Can you help me with disk layout?

[Zak95]

Dell R720xd, 12 x 4TB SAS drives, dual E5-2637vs, 256GB RAM, H710 mini IT mode, FreeNAS 11.3

This is a homelab system firstly, I do have 10gigabit networking available. I was thinking of creating 3 raid-z vdevs of 4 disks each, does this sound like the best setup for me? I plan to use iSCSI for the VMs and will have a few datasets to backup a QNAP NAS and hold VM backups with Veeam.

I have 2 x 2.5" bays in the back and can install SSDs and have a PCI-e NVMe (512GB) available too. I am not sure whether I should add a cache/SLOG/ZIL the more I read the more confused I get. If anyone does suggest a specific setup can you briefly explain why?

Thanks

 

[Heracles]

Hi Zak,

I recently answer similar question in this post. I suggest you start by reading it... A serie of Raid-Z1 vDev is not recommended for your case...

 

[Zak95]

So you would recommend 6 mirrored pairs in my case? Interesting I'll need to look into that further. Thanks.

 

[Heracles]

Indeed, with that many drives and for doing that many things at once, mirrors are the best.

Be sure to have your backups in place, just like you always need...

Have fun with your system,

 

[Zak95]

Thanks very much. Is this how you would expect it to look? How many failed disks would this tolerate? I am not sure if I've set up a striped mirror or a mirrored stripe, or possibly neither! Would this give me the equivalent performance of 6 disks combined on read and write?

 

[Heracles]

Hi again,

Yep, that is exactly what I was talking about. You have a series of many 2 drives mirrors.

The pool will always survive the first defective hard drive.
Next, out of the remaining 11 drives, you can survive the loss of 10 / 11 of them (any drive in any other mirror). So you have 91% of chance to survive the loss of a second drive.
For a third drive, you are at 8 / 11 (10 / 11 * 8 / 10 = 8 / 11), so 73%
For the fourth drive, probability is 10 / 11 * 8 / 10 * 6 / 9 = 49%.

So basically, you have about 50% chance to survive the loss of up to 4 drives. Technically, you can lose one drive per mirror, so with 6 mirrors, you can lose up to 6 drives. Probability is down to pretty low, but possible.

Because you are not 100% anymore right after the first failure, be sure to have proper backups and to keep some spare is also good so you can re-silver quickly.