right: If you modify a backed-up file in one of your vaults (using a > program that modifies the file in place rather than creating a new file the > save > the new contents in), it will modify all the other backed-up files with the > same hardlink. Your backups should be read-only for this reason. Of course, > having them read-only won't protect against disk error.
Responding to "this makes me worry": Perhaps it's worth pointing out that if this did happen (let's assume disk corruption corrupts the file data), your next backup would "fix" the problem because the modified backup file (the inode referenced by all the hardlinks in the vaults) no longer matches the source file you're backing up---regardless of whether the source file has changed or not on the source hard disk. (If the source file was changed you'd get a new copy regardless). Think of it this way: if you do a full backup on the 1st of the month, and then a differential from the 1st of the month each Sunday, and a differential from the Sunday backup each weekday (you can set this up with tar), then corrupted data in the 1st of the month backup in the restore chain would screw up your restore of data from the 31st. Whereas Dirvish "corrects" the problem the next day---even in the case of an unchanged source file which wouldn't get picked up by an incremental tar backup! Yes, with my scenario about Dirvish healing the problem I still have bad data in the previous corrupted snapshots. Are there a bunch of other methods/tools to do backups that also solve the bad data problem? Yes. But I feel safe about how Dirvish does it. My 2c. Eric _______________________________________________ Dirvish mailing list [email protected] http://www.dirvish.org/mailman/listinfo/dirvish
