TrueNAS Core, estimated raw capacity vs final free capacity

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ItsaMeDS4243

New Member
May 11, 2021
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So to start out with, I'm going to write everything here explicitly in TiB so I don't confuse myself.

I have 24 ~3.64TiB drives, in a RaidZ3 array. This in total is ~87.3TiB of space, before parity drives (and confirmed by zpool list). Minus parity drives, that puts us at around 76.4 TiB or so. TrueNAS itself, when creating the vdev, estimates about 76.37TiB. So that's all fine and good.

But when when I actually create the array, it shows up with ~67.5TiB available.

So presumably that space is being eaten up by space for checksums and what not.... but what exactly? I feel like this is something I should have come across while researching, and I still haven't found anything immediately obvious.
 

ItsaMeDS4243

New Member
May 11, 2021
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Ah, when I saw that my drives were reporting 512B sector sizes, I assumed that's what ZFS/TrueNAS would use. It seems that nowadays it forces 4K sector settings on all drives... and it sounds like that might be faster even on 512B drives?

I don't quite understand how larger sector sizes creates such a big difference in parity space used. If anything I'd assume that more, smaller sectors = more parity space... ah well.

Now I'm more torn about switching to 2x 12 disk raidZ2 VDEVs instead of a single raidZ3... I don't care about the performance gap for my use case, less free space is bad but faster rebuilds are good. I wish the record size drop down actually worked on that, I need to investigate record size changes.