Any reason to use RAIDZ2 over RAID10 with only 4 drives?

[jasn]

Hello All,

New to FreeNAS, with my first ixSystems Mini, (32GB RAM, 4x6TB WD Reds, home use), and after the requisite reading of the FreeNAS documentation, and relevant threads here, it seems to me that if you want to maximize space, and don't care about data redundancy, you could go with, no RAID or RAID0 (no protection), or RAIDz1 (a little protection).

However, for anyone else who wants a redundant configuration, isn't a RAID10 striped mirror set the optimal layout? With the same free space available, a 4 disk RAIDZ2 array wouldn't offer any advantage over the mirrored stripe set, would it? I like much about the Mini, (component quality, power efficiency, etc.), except the 4 drive maximum configuration. With 6 drives, I figure a RAIDZ2 layout would provide more free space over a RAID10 layout. Wouldn't it?

Jason

P.S. I figure to use the GUI in order to setup my RAID10 volume set, but if there's any advantage to setting it up via the command line, please free to share the commands. I am also planning on enabling encryption on the volume.

 

[Ericloewe]

RAIDZ2 is safer (any two disks can fail), striped mirrors are faster (two vdevs). With six drives, RAIDZ2 provides more space, is safer by some measures (and less so by others) and striped mirrors are significantly faster.

And for future reference: do not use the CLI to mess with pools

Things will break if you do. Stick to the GUI.

 

[Ericloewe]

Also: be aware that FreeNAS' encryption is designed to fail by rendering the data unreadable, not readable. Unless you have a very pressing reason to do full-drive encryption, don't. If you need to store a little bit of sensitive data, use an encrypted container.

 

[jasn]

Thanks Eric,

So in the case of striped mirrors, if one drive in one set fails, and any of the other drives in the other set fails, the volume is down, correct?

Having said that, replacing one failed drive in this striped mirror set won't affect the remaining drives in the same way as replacing one failed drive in a RAIDZ2 set, will it?

Thanks again,

Jason

(BTW, I will use the GUI as you instruct. The reason I asked was because of coming across this old thread while I was doing my research.)

 

[Ericloewe]

The volume is gone if you lose more than one drive per vdev, when using striped mirrors.

I recommend you read cyberjock's guide (link is in my sig), as that will answer pretty much all ZFS questions you may have right now.

 

[jasn]

Thanks. Actually one of the first things I read was Cyberjock's presentation. It's a great initial source of information, including a good description of basic terms, and it contains many good recommendations. However, it didn't answer the question I asked in my OP, (which is why I asked it), and that was, is there any reason to use RAIDZ2 over RAID10 striped mirrors, in a 4 drive (maximum) configuration?

You pointed out in your first reply that the one big advantage is that with the single RAIDZ2 4 drive vdev, any 2 drives can fail, without losing the zpool. With the RAID10 striped mirrors, if both drives in a one striped set fail, then one of the two vdevs in the zpool fails, and then the zpool is lost. That's a strong enough reason to recommend the single vdev RAIDZ2 configuration, over the RAID10 striped mirror configuration. It also seems logical that in the RAID10 configuration, if you lost one drive in one striped mirrored pair, and then you lost its mirror, both vdevs would die, and the zpool would be lost too.

The one big advantage of the RAID10 4 drive striped mirror configuration is documented in the FreeNAS documentation as primarily performance, which makes sense. However, it also seems to me that both the degraded and the resilvering mode during a single drive failure, would operate differently for these two zpool configurations. During the degraded and resilvering modes for the RAIDZ2 vdev there will be considerably more workload for all the remaining drives, than there would be in the RAID10 configuration. It seems logical to think that in the event of a single drive failure in the RAID10 configuration, there's slightly less chance to experience a second failed drive, (although certainly no guarantee).

FWIW, it was interesting for me to note that if you use the FreeNAS setup wizard, and accept the default storage choices, it always configures the RAID10 striped mirror configuration for the 4 drives.

Jason

 

[cyberjock]

Actually, if you have a given group of disks in a RAID array (ZFS or hardware RAID.. doesn't matter) your chances of disk failure after another disk just failed go *way* up. It's something like 400% for the next 4 hours.

Personally, I'd always go with RAIDZ2 with 4 disks. If you are so worried about the extra IOPs that you need 2 vdevs over one, you can do better things with L2ARC, lots of RAM, and/or an slog.

 

[jgreco]

The volume is gone if you lose more than one drive per vdev, when using striped mirrors.

For a two-drive mirror, yes. Not for a three-way mirror.