I have 3x4TB NVMe disks in a btrfs cache pool. In the Unraid GUI, it shows the size to be 6TB, which is what I expect. It shows the used space as 712GB (the amount of data I copied to the cache pool). But the free space shows as only 3.53TB, which is not what I'd expect. This seems like it might be the amount of free space on a single drive in the pool, but I'd expect it to show the total free space as ~5.29TB. Indeed, 3.53 x 3 / 2 ~= 5.29.

Edited December 27, 20223 yr by Ancalagon

It's a known issue with btrfs and an odd number of devices, the free space will get closer to correct as the pool gets filled up.

It seems like this UI would be more intuitive if it mirrored the arrays UI, where next to each drive it showed the size, used, and free space on each drive and at the bottom, the total size, used, and free space.

11 minutes ago, Ancalagon said:

next to each drive it showed the size, used, and free space on each drive

Those concepts don't apply to individual drives in a pool

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Those concepts don't apply to individual drives in a pool

Not to the same extent, in that the usage should be roughly equal between all the drives in a btrfs cache pool and if it's a RAID1 mirror, the total pool size and usage will be half of the total individual drives' usage. But there's still the concept of individual drive usage vs total pool usage, just not as relevant to see though.

 

Is this bug expected to be fixed in the next Unraid release?

 

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the free space will get closer to correct as the pool gets filled up.

This is true, in that the individual drive free space will converge to 0 at the same speed as the total free space converges to 0. The values will get closer, but never truly be accurate until the pool is completely full.

34 minutes ago, Ancalagon said:

Is this bug expected to be fixed in the next Unraid release?

This is a btrfs bug, only they can fix it, when it's fixed in the kernel it will also be fixed with Unraid.