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iSCSI Performance & File System Parameters (Record/Block size etc).


Mar 21, 2013
Would you say that advice still rings true (or less so?) on multi-vdev raidz arrays? I was hoping to somewhat mitigate the effects of fragmentation with the much higher random I/O per device than traditional HDD's over a 4 or 5 wide vdev pool.


Resident Grinch
May 29, 2011
So you figure if you were to use, say, a 5 SSD RAIDZ1 vdev instead of a 2 SSD mirror, that you'd mitigate fragmentation because you end up with 4x the storage space? There could be some legitimacy to that - ifff you're not tempted into using the extra space. But the keen observer might also note that you're using 150% more SSD's to accomplish that. Plus I don't think it'll pan out as well as you'd like, since there are various issues with space allocation of small blocks with RAIDZn that you don't get with mirroring. If it were me, and I had a choice between configuring ten 500GB SSD's as two vdevs of 5 drive RAIDZ1 (4TB) or five vdevs of 2 drive mirrors (2.5TB) I think I'd be tempted to go for the 2 drive mirrors and fill it to a higher percentage, because my suspicion is I'd still get some speed benefits out of avoiding RAIDZ and having more vdevs.

That might not hold true for non-block-access pools, at least not as much, but I'd suggest being prepared to do some serious testing prior to making any decisions...


May 30, 2019
I am about to make a new iSCSI volume on a frest FreeNAS. The volume is going to be used by a Hyper-V 3 node cluster.
I have 8x1TB SSD disks.

What is the best use of the disks:
4 mirrors with a total off about 4TB. And can those be presentet to the Hyper-V cluster as one volume?

Or a Raidz1 off all eight disks giving me about 6TB of data.

I need at least 4TB.


Feb 2, 2018
Hey JanJunker,

RaidZ1 is to ba avoided for everything because it is not safe enough. A single lost drive will put the entire pool in jeopardy when it will be time to re-silver it.

RaidZ-x is to be avoided for iSCSI because it basically offers the IOPS of a single drive.

As such, it is clearly way better to go with Raid-10, so here 4 mirrors.