Mixed Drive Mirror Pool

[Wasse]

I've been searching for hours, and haven't found an answer to this, so I thought I'd ask before I spend $500 on drives and not have it work correctly :D.

If I have 4x 5TB drives, and 2x 2TB drives, will I be able to make a mirror pool of 12GB (12GB usable, and 12GB mirror)? I'm not looking to make two separate pools, just a single mirror pool with those mixed drives. Or is a single mirror pool only an option if all the drives are the same?

PS - I may be using the wrong term when referring to it as a Pool or Volume. I'm still semi-new, so forgive me :)

 

[Arwen]

ZFS has Pools to refer to a collection of disks. Then each Pool has one or more VDEVs,
which are a specific type, Mirror, RAID-Z1, RAID-Z2, etc... FYI, mixing VDEV types in
the same Pool, like Mirror and RAID-Zx, while "legal" are highly NOT recommended.

Yes, your proposed Pool setup will work;

2 x 5TB in a mirrored VDEV
2 x 5TB in a mirrored VDEV
2 x 2TB in a mirrored VDEV

The actual amount of usable space available will be less that 5+5+2=12GigaByte. That's
due to the 1000 verses 1024 thingy, ZFS overhead and you want to run your Pool at 80%
or less. (Unless you are using iSCSI, then 50% or less.)

That said, you may experience irregular performance for both reading and writing. At
a certain point when the 2TB VDEV is full, writes will only be striped across the remaining
2 x 5TB VDEVs. Thus, many people here in the forums recommend having same or very
similar sized VDEVs, (both disk size and number of disks).

My thought is that for a home user that is willing to accept the irregular performance,
AND not complain, then you can try it. You can always change it later, (maybe not easily).
Complaints from known original server design flaws, (like this), tend to make people
annoyed around here.

 

[jgreco]

Yeah, but if you create the pool like that initially, the vdevs should fill in a balanced manner, so that's not really that big a deal. You'll see the two 5TB vdevs getting hit harder though.

 

[Wasse]

Ahhh, gotcha :). Thanks for the information guys! I'll have to do some more research into it, but it's looking like I should stick with same size drives and rethink my strategy. You guys defiantly saved me a big headache, thank you again :).

 

[Robert Trevellyan]

I have 4x 5TB drives, and 2x 2TB drives

One option would be a 6-drive RAIDZ2 pool, which would initially have the capacity of a 6-drive RAIDZ2 pool of 2TB drives (roughly 8TB usable). However, when you're ready, you could replace each 2TB drive with a 5TB drive, and your pool capacity would grow to benefit from the full capacity of all the 5TB drives (roughly 20TB usable).

Another option would be a 4-drive RAIDZ2 pool of 5TB drives, ignoring the 2TB drives (roughly 10TB usable).

Note that whatever your pool layout, it's best not to fill it past 80%.

 

[jgreco]

Those are both good suggestions, and the RAIDZ2 gives you better redundancy characteristics than the mirroring solution.