Can I safely change ashift value in running zpool ?

[SargOn]

Hi, first of all, hello to everybody. I'm new as user, but I've read you a lot for long looking for support. Thx!

My question... We have a production pve cluster (4 nodes + 2 test nodes), updated to latest 5.1. This has been running for more than 1 year and now we have jumped to ZFS (I've fallen in love...).

In node 01 ashift is set to 0 (auto?) and I see zdb shows as 12 with 4K block size in main zpool (8xSSD intel HW RAID5, perhaps in the future we change this, delegating RAID management directly to ZFS) so I gues this is ok, right ?

But in node 02, ashift value is set to 12 in zpool (same value by zdb), but block size is 512b so I understand that ashift should be 9...

Can I directly (and safety) set... ?

# zpool set ashift=9 <zpool name>

Thanks in advance

 

[guletz]

But in node 02, ashift value is set to 12 in zpool (same value by zdb), but block size is 512b so I understand that ashift should be 9...

Can I directly (and safety) set... ?

# zpool set ashift=9 <zpool name>

No, you can not doing this. You can define ashift only at pool creation time. After that ashift is a ReadOnly zpool prop.

 

[SargOn]

Ok, I was afraid of it... Thx.

Any advise about using or not ZIL and/or L2ARC in my sceneario ? (many VMs in a HW SSD RAID5) ?

Thx in advance.

 

[guletz]

Any advise about using or not ZIL and/or L2ARC in my sceneario ? (many VMs in a HW SSD RAID5) ?

Because your pool is a SSD only pool, I guess that this is not so useful. For sure ZIL is not useful at all. And the downside of using L2ARC is the fact that you will use more RAM.

 

[LnxBil]

Could you please also elaborate what type of SSDs you use, how much RAM you have and why you want to use HW-RAID?

 

[SargOn]

We're using HW RAID because we didn't know all the ZFS features and recommendations at the beginning when we were initially setting up the system (we assumed HW RAID is always better... and we were using EXT4).

The SSD disks are all 8 x SATA3 INTEL S3520 800GB 6Gb/s 3D MLC, and about RAM: 256GB.

Thanks in advance.

 

[LnxBil]

Thanks. You have a nice machine there. It is hard to find SSDs much faster than that without a hotplug PCIe slot, so I'd agree with @guletz.