I have an Ultrium 448 tape drive, LTO-2 tapes (200/400GB) and I'm using HP Data Protector as the backup client. The system is set to format and overwrite all tapes but I never manage to get more that ~150GB on a tape before it asks for another tape to be inserted.

Any ideas why this may be happening?

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    What brand tapes are you using? What color are they? Are you certain that they're LTO 2 tapes? – joeqwerty Dec 8 '16 at 12:26
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    HP - Maroon, Maxell - purple. They are labelled LTO-2 tapes. – Andrew Hoole Dec 8 '16 at 13:47
  • Do people really still use tapes for backups? What advantage does it have over a hard drive? My experience with tapes in the past was always that they were noisy, slow, and unreliable. People still used them though because of price and capacity. But in the present, when a 1TB hard drive is ~50$, is it still worth it to use tapes? – Drunken Code Monkey Dec 9 '16 at 3:32
  • Cheaper disks mean even cheaper yet tape. Now those specific tapes cost 3× what it would to buy a HDD, but open-reel tape in the abstract should proceed at the same pace as platters. – JDługosz Dec 9 '16 at 8:51
  • It's not only the costs, Its depends of the company regulations. You need to have an external hard copy (not a hard drive) of your data to comply any IT regulations (Commonly Audit Purposes). – HEMAN85 Dec 30 '16 at 14:14

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.

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    Regarding the metadata, if the system is creating OBDR tape how much would you lose or is that very dependent on your system disk size? – Andrew Hoole Dec 8 '16 at 13:51
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    The answer is "It depends!" like it or not. – BaronSamedi1958 Dec 8 '16 at 13:52
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    Totally agree, reducing backup capacity can be achieved by using backup software with built-in deduplication, such as Veeam or Backup exec. – Stuka Dec 8 '16 at 14:44
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    Also, you can emulate tapes and write backups on the intermediate HDD's and then copy backups to the real tapes by using something like Dell Ocarina or StarWind VTL. Afterward, you will have an additional copy of your data will keep system performance stable. – Stuka Dec 8 '16 at 14:52
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    200GB ~= 186GiB. That's still quite a lot more than 150GiB. – marcelm Dec 8 '16 at 21:52

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.

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    Drive is cleaned weekly and there are new tapes in the set which are also exhibiting this behavior. Problem with accepting the reduced capacity is having to swap tapes multiple times to get a complete backup and doubling the storage space required to store a full set of them. I hope I can find a solution, another post hinted that the drive may be the cause of the problem. I may have to investigate that. – Andrew Hoole Dec 8 '16 at 17:19
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    @AndrewHoole: Why is the storage space doubled? 200 GB -> 150 GB you should only be seeing an increase by about 33%. – Dietrich Epp Dec 8 '16 at 17:34
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    I guess the OPs dataset is >150GB but <200GB, thus moving from 1 to 2 tapes. – Hennes Dec 8 '16 at 20:53
  • @DietrichEpp: Sorry I wasn't very clear, I meant the physical space required to store the tapes was doubled if I let it bridge over to a second tape. – Andrew Hoole Dec 9 '16 at 11:37
  • @AndrewHoole: Shouldn't you still only need 33% more tapes, though? – Dietrich Epp Dec 9 '16 at 15:05

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.

  • Is it really common to backup individual files, rather than a single tape archive? That's the primary purpose of tar after all. – Dmitry Grigoryev Dec 9 '16 at 8:15
  • @DmitryGrigoryev well that depends on the software, I have no idea about the way HP Data protector works or the format it uses. – wazoox Dec 9 '16 at 11:50
  • I hadn't considered this and it may be an issue, I'll need to go and check what the block size is. – Andrew Hoole Dec 9 '16 at 12:46

I do not have any experience with LTO-2, but I have seen a lot of LTO-4 drives fail in a fashion that would cause a sudden 25% drop in tape capacity. Since you appear to also see exactly 25% drop in tape capacity it is possible that your LTO-2 drive is failing in the same way as the LTO-4 drives I have seen.

An LTO-4 drive will write tracks on the tape in alternating directions. A total of 28 tracks in each direction will be written before the tape is full, and each pass from end to end takes approximately 2 minutes.

The drive will read back all the data it has just written and in case the quality is not good enough the firmware will write another copy transparently to the backup software. This will lose some capacity but protect against many cases of data loss.

However as the drive heads wear down it will reach a point where the firmware will always need to write two copies of every block written, but only in one direction.

Careful measurements of the writing speed would show alternation between the nominal 120MB/s in one direction and 60MB/s in the other direction. If you can feed data to the drive fast enough to ensure that it is always the drive which is the bottleneck, then this distinctive pattern in writing speed is the most reliable way to detect a worn down tape head.

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