ZFS and a spare SSD: ZIL or L2ARC?

krad kraduk at gmail.com
Thu Feb 14 14:48:54 UTC 2019

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.

>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, as like others  have said its critical if it
fails. Your SSD also needs to have good write endurance. You can achieve
this by either spending loads and getting a high quality drive or over
capacitying something cheaper. (as bigger drives have a higher lifetime
writes). However you do this you are relying on the wear levelling working

Another thing to consider is again a separate ZiL can hurt performance,
depending on workloads and the configuration of the main pool vs the
ssd/nve used. A large zpool can have a higher write throughput than a
single puny ssd so unless you are heavily io contended you would hurt
performance in this scenario.

Sorry to paint a complicated picture but like most things there is no right
or wrong, just choices and balancing of risks.

On Wed, 13 Feb 2019 at 17:15, Steve O'Hara-Smith <steve at sohara.org> wrote:

> On Wed, 13 Feb 2019 18:02:51 +0100
> Andrea Venturoli <ml at netfence.it> wrote:
> > Now I've just got hold of a spare SSD and, although the box performance
> > is adequate, I'd like to see if it can boost it.
> > Should I configure it as L2ARC? ZIL? Both?
>         In my experience L2ARC tends to run full and ZIL uses almost none
> of the space allocated. But then my workload is read heavy - for a write
> heavy workload ZIL is probably more beneficial.
> --
> Steve O'Hara-Smith <steve at sohara.org>
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