Recovering a Wrecked RAID

Dr. Eggman writes "Tom's Hardware recently posted an article specifying how the professionals at Kroll Ontrack recover data from a RAID array that has suffered a hard drive failure, allowing for recovery of even RAID 5 arrays suffering two failures. The article is quick to warn this is costly, however, and points out the different types of hard drive failures that occur, only some of which are repairable. Ultimately the article concludes that consistent backups and other good practices are the best solution. Still, it provides an interesting look into the world of data after death."

Comments

Re:RAID5.

[Anarke_Incarnate]

RAID 5 is great, though expensive when done right. RAID 6 is better, though has less performance, as well as additional cost. Many controllers will not do RAID 6, and you lose 2 drives to parity. If your data is truly critical, you should have backups done VERY often, as well as a RAID 50. This way you are far less likely to lose data, though you have to have a stripe of at least 3 drives, in a mirror. This requires at minimum, 6 drives. There are also VRAIDs, which allow for you to lose drives until you hit the watermark of your data. This technology is usually reserved for SAN systems.

Re:FOR THE LAST FREAKIN' TIME...

[eln]

That's true, but the most common cause of data loss on a RAID system that I've seen is when a disk fails, and people leave it there for days or even weeks without bothering to replace it.

When a disk fails in a RAID, it needs to be replaced IMMEDIATELY. A RAID system with a failed disk is a disaster waiting to happen. I've been in smaller shops that don't even have spare disks around. When a disk failed, they would order a disk at that point and have it shipped.

You should always have plenty of spare disks around, and you should replace disks as soon as they fail. A double disk failure is rare, but the longer you put off replacing a failed disk, the more likely it becomes.

Re:Backing up HDDs is very hard

[operagost]

Optical discs are a joke - 4.3GB is just not enough. Larger formats exist but are relatively expensive. Tape is expensive per MB and slow, plus it isn't random access and not suited to anything but slow full backups.

Your knowledge is out of date. For example, a SuperDLT 640 backs up at 32 MB/s with compression. Slower than a disk, but not "slow". Sequential access: well that's a given. Only suited for full backups? That's news to my company. Even daily incrementals and differentials are usually hundreds of megabytes or a few GB, which negates the small spool-up time of the tape. Besides, most modern tapes now store metadata on an internal chip so that an on-tape index does not need to be searched.

use RAID 0 mirroring

RAID 0 is striping. You probably mean RAID 10 or RAID 0+1.

Re:RAID5.

[Intron]

With the two drives on separate channels, mirrored writes can be done in parallel.