Let's think about tape media quality. I know about disk bad blocks. Shouldn't tape have bad blocks too? It's a similar recording media layer after all and there's hundreds of metres of the stuff with closely packed parallel tracks. Current cartridges hold 800MB to 1 terabyte and the recording media layer has a much harder life than the easy time spent by its cousin coating a disk platter.
A disk platter is stiff and doesn't flex. It lives in a fairly clean environment inside a drive clamshell case and has a relationship that's up close and personal with its very own read/write head but, remember this, no touching allowed. The drive has a memory and only has to think about its own disk platters and no one else's.
That means it can remember the location of bad blocks on the platter surfaces and avoid them. Data just doesn't get written onto bad recording areas on the platter surfaces. Not so with tape.
It's a hard life
A tape is a long long reel which bends. It is stored in a spiral and then fed past rollers, aligned with a read/write head, flows past it and then gets wound onto a take-up spool. That's a lot of movement and the recording media layer has to be strong enough and consistent and durable enough to take it again and again.
It also has to be covered with a coating layer and a lubricating film. These have to have an even thickness and consistency so that the electrical signals and magnetic fields can travel through them between recording layer and read/write heads. They have to have a strong surface; one that doesn't get abraded by contact with rollers and read/write heads and the layers of tape behind it and in front of it on a tape spool.
Then the tape lives in a relatively dirty environment, compared to a disk platter that is. It doesn't have its own dedicated drive. Instead it shares a drive with tens, maybe hundreds of other tape cartridges. It gets moved about in a tape library or an autoloader. It gets jarred and knocked about Compared to a disk drive a tape cartridge has a very much harder life and needs to be very robust.
Now let's return to media quality. Even though a disk drive platter leads a charmed life there are still areas of poor media quality. It has a virtually totally flat surface that doesn't move and never touches anything. But tape, poor tape, it has its hard life ahead of it and it isn't stiff. How the heck do you apply the chemicals to it with anything like the same dependability as you can to disk platters?
How on earth can you avoid areas of insufficient media quality on such stuff? Areas that will get expanded as the tape flexes during its working life. The answer is that, of course, you cannot. Tapes are not perfect. But there is a big problem. Since a tape doesn't have its own drive the tape drives cannot store bad block information, information such as track 13 has a bad area beginning at 50 metres and lasting for 0.12 metres.
What the drives do is to retry when a error occurs. Every time a re-try occurs time is taken and the tape fails to meet its rated speed. Suppose you find that a backup session is taking far longer than you thought and yet the data amount is the same as on other occasions. There is nothing wrong with the server or the network links.
What do you know at this point? You know that either the drive or the tape or both have a fault. How do you check? Do you call in the tape system vendor? Do you call in the media manufacturer and have the possibility of a finger-pointing competition between it and the tape library/autoloader vendor? Oh bliss!
What you can do is to use a software product called TapeWise from Data Product Services. This writes and then reads data from tapes and verifies what the drive is doing. If you suspect a drive is faulty then it can check two or more tapes on the same drive and if the same pattern of error-strewn performance is repeated then you have a flaky drive. But if one tape is good and the other is chock full of retries and the check time is greatly extended then you have a bad tape. Retire it and don't use it any more.
Once you have known good quality drives, golden drives, you can check a tape and produce a map of its quality, a visual representation - a 3D graph - using Tapewise's Tape error mapping (TEM) technology. You can set an error threshold yourself, in terms of the number of permitted errors per amount of data and then check tapes against that level.
It can check reads and writes and rewrites and more. With a known good tape, a golden tape, you can check a suspect drive and see if it is not reading tapes well or writing them well. You can get a tape error map that is actually a drive performance map and shows where it is start-stopping or 'shoe shining'.
Such performance maps can be very good evidence to present to a tape automation vendor or media supplier as a strong indication of where poor performance is coming from.
You may not get a full-hearted reception as tape media vendors don't necessarily want to take tapes with recoverable errors back. But from your own data protection point of view you have a very much better idea of the quality of your own pool of tapes and can retire suspect tapes before their performance impinges on the quality of your data protection arrangements.
Tapewise supports most tape formats: IBM and Sun/STK mainframe formats; LTOs; DLTs; Mammoth, AIT, 8mm, 4mm and more.
It gives you sight into the performance quality of tape drives and tape cartridges and without it you have much less information. Tape automation devices can never give you the same information about the media quality of tapes, because drives are shared between cartridges. Software like Tapewise can redress the balance. Without it you are running your tape infrastructure blindfold.
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