RAID 0 with different volumes

[alacaza]

Hello everybody, I have a virtual machine with FreeNAS 11 installed on Virtual Box. I created that for doing experiments and learning to manage this OS.
As far as I know, if we build a RAID 0 with drives with different sizes, RAID would only take, from every drive, the equivalent storage to the size of the smallest drive.
However, I made some experiments and I achieved:
- Create a RAID 0 with two different volumes. The storage of the RAID 0 is the total sum of the two drives.

- Expand a RAID 0 originally composed by two drives and add up the total storage of a third drive.

How is that possible?

 

[danb35]

Create a RAID 0 with two different volumes.

You didn't create a RAID0. FreeNAS doesn't do RAID 0. FreeNAS does striped volumes. What you're seeing is exactly what's expected.

 

[alacaza]

Ok. I get it. FreeNAS called them striped volumes, but...they are practically the same, aren't they? I mean, the result is quite similar, even better because we don't have any limitation when we want to expand our striped volume.

 

[danb35]

they are practically the same, aren't they?

I guess that depends on what you mean by "practically the same." If you mean that you have a group of disks together with no redundancy, and that data is spread across them, yes. But ZFS dynamic striping works quite differently from RAID0.

 

[alacaza]

Actually, when I've googled about this topic, I found that there are many sites where the two terms are used interchangeably. So, please, could you recommend me some references about the difference between ZFS dynamic striping and RAID0?
Thanks in advance.

 

[alacaza]

Sorry, but I've seen this just now in the FreeNAS User Guide.

So, I've two questions:
- Is usable the whole storage of the pool I've been creating in the earlier images? Or just the size of the smallest disk x 2 disks?
- What happens with the rest of the storage of the bigger disk that is not usable?

 

[danb35]

That note is true of RAIDZn volumes, not striped volumes.

 

[Chris Moore]

So, I've two questions:
- Is usable the whole storage of the pool I've been creating in the earlier images? Or just the size of the smallest disk x 2 disks?
- What happens with the rest of the storage of the bigger disk that is not usable?

If you are looking for a solution that has no redundancy.
I just want to ensure you understand that.
The data will be striped across the available disks, but if any disk in the set fails, you loose access to all the data.

 

[alacaza]

Ok, I think I've understood. Manual setup on FreeNAS allows you to stripe your data across the available disks and use the whole storage of the different disks that compose the ZFS pool for that. But, like a RAID 0 system, you don't have any kind of fault tolerance on that layout.

 

[gpsguy]

Novice FreeNAS users should not use the manual setup button. Far too many have messed up their pool layout, by using the feature.

For example, one can use it to stripe a single disk to a RAIDZ2 pool. Instead of being able to withstand the loss of two disks, should that new single disk fail, they lose everything.

 

[danb35]

For example, one can use it to stripe a single disk to a RAIDZ2 pool.

...not that we've ever seen that...