space management on tape

[Frank Cherry]

Hi there,

a question how Bareos managed space on tape:

Hypothetc:

On a LTO tape are stored in this order 3 jobs:

1: 3 TB
2: 2 TB

3: 1 TB

Job 1 is deleted.
Now a new job is queued, the spooling file has a size of 2 TB.

Will now the SD despool it

a) on position 4 of the tape (append) [this is what I think]
or

b) replace position1 because the is availabe space

So, thinking about fragmentation would be one part of a backup strategy when working with tapes.

Thanks and all the best, Frank

[Spadajspadaj]

It's almost obvious if you look at possible medium states but to give you a verbose answer - the media can be read from any point but can only be appended at the end.

So if any job is being pruned/purged/deleted, it's just being "forgotten" by the database but is still present on the media where it originally was.

Oversimplifying a bit - a media life cycle is:

Purged -> Recycled -> Append -> Used/Full -> Purged again.

So, as you can see, there is no (de)fragmentation. A volume is getting appended to, then it's getting recycled. Simple as that.

With disk-based storage it's getting a bit more complicated with dynamicaly created volumes, single-job volumes and auto-truncate on purge.

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[Brock Palen]

Your hunch is correct. Tape is linear and only appended to. Even systems like LTFS only append and never write data in hold of deleted/expired data. 

This is one reason why many outlets full and incrementals on different pools and this different tapes. So they expire at similar times so the entire tape can be reclaimed. Not leaving one job with massively different retention time. 

Sent from my iPhone

Brock Palen

989-277-6075

On Dec 26, 2020, at 12:32 PM, Spadajspadaj <spadaj...@gmail.com> wrote:



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[Frank Kirschner]

Thanks and yes, working with different pools will be a good thing for controlling the usage of the tapes.

Thanks, Frank