ZFS and a spare SSD: ZIL or L2ARC?

Andrea Venturoli ml at netfence.it
Thu Feb 14 15:47:34 UTC 2019

On 2/14/19 3:48 PM, krad wrote:
> You need to be careful with L2ARC as under certain conditions it can
> actually degrade performance. This is down to you needing a data structure
> in ram to manage the l2arc. This comes at the expense of ARC. However this
> is more of an edge case when you are adding huge amounts of L2ARC compared
> to system ram, but important in the enterprise world.

This is interesting.
I understand precise measurement would be required: are there proper 
tools to do this?
Lacking them, are there "known cases" described? Like 16GiB RAM + 128 
GiB SSD (which would be my case)?

>  From your described workload ZiL might be better suited as databases and
> virtualisation often cause lots of sync writes. This is where Zil can help.
> However the picture is never simple. If you are going to have a zil you
> really want it mirrored

This was also my impression: I would probably benefit more from a ZiL 
than an L2ARC, but as I said, I've only got a spare SSD (no mirror).
Since this is more of a curiosity than a real need, I'm not risking data.

  bye & Thanks

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