Creating my pool, best practice?

[lukeren]

I have 6 x 3TB WD Red disks which I've been using for some time. I just recently got hold of 2 x 3TB SAS disks and now I'm wondering what would be the best approach.
The WD disks are currently running a RAID 5 on an Adaptec controller. I've copied the data off and I'm ready to pull the controller and get it all running in my new server (with an IT flashed controller).
Up to now I've not considered mirrors, because I had too much data to fit on a mirrored setup of my current drives. The addition of the SAS disks changes that though.

Should I create 4 mirrored vdevs? This would make it easy to upgrade to larger disks, since (as I've understood) I can just upgrade 1 vdev (2 disks).
Is it better performance to create a single vdev?

 

[m0nkey_]

Take a look at @cyberjock 's ZFS primer. It might help you decide.
https://forums.freenas.org/index.ph...ning-vdev-zpool-zil-and-l2arc-for-noobs.7775/

 

[lukeren]

It's an excellent write up, but he focuses mostly on raidz. There isn't much about mirrors and how to best go about them.
If I want to upgrade disks in the future, I'm guessing I want to go with small vdevs of 2 disks. I would just like to know if there's anything gained in using larger vdevs, say 4 or even 8 disks.

 

[Ericloewe]

8-way mirrors? That's the most insane idea I've heard in a while. Who in their right mind would throw away 7/8 of the capacity before even accounting for free space requirements?

 

[SweetAndLow]

You want to setup 4 mirror vdevs striped together. Smaller vdevs provide faster rebuild times, more performance in the form of iops and the ability to expand a vdev by only replacing 2 disks.

Larger vdevs for mirrors will give you a little more read performance and more redundency. You would never go over a 3 deep mirror.

Sent from my Nexus 5X using Tapatalk

 

[danb35]

Who in their right mind would throw away 7/8 of the capacity

Someone who needed an awful damn lot of redundancy. Should be great for read performance though (but it'd suck for writes). But I expect he's really talking about striped mirrors.

 

[danb35]

I would just like to know if there's anything gained in using larger vdevs, say 4 or even 8 disks.

Size of the vdev, within reason, doesn't have much impact on performance, but number of vdevs does--the more vdevs you have, roughly speaking, the more IOPS you can handle. Two four-disk RAIDZ2 vdevs would give you the same capacity as eight disks in striped mirrors (i.e., four two-disk mirrors striped together), somewhat lower performance, but better redundancy. A single eight-disk RAIDZ2 vdev would have lower performance yet, higher capacity, potentially worse redundancy.

 

[Linkman]

. . . Two four-disk RAIDZ2 vdevs would give you the same capacity as eight disks in striped mirrors (i.e., four two-disk mirrors striped together), somewhat lower performance, but better redundancy. . . .

This is actually what I did, two four-disc RAIDZ2 vdevs, in my FreeNAS server. I considered it a compromise between four mirror pairs and eight disc RAIDZ? for my available discs and space in the case.

 

[Robert Trevellyan]

The thing about performance and number and type of vdev is that there are different types of performance and they respond differently. In particular, IOPS is pretty straightforward, i.e. more vdevs = more IOPS. Streaming read performance and streaming write performance aren't quite so simple. That's why I always defer to the OpenZFS performance tuning page: http://open-zfs.org/wiki/Performance_tuning

 

[Robert Trevellyan]

Should I create 4 mirrored vdevs? ... Is it better performance to create a single vdev?

No, and no, i.e. it depends on you and your expectations and requirements. Unless you need to maximize IOPS, the primary benefit of striped mirrors is flexibility, which you identify in terms of the ease of upgrading.

 

[Stux]

FWIW, unless you needed lots of IOPS or the expansion flexibility, I would use 8-way Raidz2.