| I could swear there was a tape here... | Created: 28.11.2009 12:34 |
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In the last posting I already wrote about the defective
tape library gripper and what problems it caused. But
why did the gripper die?
The library is not that old, and hasn't exactly been
under heavy stress yet, so it certainly didn't die
from senile decay. For those unfamiliar with how these things work: The gripper sits in the middle aisle of the library. On both sides, there are shelves filled with tapes, and in one place there are a few tape drives. The gripper is mounted on some transport mechanism, so it can move up and down the aisle, and up and down vertically. It also can turn by 180 degrees so that it can reach both sides of the aisle. One side of the gripper assembly has a barcode scanner, so it can read the labels of the tapes, the other side has two grippers (hence the name) for taking or putting tapes. What the gripper usually does is that when the control software requests it, it goes to fetch a certain tape from its shelf, drive to the place where the drives reside, and puts it into the drive. When the tape has been read, the control software will tell it to put the tape back onto the shelf, and it will do that. The library also has an I/O slot, which can be used to get tapes in or out of the library. It's basically a shelf that can be reached from both sides - one side for the tape robot, one for the human. It all started when I told the library (remotely) to move some tapes to the I/O slot, because I wanted to take them out for demonstration purposes. However, when I arrived at the library to pick up the tapes, I was greeted with the error message "Gripper unable to pick cartridge" instead. Finding that a bit strange, I checked the inside of the library for obstructions. There was nothing wrong. However, the gripper was making strange noises, and unable to pick any tapes - in fact, it dropped some while trying to pick them. It really must have died when trying to fetch my tapes, because it had put some tapes into drives successfully just a moment before. When the support technician arrived to change the gripper, he seemed surprised how that could have happened as well - he mumbled something about the gripper being smashed into tiny little bits, everything broken that could be broken. We couldn't help the impression that in private he thought we had treated the gripper with a big hammer because we're such sick bastards. Only during the long exchange marathon that ensued (see last post) did we eventually realize the true reason. The library had gotten two additional tape drives just the week before. For putting the new tape drives in, you naturally have to make room, so you have to remove some of the storage slots for tapes. The technician that put in the new drives did that, and placed the tapes that had been there into some other slots in the library. However, he either forgot to let the library see that there are no tapes any more by letting it do a barcode scan after moving the tapes, or it didn't work properly. All slots have a special barcode meaning "there is no tape here" in the back, so when there is no tape in the slot, the barcode scanner sees the "i am empty" label instead of a tape label, and the library then knows there really is no tape. However, the front of the tape drives does not have any labels, and the library still remembered there had been tapes in that place before. Because it didn't see the "empty" barcode either, it assumed the tapes were still there, and just the barcode was bad or had fallen off. When I selected the tapes to remove, I had selected some of these tapes, and the gripper tried to grab them - and slammed into the new tape drives full speed. Luckily, these drives are a lot more stable than the gripper - else they would have had to exchange the drives as well... |
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