How to calculate resulting logical size of RAIDZ-2 pool?

[psapezhka]

Hi, I'm trying to plan infrastructure and calculate the resulting amount of usable space. I have disks quantity, size in bytes, sector size. Also, I have some non-default values of ZFS parameters, like recordsize=1m. It would be great to have some calculator or an algorithm to calc.

P.S. I've googled a couple of solutions, but all of them seem to me too approximate or abandoned.

[gmelikov]

• raidz padding https://docs.google.com/spreadsheets/d/1tf4qx1aMJp8Lo_R6gpT689wTjHv6CGVElrPqTA0w_ZY/edit?pli=1#gid=1576424058
• slop space https://openzfs.github.io/openzfs-docs/Performance%20and%20Tuning/Module%20Parameters.html?highlight=slop#spa-slop-shift

[jumbi77]

Not sure, but isn't slop space now upper bounded to max 128G (#11023) ? If yes, maybe the referenced doc should be edit to "default: 3.2% with an upper limit to 128G ?)