FreeNAS 101 - DON'T use hardware RAID???

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Ed Flecko

Sep 19, 2011
Hi folks,
Do I understand something correctly - if you plan on installing FreeNAS and, effectively, making a bunch of disks into a DIY SAN, you should NOT use a hardware RAID card?...all you need to basically do is get the host machine to SEE all of the drives (which is why cheap SAS cards are so handy), and ZFS will handle the RAID that right?

Also, does it matter if all of the disks are the same size? What happens if they're NOT all the same size?

Thank you!


William Grzybowski

May 27, 2011
Yes, that is right.

If the drives are not the same size, the final available size in the pool will be like all the disks were the size if the smaller


Jul 27, 2011
More specifically...

-Using a hardware RAID card isn't NECESSARILY a bad thing; I've had my share of bad luck with low-quality SATA controllers, so if you do have a good hardware RAID card, that's not intrinsically a bad thing. What *is* a bad thing, however, is setting up a RAID array on the controller instead of letting ZFS do the RAID. As long as the add-in controller is set to "JBOD" (Just a bunch of disks) mode, you're fine. What ZFS needs is to have each physical volume show up as a logical drive; you'll find yourself in trouble if you have a spanned/RAID volume.

-ZFS can technically make a RAID array out of different sized disks, but your array will only be as large as the smallest disk times the number of disks in the array. For example, say you've got 6 hard drives. One is a 250GB, three are 500GB, and 2 are 1TB. You can have a RAIDz array using these disks, but your total capacity will only be 1.25TBytes if you put them all into the same 'pool'. This number is achieved because the smallest drive in the lot is 250GB, and as such it will only use the first 250GB of each of the remaining drives, less one drives' worth of capacity for parity data. If that's alright with you, then by all means go that route. An alternative is to make multiple pools for each capacity group, or do a little cocktail napkin math to see where the ratio works out best. In the given example, ditching the 250GB will yield 2.0TBytes of space (500GBx5 drives - parity data).

Hope it helps!

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