Why even use RAID?

Started Oct 6, 2009 | Discussions thread
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Re: Why even use RAID?
In reply to Justme, Oct 9, 2009

What you were told is probably related to how different drives handle errors. I may not be quite accurate, but my understanding and an easy way to view things....

When a "consumer" level drive experiences an error reading data, it attempts to keep reading the data so the data may be written to another area of the disk, then marks that portion of the disk as bad, then continues. This process can sometimes take quite a while. If it takes too long, many (most?) RAID controllers see that the drive hasn't responded within the controller's timeout, and takes the drive offline. If you were to unplug that drive and plug it back in, the controller would probably rebuild the array and everything would be fine. However, while that drive is offline, another similar delay or drive failure would result in the loss of data. For non-redundant arrays such as RAID 0, data is already lost.

"Enterprise" level drives have firmware which stops this internal data recovery effort after a fairly short timeout - something less than the RAID controller timeout. At that point, the drive will mark that area of the disk as bad, and return an error to the controller. This allows the RAID controller to detect a data error (rather than a non-responsive drive), so the controller will initiate error recovery and rebuild the array/data. Western Digital calls this "time-limited error recovery" (TLER), and Seagate calls it "error recovery control" (ERC).

In practice, I'm not sure how much of a concern this is for most people. Drive errors are relatively rare, and simultaneous multi-drive errors are even more rare. With any data storage solution, you should always have backups.

I found a Wikipedia article about WD's TLER:

I'm not sure "the way to go is to use a non-RAID setup when it comes to a home PC setup". A drive may be able to recover from a bad disk sector, but a severe error like a head crash will result in loss of the drive. In that situation, everything is lost when a RAID setup may have allowed operations to continue. For instance, if I have all my photos on a drive and it fails, I have to go to my backups and start a recovery process. If I have my photos on an array with some redundancy (RAID 1, RAID 5, etc), I can still operate if a drive fails.

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