FreeNAS: Open Source Storage Operating System

Post by baruch » Tue, Aug 30, 2016, 7:20 PM UTC

cbsmith wrote: post #180

baruch wrote: post #174

Earlier quoted context omitted.

Adam Leventhal uses the wrong interpretation of the BER value so I wouldn't take his words at face value. The whole argument of Adam is by the disk BER and not by density. I do assume though that the RAID array does media scrub (BMS) periodically, if you don't you are at risk anyway and I'd call that negligence in maintaining your RAID. If you do use scrubbing the risk that another drive has a bad media spot is low a…

So why would one ever bother with RAID-6 (or its moral equivalent with RAID-Z)? I've yet to hear someone justify it from a requirement being to actually survive two drives failing simultaneously.

The main problem is not two drives failing simultaneously but rather one failing shortly after the other. The more drives you have the more the likelihood of that increases, the rebuild time also increases with large drives and so you are more open to such a second disk failure. If you take the MTTF of the drive (or the mostly equivalent probability of failure) you find the risk increases since the MTTF of the drives does not increase at all with size, it is rather constant at least as it is specified by the vendors (HDDs are usually 1.2M hours). The more drives you have the more likely one of them to have a failure and once one fails and since the rebuild time increases with size your chance of failure during rebuild increases as well.

Some systems mitigate that by having a more evenly distributed RAID such that the rebuild time doesn't increase that much as the drive size increases and is actually rather low. XIV systems are like that.

Post by cbsmith » Wed, Aug 31, 2016, 12:43 AM UTC

baruch wrote: post #181

cbsmith wrote: post #180

Earlier quoted context omitted.

So why would one ever bother with RAID-6 (or its moral equivalent with RAID-Z)? I've yet to hear someone justify it from a requirement being to actually survive two drives failing simultaneously.

The main problem is not two drives failing simultaneously but rather one failing shortly after the other. The more drives you have the more the likelihood of that increases, the rebuild time also increases with large drives and so you are more open to such a second disk failure. If you take the MTTF of the drive (or the mostly equivalent probability of failure) you find the risk increases since the MTTF of the drives…

> The more drives you have the more likely one of them to have a failure and once one fails and since the rebuild time increases with size your chance of failure during rebuild increases as well.

That was exactly what I was saying. Given the same sustained transfer rate, the bigger the drive, the longer the rebuild time, hence the greater the chance you'll have a failure during the rebuild. While you might think they would, throughput increases have not grown to match increases in bit density & storage capacity.

SSD's have helped a bit in this area because their failure profiles are different and under the covers they are kind of like a giant array of mini devices, but AFAIK they still present challenges during RAID rebuilds.

Re: FreeNAS: Open Source Storage Operating System

Re: FreeNAS: Open Source Storage Operating System