overwriting full backup in an incremental backup job

[mk128935]

I have a customer who has a Windows server with a Tanberg Quikstation 8 emulated tape library attached. They want to run two backup jobs (one job taking full backups only, the other job taking incremental backups) and store the data on two RDX cartridges. When the second RDX cartridge becomes full, they want to overwrite the data on the first RDX cartridge etc. See attachment for what I mean.

For the second job, when the second RDX cartridge becomes full and overwrites the full backup on the first cartridge, will the incremental backup job continue to run? (I think yes). Can they restore files from the recent incremental backup even though the full backup was overwritten? (I think yes). One concern I have is how long can they operate the second (the incremental backup) like this having overwritten the initial full backup? Are there any concerns such as data integrite regarding this? We don't have always incremental backup so I would think this would be a concern.

I appreciate the further comment on this.

Thanks.

[Colin_Weaver]

Disk based backup sets do not get overwritten, they get deleted (reclaimed) once they meet a combination of some DLM deletion/reclaim rules and the time based expiration criteria you set (and in the case of RDX when a backup jobs starts) We have no criteria that forces an overwrite when a cartridge is full.

There are also rules that link the incremental backups to the full backup. These rules means that the full cannot be deleted if there is a chain of incremental backups linked to it, if all incremental sets in the chain have not expired. You will also have problems with this rule if the incremental sets are on a removed/offline RDX cartridge when the full that you want deleted is on the active/online cartidge.

The last recovery chain (last full plus any linked incrementals will also not be deleted until a new recovery chain (so new full) has been run), although this can be on a different cartridge

One final point - GRT incremental sets require that the complete incremental and full chain is online when the next incremental runs so again means your strategy may have a problem.

The normal recommendation for RDX use is therefore run only full backups OR if you must run an incremental setup then these have to be written to the same cartridge as the full they depend on. Once you start with this as a concept, then you have to look carefully at the size and number of RDX cartridges in your rotation (taking into account last recovery set rule) as well as the expiration times of you full and incremental sets

Soem further info on RDX here:

https://vox.veritas.com/t5/Backup-Recovery-Community-Blog/Backup-Set-retention-and-expiry-Handling-o....

[Colin_Weaver]

and info on DLM

https://www.veritas.com/support/en_US/article.TECH214945

[mk128935]

Thanks for the response. Sorry I should have been more clearer. Although this uses an RDX cartridge, the QuikStation 8 runs in tape emulation mode so it behaves just like a tape libarry. So the DLM rules wouldn't apply here but the tape media rotation rules would apply I would think.

[Colin_Weaver · Accepted solution]

Hmm Ok - so if using in tape emulation mode, we still don't do overwrites based on it filling up - it is still based on a timing that starts when the last data is written to the tape (and based on your PPT content you would have to allow an append period on the tapes for them to get close to full)

We also have no way for the next job that starts to identify that the tape will fill during the activity of that instance of the job - so spanned sets will happen in your setup where the backup data ends up on two tapes/cartridges

Oh and the reason for locking the full from being overwritten when there is a dependent incremental set now has to be managed manually by you as when a tape is overwritten it is the whole tape and not a specific backup set on the tape that is destroyed.

GRT incremental sets still rely on the chain of incrementals back to and including the last full for a restore.

Standard file system (non-GRT) sets of drive letter backups allow the resTore of files from just the specific incremental set (although for the the specific type of restore known as a point in time restore you do need the complete incremental chain back to the full)