To recap the earlier post, the issue is the unrecoverable read error (URE) rate of SATA drives used in consumer storage arrays. With an unrecoverable read error rate of 10^-14, you could expect a failed block read once every 12.5 TB or so.
If you had a 8 drive array with 2 TB drives with one failure your chance of having a unrecoverable read error would be near 100%. That second unreadable block during a RAID5 recovery is enough to destroy the RAID group and wipe out all the data on it. Not good!
Even with a four drive RAID5 - and 2TB drives - you would have around a 40% chance of a rebuild failure. Better, but not good enough.
The combination of the increasing capacity of SATA drives, the constant unrecoverable read error rate and the number of drives in the RAID stripe that led to the prediction that RAID5 would no longer be viable in 2009.
A couple of years ago I started seeing consumer drives spec'd at 10^-15, a rational response to the RAID5 problem. With a tenth of the URE rate consumer RAID5 arrays would be fine.
But reviewing current 3.5" SATA drive specs from HGST, Seagate and WD and guess what? They are all back to 10^-14.
Which means that consumer RAID5 arrays can't be trusted to store your data reliably. Quickly, yes. In large chunks, yes. More simply than individual USB drives, yes.
But not more reliably than a single drive.
Yet RAID is not only about availability. Its other advantages are important and, for most, possibly more important.
- Performance. Striping data across multiple drives can dramatically increase bandwidth for large file apps like video editing.
- Capacity. Putting 4-12 drives in a RAID gives a large virtual disk that is much larger than any single drive.
- Management. After the often painful setup process - and until something breaks - RAID arrays are simpler to manage than individual disks.
Storage Bits take
It seems that people use small RAID5 arrays more for convenience rather that data availability. Either that or they really don't understand how vulnerable their data is - as one business found out recently - and prefer the bliss of the RAID5 delusion.
Many are still using small RAID5 arrays with 10^-14 error rates - me too! - and RAID5 seems to work fine. But adjustments should be made to account for the unchanged error rates.
- Always maintain a minimum of 2 copies any data stored on a RAID - 1 on the RAID and 1 elsewhere.
- Where there is a drive failure pull any unbacked up data - latest documents that aren't backed up - off the RAID before replacing the failed drive.
Since RAID arrays are more complex than individual drives, they are more likely to fail. But until they do they are more convenient, faster and larger than any single drive.
Comments welcome, of course. Few consumers use RAID arrays. If you do, do you think more people should?