IT Administration

The case against RAID 5, why use RAID 6 instead

For many years RAID 5 used to be best practices and golden standard for any organization, who cares about smoothness of their operations, especially when downtime means losing money. RAID 5 is cheap and when the hard drive in your array fails, you can replace it without data loss. You just swap the drive manually or you can have installed spare drive already in your storage device exactly for this situation. Users often won’t even notice something is wrong. RAID 6, on the other hand, is still seen as overkill with bigger overhead and with worse performance.

Disk cost

RAID 5 needs one more drive, which you cannot use for storing data. If any of the drivers fails, you can rebuild it without data loss.

RAID 6 needs two more drivers, bud you can lose 2 drives at once and you will still be able to rebuild the array successfully.

Speeds

The reading performance of RAID 5 and RAID 6 is basically the same.

The writing performance is a little bit more complicated, RAID 6 can be more power hungry (33 % more CPU power) and also little slower in some scenarios. But the difference can be eliminated using cache in your storage device.

So what’s the problem?

It’s the security. You are using RAID, so you obviously want no downtime –  restoring from backups takes precious time and you have to bring your information systems offline in most cases.

Many people think the chance of 2 disks failing at once is slim. It happens all the time, believe me. The drives have much bigger capacity today so the rebuild often takes days. When another drive fails in this period and you are using RAID 5, your data are gone.  Also, the rebuilding process is pretty disk-intensive and it can cause another failure. You have probably bought the disks at once, they can come from the same manufacturing batch and have the same defects. Hell, you probably installed them and started to use them on the same day!

There is another big problem, which wasn’t such a big deal years before. Storage is cheaper nowadays. With vast arrays of many terabytes, rebuilding times are not only much longer, but there is a higher chance to encounter URE (Unrecoverable Read Error) and again, lose your data.

One recommendation holds well against the time. RAID doesn’t replace backups. Have good backups, folks.

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