Some questions about vdevs and pools before installing freenas for the first time

[Mr. Bigglesworth]

Hello

I've been lurking in the forums for a while in order to figure out if Freenas is right for me.
I originally thought that Unraid would suit my needs better, but examining the different workings of Unraid vs Freenas I'm not so sure anymore
I think I've gotten most of my questions answered but there are some things I'm still a bit unsure about:

Reading Cyberjocks slideshow here made me understand the following:
- Once I create a vdev with any number of disks I *cannot* add disks to that vdev at a later time.

So, this means if I want ot maximize the available space in a pool, I should just fill my controller to capacity (In the case of M1o15 that would be 8 disks) add all 8 disks to 1 vdev and set it for RAIDZ2 or RAIDZ3 depending on how low I want the risk of a complete vdev failure.

I currently have 3 x 4TB WD RED with 1 disk for parity.

Another way of doing the above could be to buy 1 extra disk now, create 1 RAIDZ2 with the 4 disks. add it to a pool and perhaps at a later time add 4 more disks in a RAIDz2 vdev

I've also seen that you can have alot of 2 disk mirrored vdevs in 1 pool.

Technically this would protect me from 1 disk failure in each vdev, the rebuild time should be faster, but it just seems that a 4 disk RAIDZ2 vdev is better protected.
However, 2 disk vdevs seem more flexible when it comes to incrementally upgrading your storage capacity.


My question is: What is the best scenario for home use? 2 RAIDZ2 vdevs with 4 disks in each or 4 mirrored vdevs with 2 disks in each?

Also, what do you guys use for backups? I've been looking at an online solution like crashplan

 

[Mirfster]

What is the best scenario for home use? 2 RAIDZ2 vdevs with 4 disks in each or 4 mirrored vdevs with 2 disks in each?

For me personally RaidZ2 is the best balance of Speed, Redundancy and Space. I use it both in home and business use. Of course if I were looking at a use-case for something more demanding (iSCSI); I would consider other options.

Edit: You could even consider RaidZ3 so only 3 disks would be allocated for parity as opposed to 4 when using two x 4 RaidZ2 vdevs.

Edit2: Meant to say RaidZ3 vdev of all 8 disks...

 

[gpsguy]

You *could* also do 3-way mirrors. It gives you all the benefits of traditional mirrors and still allow for 2 disk failures in any given vdev.

With the equivalent of two disks devoted to parity, it does comes at a cost.

However, 2 disk vdevs seem more flexible when it comes to incrementally upgrading your storage capacity.

 

[Robert Trevellyan]

Great to see a new member completing their homework :)

Technically this would protect me from 1 disk failure in each vdev, the rebuild time should be faster, but it just seems that a 4 disk RAIDZ2 vdev is better protected.
However, 2 disk vdevs seem more flexible when it comes to incrementally upgrading your storage capacity.

Exactly. Only you can decide how to trade reliability against flexibility.

 

[Mr. Bigglesworth]

You *could* also do 3-way mirrors. It gives you all the benefits of traditional mirrors and still allow for 2 disk failures in any given vdev.

With the equivalent of two disks devoted to parity, it does comes at a cost.

Could you explain the 3-way-mirror to me? I've been trying to google it, but I keep getting directed to microsoft storage spaces that support it with 5 disks, but I have a hard time understanding how a 5 disk 3-way mirror works compared to RAIDZ2 with 5 disks for example.

I've bought an extra disk, and I'll most likely end up doing 4-disk RAIDZ2
At a later time I'll decide if I'll buy 4 additional disks and create a vdev, or if I should perhaps break up the old vdev and create a new 8 disk RAIDZ3 vdev, but I'll cross that bridge when I get to it.

 

[gpsguy]

Disregard what Microsoft is saying ... with FreeNAS, you only need 3 disks for a 3-way mirror.

Imagine a traditional mirror with an extra disk of redundancy. Like RAIDz2, with a 3-way mirror you could lose 2 disks and your vdev would still be intact with the single remaining disk. But, it comes at great cost, since for every disk of storage, one has two additional disks devoted to parity.

For example, with 4 x 4TB disks in RAIDz2, you have ~8TB of available space. If I needed that same ~8TB of space with 3-way mirrors, I'd need 6 x 4TB disks. One of the sweet spots with RAIDz2 is 6 disks. In the example above, you'd have ~16TB of space. With 3-way mirrors, I'd need 12 x 4TB disks.

For your use case, RAIDz2 is fine.

 

[Mr. Bigglesworth]

Disregard what Microsoft is saying ... with FreeNAS, you only need 3 disks for a 3-way mirror.

Imagine a traditional mirror with an extra disk of redundancy. Like RAIDz2, with a 3-way mirror you could lose 2 disks and your vdev would still be intact with the single remaining disk. But, it comes at great cost, since for every disk of storage, one has two additional disks devoted to parity.

For example, with 4 x 4TB disks in RAIDz2, you have ~8TB of available space. If I needed that same ~8TB of space with 3-way mirrors, I'd need 6 x 4TB disks. One of the sweet spots with RAIDz2 is 6 disks. In the example above, you'd have ~16TB of space. With 3-way mirrors, I'd need 12 x 4TB disks.

For your use case, RAIDz2 is fine.

Thanks, makes sense then - it is similar to what I believed it was, but the Microsoft definition had me somewhat confused.