How to use the full LTO-2 Tape backup capacity of 200/400GB?

Solution 1:

200 GB (marked on the tape) = 200,000,000,000 bytes = ±150 GB (real, 2^30) usable + some metadata.

If you use backup app that has own compression, then there's no hardware compression working with your tape drive, so 200 -> 400 doesn't happen.

IMHO everything works as expected.

Solution 2:

As BaronSamedi1958 said, if you are counting binary gigabytes (GiB) then the capacity of a 200 GB tape is about 186 GiB (200 / 1.024³). This is why I encourage people to use real gigabytes (1,000,000,000 bytes) everywhere except when buying RAM.

However, short tapes occur for a number of reasons.

• If the tape or drive is dirty, then some blocks will fail to write. The drive will simply continue and write the block in the next available space on the tape. This will result in degraded capacity.

• If you cannot supply data to the drive at the full write speed, this can also result in degraded capacity. However, LTO-2 is only 40 MB/s so one can hope that this isn't your problem.

Make sure the drive has been cleaned recently and use a fresh tape. See if the problem persists. This could also indicate a fault in the drive.

On the other hand, tape is cheap and the best solution might be to simply ignore the problem and live with 150 GB per tape.

Solution 3:

Also take in account the blocking factor. If you're using for instance a 128K block size, and you're backing up many small files, as each file occupy at least one block on tape, you end up with a lot of wasted space. Typical disk block size is 4K; on tape for decent performance you'll rarely use less than 32K.