Dangers of RAID 5 Array with SATA Drives
For many in the tech industry, RAID 5 has always been the recommended RAID level when creating new arrays. When I was first deploying a new iSCSI SAN for xByte, I found strong recommendations against using RAID 5. I took the advice and deployed a 17 disk RAID 6 array. With 17 10K SAS drives, performance was more than we needed. Now with added role of Sales Engineer, I wanted to revisit the dangers of RAID 5 more completely so we can give the best advice possible to xByte’s customers.
RAID 5 arrays protect against the failure of 1 disk in the array. When (not if) a drive in the array fails the drive must be replaces and the redundancy of the array must be rebuilt on the new drive. This requires reading every bit of data on the remaining non failed drives. Typically SATA drives will encounter an unrecoverable read error rate (URE) once every 10^14 reads. During the rebuild process it is VERY likely that one of the disk in the degraded array will experience an URE. When this happens the RAID controller will detect that a 2nd disk has failed and will fail the entire array.
RAID 10: Often RAID 5 is made up of 3 online drives plus a hot spare. For 1TB drives that will give a usable space of 2TB. Using all 4 drives in a RAID 10 configuration will provide the same 2TB of space and better performance. The drawback is that the array will be degraded until the failed drive is replaced and the data copied from its mirror.
RAID 6: For larger arrays, consider RAID 6 with or without a hot spare. A RAID 6 array will withstand 2 drives failing. Because 2 drives are used for parity, there is a performance penalty vs. RAID 5. For the Dell PowerVault MD3620i this penalty is very small. For xBytes 17 drive array the performance of a 17 drive RAID 6 is about the same as a 16 drive RAID 5 + 1 hot spare.
Enterprise NL-SAS: While the NL-SAS is essentially SATA drives with a SAS interface, the enterprise NL-SAS have a better URE. I would still recommend RAID 10 or 6 with NL-SAS.
SAS: Enterprise SAS drives have much better URE and are if RAID 5 is absolutely required, are the best choice.
With RAID 5, most of the concern is when it’s used with large SATA drives. However, we will be in the same situation with SAS drives in another 5 years unless the reliability of hard drive technology makes a major leap forward.
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