RAIDZ2 drive count for >10TB usable storage.

[Atreiess]

I'm putting together a FreeNAS server and before purchasing drives I have one question please :

Would 5 x 4tb or 6 x 3tb drives be a better configuration for RAIZ2 with >10Tb of usable storage. Both will achieve my capacity amount and I would prefer to use less drives but if 6 drives has a notable benefit I can build around that goal.

Thanks.

 

[Evertb1]

I'm putting together a Freenas server and before purchasing drives I have one question please :

Would 5 x 4tb or 6 x 3tb drives be a better configuration for RAIZ2 with >10Tb of usable storage. Both will achieve my capacity amount and I would prefer to use less drives but if 6 drives has a notable benefit I can build around that goal.

Thanks.

If you aim for 10 TB practical usable storage: you won't get that. 5 x 4 TB will give you about 8.2 TB, 6 x 3 will give you about 8.3 TB. You need to take 20% free space limit in account to let ZFS function efficiently. 6 disks are a bit better (about 50 %) then 5 disks (about 45 %) if it comes to the percentage of available space from the raw space. A final note: an even number of disks (multiples of 2) are supposed to be a better configuration for RAID-Z2. To be honest: I run 5 disks and have no problems with the performance. Of course the price of the disks are an important part of the equation.

Afterburner: In my part of the world at my favorite shop 6 x 3 TB (WD RED) would cost me 567 euro (about 697 US $). 5 x 4 TB (WD Red) would cost me 624 euro (about 767 US $). Not hard to figure out what I would do. I gues you would pay less in the US.

 

[Atreiess]

Just to clarify the 20% you are talking about would still show as free space but as you consumed that last 20% of free space it would hamper the system? As opposed to parity and formatting space that does not show as usable at all. Or is the calculator at http://wintelguy.com/zfs-calc.pl not accurate unless the last box is checked for free space limit to incorporate what you stated?

 

[Evertb1]

Just to clarify the 20% you are talking about would still show as free space but as you consumed that last 20% of free space it would hamper the system? As opposed to parity and formatting space that does not show as usable at all. Or is the calculator at http://wintelguy.com/zfs-calc.pl not accurate unless the last box is checked for free space limit to incorporate what you stated?

Yes that 20% will show as free space but still you need to take that 20 % free space limit in account. ZFS will not stop functioning but the bets are off for the perfomance. That calculator of wintelguy is the one I use myself. And yes I check that 20% box. Otherwise you see that the ZFS usable storage capacity equals the practical usable storage capacity. And that would be a bit of fooling your self.

 

[Atreiess]

Gotcha, thank you very much for the help.

 

[Arwen]

Note that you can even go down to 4 x 12TB disks in a RAID-Z2. This is better than a Mirror configuration, in regards to reliability, (but not IOPS). You can loose any 2 disks and not loose data. (In 2, 2 way Mirror configuration, lose both disks in a Mirror, and poof, your entire pool is gone.)

Of course, 12TB disks might not get your storage goals.. just giving an alternate example.

 

[Evertb1]

Note that you can even go down to 4 x 12TB disks in a RAID-Z2. This is better than a Mirror configuration, in regards to reliability, (but not IOPS). You can loose any 2 disks and not loose data. (In 2, 2 way Mirror configuration, lose both disks in a Mirror, and poof, your entire pool is gone.)

Of course, 12TB disks might not get your storage goals.. just giving an alternate example.

I never liked the economy of a 4 drive Raid-Z2 pool. You utilize around 37-38 % of your raw storage space. The reservation for parity and padding kills you. With 6 drives you are doing a lot better. Financialy as well. Take your example of 12 TB drives. Four 12 TB Iron Wolfs would cost me 1596,00 euro, while six 6 TB Iron Wolfs would cost me 1049,40 euro (prices of today). The 6 x 6 TB would deliver a slightly better practical usable storage as well (16.65 TB against 16.21 TB).

 

[Arwen]

I never liked the economy of a 4 drive Raid-Z2 pool. You utilize around 37-38 % of your raw storage space. The reservation for parity and padding kills you. With 6 drives you are doing a lot better. Financialy as well. Take your example of 12 TB drives. Four 12 TB Iron Wolfs would cost me 1596,00 euro, while six 6 TB Iron Wolfs would cost me 1049,40 euro (prices of today). The 6 x 6 TB would deliver a slightly better practical usable storage as well (16.65 TB against 16.21 TB).

Yes. Just reminding people that there are valid options that can work under some conditions.

For example say you build a 6 disk RAID-Z2 on system with 10 SATA ports. Later you want to add more disks in another vDev. Without buying a HBA or new system board, that's only 4 new disks. And to maintain reliability, you would have to use RAID-Z2 for those 4 disks.

But, yes, less cost effective.