Enterprise-class hard drives have a few features that make them more appropriate for use in RAID arrays. One well-known technology is Western Digital's Time Limited Error Recovery (TLER). Samsung has something similar in their Command Completion Time Limit (CCTL), and Seagate calls theirs Error Recovery Control (ERC).
What's the big deal about TLER/CCTL/ERC? Feel free to hit the links above if you would like the long-winded manufacturer's answer. The short of it is that these hard drives will "give up" fairly quickly when they experience a read error.
I'm sure you're thinking "WHAT??? IT GIVES UP MORE QUICKLY?" Yes, in a RAID array, giving up quickly is a good thing(tm). Let's look at two different scenarios:
Scenario 1: You spent all week working on a paper for school. The document was stored on your hard drive in a sector that's just starting to develop a problem. The next day when you go to print it, the hard drive is unable to read from the sector where your paper was stored.
Pop Quiz: What do you want your hard drive to do?
- A.) give up after a few seconds and say “sorry, you’ve lost your paper”
- B.) be heroic, keep at it, attempt reading the failing sector over and over again until the data is recovered, no matter how long it takes.
Yes, “B” was the correct answer, and that’s exactly what consumer-class hard drives do. When they experience a read-error, they keep at it. I'm not sure how long, but as far as you're concerned, it can take as long as it wants because you need that data.
Scenario 2: Same situation, but instead of storing your paper on a consumer-class hard drive, you store it on a RAID array using enterprise-class drives with TLER/CCTL/ERC. When it comes time to print the paper, one of the hard drives is unable to read one of the sectors where your paper is stored. This isn’t a problem because the drive will give up after a few seconds. The drive will notify the RAID controller that it couldn’t read a sector. The RAID controller will then recreate the missing data from the other drives. You print your paper and you’re off to school. In this situation, giving up quickly is a good thing. You only had to wait a couple of seconds and you were able to print your paper.
In Scenario #1, while the drive was attempting to get your data, the computer is unresponsive. It acts like it's frozen, and it can't do much else until it's able to complete reading your file - and that's okay. You want that file and there's only one place to get it.
In Scenario #2, delays like this are completely unacceptable. Can you imagine if 100's of employees had to sit around twiddling their thumbs for who-knows-how-long because "the server" (with a consumer-class drive) took heroic measures to read a failing sector? Lost productivity. What about 100's of customers on your website trying to buy stuff? Lost revenue. Either way you want that drive to quickly give up so that the RAID array can do its thing and your server can get back to supporting your business.
There's one more scenario, the one where you use consumer-class hard drives in the RAID array. That's a longer story and one I'll save for my next post. Let's just say that a drive taking "heroic measures" isn't awarded any medals by the RAID controller. Instead it is taken out back and shot in the head. It's not a pretty sight.
0 comments