Volume vs dataset used storage

[rdybro]

Hi.

I got a question about volumes and datasets on my newly installed FreeNAS box.

I created a volume called "volume01". Then FreeNAS auto-created a dataset underneath this also called "volume01".

All my data is placed in datasets created under the dataset "volume01".

Now, if I add all the used storage for my sub-datasets it adds up to approximately 2,6 TiB, which is exactly the same as the dataset "volume01" states is used on the system. So far so good.

The volume "volume01" though, states that 4,4 TiB is used, even though it only contains the dataset occupying 2,6 TiB. Can anyone explain to me why it is doing like this?

And another question: The volume "volume01" states that there is 9,2 TiB available storage, which is correct (5x 3 TiB disks in RaidZ2), but the dataset "volume01" have only 5,2 TiB available storage, why is that? Is it limited to 5,2 TiB then? What about the remaining 4 TiB then?

Thanks in advance for your answers. Regards

 

[danb35]

I thought this was in the manual somewhere, but I can't find it with a quick scan. So here goes...

Every ZFS pool contains a dataset with the same name as the pool. I call it the root dataset, though there may be a more appropriate term. Prior to version 9.3, FreeNAS didn't show that dataset on the storage screen. It was still there, but just not shown. Beginning with 9.3, it does. So, why the apparent discrepancy in capacities?

The first entry is for the pool itself. FreeNAS shows the total space used and remaining ("Available" means remaining space, not total space) on all your disks, including those used for parity. You have 5 x 3 TB disks, with a total capacity of 15 TB, or ~ 13.5 TiB. Your volume shows 4.4 TiB used + 9.2 TiB available = 13.6 TiB. This is as expected.

The second entry is for the dataset. This entry shows your "net" capacity--the capacity after parity is accounted for. RAIDZ2 uses two disks' worth of parity, so you'd expect your net capacity to be roughly 3 x 3 TB = 9 TB or 8.1 TiB, minus filesystem overhead, reserved space, etc. Your dataset shows 2.6 TiB used + 5.2 TiB available = 7.8 TiB total. This, again, is as expected.

 

[rdybro]

I thought this was in the manual somewhere, but I can't find it with a quick scan. So here goes...

Every ZFS pool contains a dataset with the same name as the pool. I call it the root dataset, though there may be a more appropriate term. Prior to version 9.3, FreeNAS didn't show that dataset on the storage screen. It was still there, but just not shown. Beginning with 9.3, it does. So, why the apparent discrepancy in capacities?

The first entry is for the pool itself. FreeNAS shows the total space used and remaining ("Available" means remaining space, not total space) on all your disks, including those used for parity. You have 5 x 3 TB disks, with a total capacity of 15 TB, or ~ 13.5 TiB. Your volume shows 4.4 TiB used + 9.2 TiB available = 13.6 TiB. This is as expected.

The second entry is for the dataset. This entry shows your "net" capacity--the capacity after parity is accounted for. RAIDZ2 uses two disks' worth of parity, so you'd expect your net capacity to be roughly 3 x 3 TB = 9 TB or 8.1 TiB, minus filesystem overhead, reserved space, etc. Your dataset shows 2.6 TiB used + 5.2 TiB available = 7.8 TiB total. This, again, is as expected.

Makes perfect sense, thanks a lot for your thorough explanation, it helped a lot

I looked through the documentation myself prior to posting, and couldn't find it either. Though as you can tell I was a little off in my assumptions, which possibly could have made me look for some other things.

Thank you! :)