Thursday, March 10, 2011

RAID-5 is the Devil

And I’ll tell you why.  But first, the TLDR takeaway:  If you have a storage array of anything over a terabyte or two DO NOT use RAID-5, use RAID-6 or something with superior fault tolerance.  If you remember anything from this post remember that or you may find yourself in the position I’m in currently, lost data and all.

It really comes down to one little performance specification that until now I hadn’t ever really noticed: “Unrecoverable read errors”.  For the latest Seagate home drive that statistic is 1 in 1014, for a standard SATA Enterprise class drive that number is an order of magnitude better, 1 in 1015.  Remember those numbers and now read this excellent article as well as one of the referenced white papers from NetApp, and perhaps finish off with this StorageMojo post

See the problem now?  My backup server sure did when it ran afoul of a double drive failure.  Perhaps it was just bad jujus, but my 6x1TB drive array (with hot spare) had a drive fail – and hit “Unrecoverable read error” while reconstructing the data to the hot spare.  So now I’m working at evacuating the existing data and restoring from backups.  To save yourself that trouble it’s worth remembering:

For large arrays, RAID-5 is the devil.