Free space in DDS tape

[nck2pg2]

How do I know the free space available in DDS tape? Recently I have some problem with automated backup. Suspected the tape is full. Please advise.

[Vincent Farrugia]

Hello,

The answer is that you can't really say. You can know the amount of data put in a cartridge, but not how many metres of tape you have left.

You never know what amount of compression the backup used. On a DDS3 tape cartidge, you can hold up 12Gb of data uncompressed. So actually, if you store 12Gb of data in such cartridge, you could have used either a full cartidge, or half, or 3/4, or 1/4... you can't really say. It depends on the data itself with how much compression it stored the data.

Even if you know the amount of tape in metres left in your cartridge, that won't help either, cuz the same rule applies. If you have let's say 20 metres left, you can't say how much amount of data you can store on this amount of physical tape because of compression.

HTH,
Vince

[Michael Tully]

Basically it is not possible to work out due to the compression being used. If a tape is full the program generating the backup will actually ask for a additional tape depending on the program. If you are using a preogam suite like omniback you can work how much data is on a tape. Other programs may not.

[Stefan Farrelly · Accepted solution]

This is difficult. I think the best way is if youre using fbackup, it reports at the end of the backup;

total blocks written to output file /dev/rmt/2m: 54756166

So from this we can calculate how much data was written to tape; 54756166 * 512 = 26,780 Mb (26 Gb). We know from a DDS drive and the length of tape used how much space can be written to it, eg;
DDS2 120m tape = 4Gb
DDS3 125m tape = 12Gb
or in my example above;
DLT7000 CompactIV tape = 40Gb

so from this you could get a pretty good idea how close to filling up the tape your last good backup was (if you had a backup failure I dont think it displays the total blocks written message).

[Stefan Farrelly]

oops, my example for a DLT7000 with a compactIV tape should of course be 35GB, not 40.