Reliability of tape backup

[ncb]

Hello,

We are testing the tape backup functionality of PBS. For now, if i understand correctly, PBS is sending the chunks of the selected datastore or namespace to the tape.
These chunks seem to be gathered in groups of about 4.3 Gb on the tape, along with some other files containing catalog data (btw, is there more information somewhere on the data structure of what is stored on the tape ?).

One of our questions that remain open is : What would happen if a piece of the tape is unreadable when trying to restore ?
Is the filesystem restored with some files missing or is all the data stored on this tape lost ?

In our current system we are backuping the files of our systems directly to tape, so if we lose a part of the tape, we only lose the files stored on that part of the tape.

Thank you for your help.

 

[dcsapak]

(btw, is there more information somewhere on the data structure of what is stored on the tape ?).

only really in the code itself, not as 'human readable' documentation, but the basic layout is rather simple:
we have 'chunk archives' (~4GiB of chunks) and 'snapshot archives' (one per snapshot)
the chunk archives always come before the snapshot archives in which they are referenced, so that all snapshots that were restored are valid

if a chunk is damaged on tape, all snapshots that reference this chunk are invalid (and thus gone)
if a snapshot is damaged, only that snapshot is gone

in any case, you can have multiple media sets in parallel, so that you don't have to rely on a single tape for your restores
and since the backups are deduplicated also on tape, you can store much more snapshots on a singe media-set than if the snapshots were 'full' backups each time