The RAID5 delusion

RAID5 isn't for protecting your data – it is for keeping your applications running when there is a failure. And it is no substitute for backup.
Written by Robin Harris, Contributor

Case in point
I spoke to the head of small company – about 25 employees – who had suffered a RAID5 drive failure. The 4TB RAID was used for file sharing.

A drive failed, reconstruction failed and vendor phone support was disastrous. All data was lost.

But the worst of it was that there was no backup. They believed that RAID5 would protect their data. They were wrong.

What RAID5 is for
RAID5 does offer some data protection assuming it works. But it's main purpose is to protect access to your data. This is why it is popular in enterprise applications where maintaining data access during a failure is of vital concern.

But these arrays are always backed up so that if there is a catastrophic array failure – a not uncommon occurrence – the data is still recoverable despite the interruption in service.

That's how it played out with the small company. After the drive fail they still had access to their data. But when they replaced the drive the rebuild did not go as expected. They were stuck.

If they had stopped there and made a backup they probably could've saved all their data. But they thought the RAID was there to protect their data. Oops.

The Storage Bits take
Most enterprise RAID today do not use RAID5 because the likelihood of a second failure during the rebuild – increasingly lengthy because of growing drive size – means that the likelihood of a second failure during rebuild is too high for comfort. Instead they use RAID6 - and hyperscale Internet services use even fancier erasure codes that can survive 4 failures.

Note that this does not mean a second drive has to fail: it can be as simple as an unrecoverable read error on a remaining drive that totally pooches the rebuild. Then you have to go to your backup - assuming you have a backup.

RAID was a wonderful advance 25 years ago. But the catchy name is no substitute for a backup and archive strategy.

Comments welcome, as always. That said, how do you archive your personal and small business data?


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