When to change a hdd in a raid/NAS?

When using a Network Attached Storage, when is it the best time to replace HDD (with RAID)? Upon failure, or proactively?

I have a Synology DS713+, with two HDD from Western Digital (WD Red, acquired at the same time), put in Synology Hybrid Raid. S.M.A.R.T. tests show no error for the moment.

I have been told that, with the current setup, my two HDD will likely fail at the same time, sometime in the future (or close enough), hinting that I should have one HDD replaced sooner, even though it is not dead yet.

Is this a current practice in RAID environment to change HDD asynchronously, in order to avoid havoc?

Marc-Olivier Titeux

Posted 2014-02-25T10:30:43.030

Reputation: 171

FYI RAID is an availability solution, not a backup solution. You should be backing this data up off the RAID somewhere. – LawrenceC – 2014-03-05T19:58:32.413

Thanks for adding the precision. I'm aware of it (what is a RAID, etc). Some sensitive data are replicated by hand on a USB HDD that I use only for this. But, would you mind to elaborate the "do not use RAID in backup system" stance? Thanks! – Marc-Olivier Titeux – 2014-03-05T20:30:28.840

1Just because a volume is RAIDed doesn't mean you shouldn't back it up on another physicaly separate volume - since you're doing that, sounds like you're doing the right thing. :) – LawrenceC – 2014-03-05T21:50:02.823

Doesn't matter too much since the rebuild time is the same regardless. – kobaltz – 2014-03-05T22:12:19.390

Answers

There's no hard rule since you can't really predict when the drives will die, other than probably after the warranty expires. (Not just a joke; they are engineered that way.)

So after the NAS has been in use for a few/several months, wait for a sale on WD Reds. Buy one and swap it in during some slow time that's convenient. You have now staggered the age/lot of the drives in active use, and have a spare to swap in, which also has its own snapshot of the data that might come in handy, like if the RAID recovery doesn't actually work. Better to find out when it's not a real panic situation with some looming deadline.

Edit: as for using the same model, here's an excerpt (in a response by the author to a comment) from a Synology blog post. Sounds right to me:

Personally, I’ve always used drives of the same model to maintain consistency of data communication throughout the entire array. If possible, I would try to buy drives from a different manufacturing batch – but that is difficult in today’s world.

For the strategy of mixing brands/models of drives, at the very least, I would recommend keeping the same speed, cache, and drive size.

Ken

Posted 2014-02-25T10:30:43.030

Reputation: 7 497

Thanks for the answer. As an aside, should I stick to the same model if I want to replace one? – Marc-Olivier Titeux – 2014-03-05T20:39:18.027