One of the adages of the storage industry has been “Fibre Channel and SCSI drives are more reliable than SATA and PATA drives”. This has always confused me. The technology in the spindles just doesn’t change that much, and in the past the difference between the SCSI and ATA models of a drive may have been as little as different drive electronics on the same spindle.

How could SCSI drives have been more reliable? Could it have something to do with them costing three times as much for the same amount of storage? Hmmm…

It used to be easy to find comparable drives in SATA and SCSI flavors, but that’s become increasingly difficult with the advent of 10K and 15K RPM drives. The drive manufacturers have created a false segmentation in the market, where 10K and 15K RPM drives are only available in SCSI, FC and SAS flavors, and almost never in SATA. Western Digital was the lone company that broke the rules of this cabal, but they seem to have been shamed back to offering only a single model, the VelociRaptor. Let’s hope the FTC decides to start looking into this.

So, today your performance requirements may lock you into paying extortionist prices for SAS drives. But, where we can make comparisons, the reliability canard doesn’t seem to make sense. I looked at the data sheets for all of Seagate, Hitachi, and Western Digital’s current drives, and they all have MTBFs quoted in the 1 million to 1.4 million hour range. Two recent studies have shown that there’s no observable difference in failure rate of SATA vs. SCSI/FC drives, although both are far less reliable than the manufacturers’ quoted figures. There’s no enhanced spindle reliability with the pricey drives.

Where there is a difference between models, though, is with our old friend, the bit error rate. The bit error rate is the rate at which a block just can’t be read from the disk, due to not being able to recover data from the PRML and ECC codes on the platter. The whole drive doesn’t fail, but you can’t read that block. In a RAID system, this triggers reconstruction of that block from the remaining drives.

As I describe in the video “The Trouble with RAID”, this bit error rate is the biggest problem with RAID technology today. In the event of a drive failure in a RAID 4 or RAID 5 set, every remaining drive must be read perfectly from start to finish or else data will be lost. With a 7+1 set of terabyte drives, this means 7 TB must be read. A bit error rate of 1 in 10^14 means that there’s a 44% chance that can’t be done.

With Seagate, every SCSI/FC drive has a bit error rate of 1 in 10^16, except for the Cheetah 10K.7, which is at 10^15 but maxes out at 300 GB. With Hitachi, every SCSI/FC drive has a bit error rate of 1 in 10^16. With Seagate, Hitachi and Western Digital, every SATA drive has a bit error rate of 1 in 10^15 or even 10^14, even with terabyte capacity drives! Not one exception.

This means that if (and only if) you’re using RAID, SCSI drives are hands-down more reliable. SCSI drives have a better bit error rate, and are smaller in capacity (so less data needs to be read for a RAID rebuild). In the worst case, the Cheetah 10K.7 drives, there’s only a 1.7% chance that a block will be lost in reconstruction. In the SATA worst case, the Hitachi Deskstar 7K1000.B or the Seagate DiamondMax 22, there’s a massive 44% chance the RAID rebuild will fail.

This just goes to re-emphasize that in large enterprise archives, RAID technology just doesn’t cut it anymore. That’s why Permabit developed RAIN-EC and our other on-disk data protection technologies; only these can provide the level of reliability required for large-scale, long-term data storage.

The other big mystery, though, is why these vendors aren’t shipping SATA drives with better bit error rates. Unlike spindle reliability, bit error rate is largely a software tweak — just use a few more bits of the disk for ECC purposes. Take that terabyte drive down to 750 GB of capacity and make it ten times more reliable. Drive vendors could do this tomorrow.

The false segmentation of drive interface and spindle speed is alive and well today, with Western Digital appearing to have largely given up. Is there a false segmentation on bit error rate too?