Help understanding available storage

[Huib]

Hi Guys,

I'm a bit 3 confused and I hope you guys can explain something to me.

I have a mirrored vdev of two 3 TB drives and a vdev of 3 drives in raidZ 1 also each 3 TB

It's my understanding that the mirrored vdev should have 3TB(about) available as the second disk is used for redundancy when empty
It is also my understanding that the raidz1 vdev should have about 6TB available since the 3rd drive is used for redundancy.

What I don't understand is the following:
On the mirror freenas reports about a TB of used space and about 1.7 of free space. Close enough for a total of 3 TB when empty.
On the raidz1 freenas reports about 2 TB used and 6.17 of free space. This is misleading in my opinion since due to redundancy it should be 4 TB of usable free space.

At least is is inconsistent. Why is the usable space reported for the mirror and the physical space reported for raidz1?
Is there a method behind this madness? Or am I missing something obvious?

I'm using freenas 11.2B2 and this is the case in both the new and the legacy UI

Looking forward to getting schooled!

 

[garm]

This is explained in the manual

 

[Huib]

Thanks for that but I still think it's inconsistent.

According to the manual the dataset is presented twice. The first time showing the free space with free parity space and the second time showing the implicit available storage.

looking at my pools I see this:

Pool D is a mirror. If the first free space report is the free space with parity, this should be more in the lines of 3*2 drives = 6TB, minus the 1TB written twice leaving 4 TB of free space including the parity. However it is verry close to the 1.6 TB reported for the actual usable space of the pool so the statement in the manual apears incorrect to me.

Pool W is a raidz1 pool with 3 3TB dirves.
Here you can see that the first listing of the pool shows 6.17 TB and that's obviously including parity.
The actual available space is reported as almost 4 Tb and that makes sence with the used space.
This is in line with the manual

So I still don't understand why a mirrored pool is represented differently as a raidz1 pool.

What am I missing here?

 

[Ericloewe]

You're missing that disks are sold in powers of 10 and OSes report space in powers of two.

3 TB disks: 3*10^12 bytes = 3 000 000 000 000 bytes
3 TB (TiB, formally): 3 * 2^40 bytes = 3 298 534 883 328

You'll notice that the value as reported by the OS is approximately 0.909 times the manufacturer's quoted number.

 

[Huib]

Hi Eric Loewe,
I'm aware of that but it doesn't compute. 0.909 does not create a difference of 2 TiB on a 2 x 3 TB drive

My question is:
Why is the free space on my mirror (D) reported without the parity (unusable) space and why is the raidz1 (w) reported with the parity space. All my drives are 3 TB.

If you check the numbers in my screenshot you will see it's inconsistent.

 

[garm]

Mirrors don’t have parity, they just write everything to both disks. The space shown for mirror vdevs are different then for RAIDZ vdevs. For RAIDZ pools you will lose storage to parity, but in mirrors it’s only metadata that cuts available storage from the vdevs.

 

[Huib]

OK! That makes sense...

I expected the mirror to report the sum of the unused space on both disks but as it isn't strictly parity space I can understand how this works differently.

Thanks a bunch for that insight!