SSD size for hybrid pools in TrueNAS 12


I was just playing around with the BETA1 of TrueNAS 12. So I built a pool from 2x 1TB rust and a 240GB SSD for the meta data. Standard lz4 compression.

Question is, how much of SSD space do you need for a meta data SSD?

I copied about 9.87GB of small files to the new hybrid zPool. The file count in macOS was 9642. The files were mostly from my mac, many small files: Text documents, apps, stuff you have on your hard drive. I tried to mainly copy small files to the pool, to get like the “worst case”. If I look at iostat I get the following allocation:

  • mirror 9.72Gb (2x 1TB HDD)
  • special 0.156Gb (SSD 240Gb with meta data)

The meta data on the SSD uses around 1,6% of the total pool usage. Correct me if I calculated something wrong, but that is not very much! 🤓 This is a factor of 63,31. So in theory, if you fill up the drive with the same ratio of file size, a 240GB SSD would be sufficient for a 15TB pool! If I look at my FreeNAS pool, my files are much bigger in average. I use 4x 4TB WD Red drives in a RAID10 setup. My guess is that for my pool, a 120GB SSD should me more than enough, probably even 64GB of SSDs.

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