Freenas RAID-Z Parity drives

[virtuo]

I picked up a couple of those HP n54L Microservers while the cashback offer was on and was thinking about stuffing one full of RAM and HDDs and running it as a freeNAS appliance.

I think technically the servers can run 6 disks, but only have internal space for 5 (4 cold swaps and the ODD bay), so my plan was to stick 5x WD Red 3TBs in there.


I have read a little about RAID-Z and think that this is the way I want to go. The problem is I'm not totally sure how much resilience/storage I will have available in the various RAID-Z configurations, mainly due to not being sure how the parity drives work.


Using the Freenas hardware recoomendations wiki (http://doc.freenas.org/index.php/Har...ecommendations), this is what I've arrived at - I think my Googling is hampered by the number 5 as most results are about "RAID5"..

Assuming 3TB drives (and ignoring the "actual" capacity being lower for simplicity)

RAID-Z1 - 4x data drives + 1 parity drive = 12TB storage and resilience to 1 disk failing.

RAID-Z2 - 2x data drives + 2 parity drives = 6TB storage and resilience to 2 disks failing.

RAID-Z2 - 4x data drives + 2 parity drives = 12TB storage and resilience to 2 disks failing. The docs say RAID-Z2 needs 4, 6 or 10 drives, so going with 6 which is going to be tricky in the n54l.


Am I right in assuming that for RAID-Z1 there is only ever 1 parity drive, and RAID-Z2 there are 2? Or does the number increase as more data drives are added?

So, does everything seem about right, or have I got a bit mixed up in my assumptions?

[peterb]

There is an article about RAID Z2 here

http://dtrace.org/blogs/ahl/2006/06/...parity-raid-z/

I haven't used zfs filesystems, but it appears that you get double parity through software and some of the fundamental features of the filesystem, without loss of storage space.

As with most RAID systems (above RAID 1) the parity data is itself striped across the array.