raidz out of space

[rod86]

Hello, i create one raidZ with 5 disks of 16To. I have a raid with 79,99To from size.
When i want to add a hard disk to my VM, i can't add more 35To.

Thanks for your help

 

[Dunuin]

Thats because of padding overhead. You need to increase the volblocksize from 8K to something like 32K. Otherwise you will loose 50% of your raw capacity (20% parity overhead loss + 30% padding overhead loss). In other words...everything written to a zvol will consume 160% space. You can change the blocksize for newly created guests in thr webUI at "Datacenter -> Storage -> YourZfsStorage -> Edit -> Blocksize". After that you will have to delete and recreate all VMs (easiest way to do this is to backup and restore all VMs or to migate them between guests).

See this article if you want to undestand the padding overhead: https://www.delphix.com/blog/delphi...or-how-i-learned-stop-worrying-and-love-raidz

 

[rod86]

Thanks for your answer. But i have this message when i increase my blocksize to 32.
failed to update VM 102: zfs error: cannot create 'raid-nas/vm-102-disk-0': 'volblocksize' must be power of 2 from 512B to 1M (500)

 

[Dunuin]

Did you really use "32K" and not just "32"? 32K is power of 2 and between 512B and 1M.

 

[rod86]

thanks i just write 32 but with 32k it's ok
Thanks a lot

 

[Dunuin]

Also be aware of that performance of everything that writes blocks smaller than 32K will be terrible (like MySQL or postgres DBs).

 

[rod86]

To have the best possible speed, what blocksize do you recommend?

 

[Dunuin]

You have to choose when using a raidz1/2/3 if you want to waste capacity with better performance for small blocks or the opposite. Good performance and alot of capacity isn't possible.