it can be very useful to have both the failing disk and the replacement disk in the system during resilvering. In general resilvering is a time-consuming and stressful activity.
I read that you can lose another disk during this stressful process.
So lets say we have RAID2z - we can lose 2 disks. But it takes a min. of 4 disks.
We just lost one, or its nearly dead. So we can afford to lose 1 more.
We start the resilver process, and now 1 more dies.
Now we're down to nothing.
Or, what if the admin makes a mistake, we're down to nothing.
You almost need a RAIDz3 for peace of mind. So now I'm talking 5 disks, minimum.
Unless you go with mirrors. They are much easier (less stressful on the system) to resilver.
With a 4-way mirror, you can lose 3 disks.
So far I've gathered the following:
(somehow) better for data intregrity (not sure how / or if true)
harder on the system to resilver
main benefit is space, while mirror is performance (at the expense of space)
lower performance that mirrors
higher system resource use than mirrors
cannot add to an existing RAIDz.
easier on the system to resilver
main benefit is performace, while RAIDz is space (at the expense of performance.)
higher performance than RAIDz
lower system resource use than RAIDz
*can* add to an existing mirror.
*can* remove drives from an existing mirror.
*can* expand space in an existing mirror.
Overall more flexible when expanding a system.
And another one - if I lose everything but 1 drive, the whole system is on that 1 drive, which I assume I can access as 1 drive (?).
I think RAIDZx has a lot more of the "cool" factor.
But plain old mirrors may have a lot to offer for the small business type of server.
I can do a LOT of business inside 2TB worth of space.
my 2cents worth...