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The holding disk contains the tape images, when a full backup have been made.

Can these be deleted, or are they needed when creating incremental backups?

Update

Another way to ask the same. When a full backup have been made. How does Amanda know which files have changed, and should therefore be backup in next incremental backup job?

I would imagine that Amanda would compare each file to be backed up against the ones in the tape images on the holding disk.

Or does Amanda make a checksum of each file it backs up, and if the checksum have changed on a file, then it is included for incremental backup?

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The holding disk shouldn't contain any dumps at all once any (incremental or full) backup is completely written to tape. If it does, you need to review the report that amdump generated and find out where the problem was, so that you can get all of your backups on tape. –  Slartibartfast Jul 16 '11 at 21:29
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2 Answers 2

up vote 2 down vote accepted

Short answer: the holding disk files are not used in determining what goes into an incremental backup. They're optional features of Amanda in any case (when disk and tape capacity were much closer in size, and disks were expensive, you'd probably just go straight from disk to tape).

Amanda's FAQ "How are backup levels defined and how does Amanda use them?" is useful here. Particularly, there's a section at the end that basically says, if you're using dump, Amanda uses dump's existing methods for determining what goes into an incremental. If you're using gnutar, it uses gnutar's comparable methods. If you're using some other tar, I guess it would examine modification dates, but that's not relevant on a Linux system.

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Thank you very much for the links. Very interesting FAQ! About backing up straight from disk to tape. Aren't you concerned about the shoe shining effect? en.wikipedia.org/wiki/Tape_drive#Performance_problems –  Sandra Jul 16 '11 at 17:48
    
Given buffering and relative speeds, I don't know if it's an issue. For what it's worth, I've always used holding disks with Amanda, simply because it takes me several days to generate enough incremental changes to fill a tape. So rather than use 30 tapes to get daily incrementals over a month, I can alternate between two tapes that back up the holding disk daily, and flush the holding disk to tape when its contents grow larger than one tape's capacity, along the lines of the last part of this page. –  Mike Renfro Jul 16 '11 at 18:23
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I'm not quite sure what you're asking but differential and incremental backups are based on the archive bit on the file that's being backed up, not on the file that they're backed up to (tape, backup file, etc.).

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If Amanda doesn't need to the full backup tape images in the holding disk. Then how does Amanda then know if a file have changed? I mean, does it make a checksum of each file to see if the content have changed since last backup? –  Sandra Jul 16 '11 at 14:51
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No, it looks at the archive bit. –  SpacemanSpiff Jul 16 '11 at 17:19
    
@SpacemanSpiff : What is the "archive bit"? I have never heard of it before =( Is it something all Linux file systems have? –  Sandra Jul 16 '11 at 17:24
    
So the archive bit is a Windows feature, that Linux doesn't have. netadmintools.com/art399.html –  Sandra Jul 16 '11 at 17:54
    
Then it's keeping some kind of catalog and choosing to backup if the file's "last modified" time has changed. –  SpacemanSpiff Jul 16 '11 at 19:18
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