Recovering limited space from a tape.

[cnx_anup]

I have NBU 7.6.0.4

I got 2 queries.

Q1. Please let me know what the first job does ? It says media contents and is running for a long time.

Q.2. Is it possible to recover space on a tape by expiring few backups and use that space for a small backup job ?

[Marianne · Accepted solution]

1. It mounts the tape and scans sessions on tape (list of images) and produce a list in the window where you started the process.

2. No. Not possible.
Only when ALL backups on a tape has expired can it be overwritten from the beginning.

Have a look at this excellent blog:

Understanding how NetBackup writes to a tape. 

[revarooo]

Q1. Please let me know what the first job does ? It says media contents and is running for a long time.

someone ran: bpmedialist -mcontents -m <mediaid> to view what images were actually on the physical tape

Q.2. Is it possible to recover space on a tape by expiring few backups and use that space for a small backup job ?

No. Every single image on a full tape must be expired before you can re-write to it. You cannot expire a few images (leaving some images on tape) then write to those areas on tape where the images were removed.

NetBackup does not remove images from tape. They are permanently there until the last image on the tape is expired, then the tape can be overwritten.

[sdo]

1) Re media contents report, see:

http://www.veritas.com/docs/000087036

.

2) No, if there are remaining images after those to be expired.  Yes, if there are no remaining images after those to be expired, but only after media expiry/cleanup has run, which is typically once every 12 hours.

[cnx_anup]

Thank you.

[cnx_anup]

Thank you.

[cnx_anup]

Hi Marianne, thanks for that link. I understand the image expiry part that the new backup will not write to the portion were image is expired. I dont understand about the case were a backup image has failed.

According to the blog,

BOT | server1part1 | Server2part2 | Server1part2| blank space | EOT

Suppose Server1part2 is the D drive of Server 1 which is 800 GB. That particular child job failed. Say there was a connectivity issue after backing up some 300 GB.

Now I'm re-running the child job that failed. Is it going to be like this ?

BOT | server1part1 | Server2part2 | expired image (failed) | Server1part2 | blank space | EOT

Is expired image (failed)  going to consume space on the tape. ie 300 GB ?

[Marianne]

It all depends on whether your backups are multiplexed or not.

If one backup in a multiplexed backup fails, NBU discards/expire the backup but the space written on tape by that backup cannot be reclaimed.

If only one backup is written and it fails, then NBU will discard all information about the failed session and overwrite it when backup is restarted or another backup needs to append.

Have another look at this explaination in the blog regarding multiplexed backups:

Now look at it with multiplexing.

you use the tape - it gets an assigned date.

BOT|server1part1|server2part1|server1part2|server2part2|blankspace

if the backup for server1 fails or if the backup image expires then you have this

BOT|expiredimage|server2part1|expiredimage|server2part2|blankspace

remember tape is linear it cannot go back and write in those spaces where the failed image went.

so you do lose space on a tape when a backup fails or expires.

Another blog by the same user:

NetBackup and just how Multiple streams to a tape drive works. 

I have added a picture that used to be in older NBU training manuals.

If you have a look at the picture in my post, you will understand that if Client A (red) backup fails, that those tape blocks cannot be overwritten by a subsequent backup and this space will b lost until all images have expired and can be overwritten.

So, for capacity planning, add some extra tapes to make provision for bad tapes and failed backups.