When to add a L2ARC SSD

[STREBLO]

I noticed on the FreeNAS blog they mention that you will benefit from an L2ARC if your cache hit ratio is below 90%. Looking in my reporting tab I noticed my hit ratio was 70%

When is it recommended to add an SSD as an L2ARC? Considering my hit rate is so low does that mean i'm probably losing performance? I have 32GB of ram and a 6x 6TB HDD pool in RAIDZ2; however, I'm only using 7.8TB (24%) of the 36TB.

In case the other stats are needed:

 

[jgreco]

Perhaps, but we usually suggest going to at least 64GB of RAM before adding L2ARC. In certain situations a smaller memory system might benefit from it.

However, L2ARC is primarily helpful when your system is so busy that it is unable to service requests as quickly as they're coming in. If your pool is not swamped, adding L2ARC will rob the system of some ARC space plus it won't actually deliver you any noticeable performance gain. The exception to this would be database or VM storage, but there again, 64GB or more RAM *first*, then L2ARC.

 

[STREBLO]

Perhaps, but we usually suggest going to at least 64GB of RAM before adding L2ARC. In certain situations a smaller memory system might benefit from it.

However, L2ARC is primarily helpful when your system is so busy that it is unable to service requests as quickly as they're coming in. If your pool is not swamped, adding L2ARC will rob the system of some ARC space plus it won't actually deliver you any noticeable performance gain. The exception to this would be database or VM storage, but there again, 64GB or more RAM *first*, then L2ARC.

Unfortunately my motherboard is already maxed out at 32GB.

Would you would say if my performance is okay currently that I'd be better off just leaving it as-is? I'm not currently running any VMs, I just wanted to make sure I wasn't starving my system somehow it with a lower hit rate, especially if it could be improved by simply adding an SSD. I knew it wasn't cut and dried in determining if one should be added at all which is why I asked

 

[jgreco]

In general, a single user system would very rarely benefit from it, unless maybe you were doing something like serving BitTorrent on fiber.

 

[Robert Trevellyan]

Would you would say if my performance is okay currently that I'd be better off just leaving it as-is?

Emphatically, yes.