My question is,
1) Can we set netbackup to backup to the same tape untill the tape is almost full/full then only we expired it into our backup policy?
2) What will happen to the image that already expired and the tape been reuse to write another backup image? Can we still retrieve the old image from the begining of the tape?
Your cooperation are highly appriciated..
NetBackup does not work like that.
NBU will keep on appending backups with same retention until the tape is full, then load a new tape to continue the backup.
Image expiration is done according to retention level that was selected in the Policy schedule.
Only when ALL images have expired can the tape be overwritten from the beginning.
When tape is overwritten from the beginning, the tape label is firstly overwritten.
This is similar to a quick format, meaning that nothing from previous backups can be recovered.
This exellent blog helps to understand how NBU writes to tape:
Further to Mariannes outstanding answer.
You can get the number of partially full media in the olume pool settings, if set to 1, only 1 tape can exist that is 'partially' full. This has the effect of not allowing any other tapes to be used until that tape is full.
The problem with this, is if you have say 6 tape drives, as only one tape can be 'partially ful', only one tape drive would be used. You should therefore set this value to the number of tape drive you have, if you wish to write to multiple tapes at the same time. This is set per volume pool, so if you write to multiple different pools, this would still happen.
I was thinking about this, and IF YOU DO NOT EJECT OR VAULT TAPES, this IS what netbackup does.
NetBackup will write to tapes until they are full and then request a new tape.
As images expire, you may find that earlier space on a tape is actually expired data, and you CAN reimport those backups and unexpire them until the tape is reused.
It really will depend on how many drives you have, and how many retentions/volume pools you configure for a count of how many parially full tapes there will be.
You can set a max partially full tape value to limit how many per pool.
I have a specific pool for long term tapes - 7yr or infinite, and even though I have 18 drives, I only have one or two partial tapes in these pools. They write until full and I vault them to iron mountain only when full.
my short term tapes that vault every day, however, I send out 18 partially full tapes every day.