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I am going to purchase a HP Microserver gen8 system for a home fileserver and it contains 4 drive bays.

I will use those four drive bays to create a single zpool for mass storage.

My goal is the ability to survive any two drive failures from that four drive pool.

I know I can do this with a four drive raidz2 pool - that would allow me to lose any 2 of the 4 drives and still be healthy.

But it seems like a lot of overhead and resilver stress to run a raidz2 pool when I don't really need it ...

So, is there some other pool configuration that will fulfill that "any 2 out of 4" requirement ? Two mirrors joined as a single zpool does NOT fulfill it, since I could lose two drives in the same mirror and lose the entire pool.

Thank you.

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  • Is RAID 6 an option? Like RAID 5, but with two parity drives. – Xalorous Aug 30 '16 at 17:22
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    ZFS RAIDZ2 is practically the same as RAID 6. – Tero Kilkanen Aug 30 '16 at 17:27
  • I was recently debating between raidz2 and mirrors. This article swayed me to mirrors because of the faster time to rebuild (shortening your window for a second error trashing your pool). Something to think about. – Dominic P Aug 30 '16 at 17:38
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My goal is the ability to survive any two drive failures from that four drive pool.

RAIDZ2 is the only choice you have in this scenario.

I know I can do this with a four drive raidz2 pool - that would allow me to lose any 2 of the 4 drives and still be healthy.

Indeed.

But it seems like a lot of overhead and resilver stress to run a raidz2 pool when I don't really need it

When using decent hardware the overhead and stress should be negligible. Sure, resilvering RAIDZ2 takes longer than resilvering a mirror. If you're really concerned about that consider using faster drives (e.g. flash storage).

When you decided you need to survive “any 2 out of 4 drive failures” you need RAIDZ2.

So, is there some other pool configuration that will fulfill that "any 2 out of 4" requirement ?

No, RAIDZ2 is the only choice with four drives.

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In addition to the already mentioned raidz2 configuration, you can configure three disks as triple mirroring and either keep the fourth as a spare disk, or use it for non critical, temporary files.

Of course, that is not going the best choice as far as capacity is concerned, but you'll have the fastest solution.

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Is RAID 6 an option? Like RAID 5, but with two parity drives. You're going to lose half your storage space with only four drives. RAID 1 survives 1 disk failure, but costs 50%l. RAID 5 survives 1 disk failure and costs 25%. The stress of rebuilding a RAID member can cause other surviving disks to fail (They have the same use hours on them). If you can't afford a storage cluster or whatever, pick between mirrored and RAID 5, and backup regularly. When a drive fails, RAID keeps you running, ASAP (hours not days) get a backup then replace all of the drives in the array and restore from tape.

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  • He's using ZFS, so there is no traditional RAID. And he already considered RAIDZ2 which comes closest to RAID6. So the answer to your question is “No, RAID 6 is not an option”. – Marco Aug 30 '16 at 18:30
  • Software raid on top of jbod, got it. Good backups are always a good idea. – Xalorous Aug 30 '16 at 18:33
  • Actually it's a fault-tolerant journaling filesystem on top of a bunch of disks. How does zfs handle disk failure? – Xalorous Aug 30 '16 at 18:43

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