Size for dataset/zvol creation, before compress or after compress?

[Jimm Chen]

When creating a dataset or zvol, we have an option to assign a size and an option to assign whether using compression(lz4 typically). So the question is, if I assign a size of 100G, does it mean 100G before compression or after compression? The FreeNAS document(at least the bundled htmls) is not clear about this.

I think before compression seem more reasonable, because it keeps the same semantic when you turn compression on/off on the fly? Right?

BTW: Is it really safe to turn compression on and off on the fly?


[FreeNAS 9.3]

 

[dlavigne]

It means 100GB. Compression happens on the fly. If there's not enough space to complete a write, it produces an error, regardless of whether the write is compressed or not. Changing compression on the fly is fine as it only affects newly written data.

 

[Jimm Chen]

Please be explicit. It means 100GB before compress or after compress?

 

[dlavigne]

I am being explicit. It means 100GB. Compression has nothing to do with it. It either has room to complete a write or it does not.

 

[mav@]

It is 100GB before compression. After compression it may occupy less space on pool, leaving additional free space available for other purposed or just helping ZFS allocation algorithms to reduce fragmentation.

 

[Jimm Chen]

dlavigne,BEFORE and AFTER makes difference. Then I conclude you don't understand my question.

mav@'s answer is explicit, thank you.