[Solved] RAID for 3 failing drives?

thediamondgames · 2016-04-20 14:22:33

Hello everyone,

I wanted to ask if there was such a (software at worst case) raid system, which would support 3 failing drives, but would make as much space as possible. I am talking about something like raid 5/6. Is it possible to do so with 6 drives or would my best be would be to just use raid 0 with lvm? And does arch linux support it?

Thanks in advance,
Aurimas

Last edited by thediamondgames (2016-04-21 07:52:11)

frostschutz · 2016-04-20 14:49:31

Well, I guess there is RAID-Z3, in ZFS...

But in general, no, there is no such thing in a classic RAID setup. RAID6 is closest, it allows any 2 drives to fail. And with only 6 disks, allowing 2 drives to fail is plenty. Personally I'm happy with a 7 disk RAID-5.

What you can always do is add spares, so RAID6 + 1 spare, the spare would do nothing until a drive fails, and the RAID would then immediately sync the spare in. That's not real 3 drive redundancy but as close as you get with traditional RAID.

However such spare setups are usually only useful in larger environments, for example if you have several RAIDs and all of them share one spare between them to offset the first failure (until the tech has time to replace the disk the next workday).

If you absolutely must have real three way redundancy, what you can do with 6 drives is create 2x RAID-1, having 3 drives each. That's real three way redundancy but it actually wastes 4 disks on "redundancy"...

If it was me, I'd go with RAID-5 (maybe RAID-6 if it's 8+ disks) aaand crank up the monitoring. Regardless of how much redundancy you have, it's completely useless if you don't detect disk errors early and replace disks immediately. So do SMART montoring, selftests, replace disk with the first reallocated/pending/uncorrectable sector, run RAID checks and check for mismatch_cnt == 0.

You should also consider: RAID is not a backup, if your data is important you must have backups. If you're going to have backups anyway, how much do you want to invest in the super rare case of having more than one drive fail at once? The probability that it ever pays off is very low. Worst case you'll have to spend a weekend restoring your backups.

frostschutz · 2016-04-20 14:59:28

Argh, never mind the RAID-1 idea, of course with a 3 disk RAID-1 only two drives may fail which is the same as RAID-6.
My mistake, to allow 3 drives to fail you'd need 4-disk RAID-1. 4 disks to get 1 disk worth of space. Not particularly useful.

Last edited by frostschutz (2016-04-20 15:00:16)

aiBo · 2016-04-21 06:40:12

A RAID-10 with 3 spans would allow for 3 disks to fail. It would have a write rate boost, due to the RAID-0 component and would provide usable capacity of three times the disk size. With mdadm this setup is possible.

thediamondgames · 2016-04-21 07:51:38

Thank you very much for your replies! Marking as solved.

WorMzy · 2016-04-21 09:08:01

Mod note: Moving to Kernel and Hardware.

Arch Linux

#1 2016-04-20 14:22:33

[Solved] RAID for 3 failing drives?

#2 2016-04-20 14:49:31

Re: [Solved] RAID for 3 failing drives?

#3 2016-04-20 14:59:28

Re: [Solved] RAID for 3 failing drives?

#4 2016-04-21 06:40:12

Re: [Solved] RAID for 3 failing drives?

#5 2016-04-21 07:51:38

Re: [Solved] RAID for 3 failing drives?

#6 2016-04-21 09:08:01

Re: [Solved] RAID for 3 failing drives?

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