When to use L2ARC and what to use

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Tino Zidore

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Hi

I have 2 FreeNAS servers running FreeNAS 11-U4.

One is based on the Supermicro X10DRi-T and the other on Supermicro X9DRW-iF.
Both have 256 GB RAM.

The X9DRW-iF server has a JBOD chassis with 45 x 4 TB disks zpooled into 22 vdevs with 2 disks and a LSI 9207-8e.
Question 1:
Does it make sense to attach a Kingston Hyper Savage SSD disk of 960 GB as L2ARC?

The X10DRi-T server has an internal HBA LSI 9300-8e with 10 x Samsung EVO 850 1 TB zpooled as striped(just for speed, no security)
Question 2:
Does it make sense to attach an Intel P3600 PCIE SSD disk of 800 GB as L2ARC?

Additional questions:
How can I test this on the server?
 
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joeschmuck

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I think you need to decribe your use case and exactly what you are trying to get out of your machine. I assume you want some better performance but you should describe what your performance is now as well and what you need.

To see how well it's working you should look at the ARC Hit Ratio but you won't know if this is doing some benefit for a while after you install these devices. It takes time for the L2ARC to figure out what should be loaded.
 
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Other things to consider:

  • Don't know what your network speed is, but if it's gigabit, it seems all of that would be moot, since any one of those arays would crush gigabit speeds, even 10Gb should be maxed.
  • If you're looking for more IOPS, the striped SSD pool should have that covered, along with crushing 1/10Gb networks. The larger HDD pool might benefit from an L2ARC for IOPS if it was an NVMe drive. But don't know what kind of performance you're getting already.
  • Latency would be improved in both pools with an NVMe drive, not the SSD, but this would be a rather specific workload.

For Question 1: I don't think it makes much sense. I can't imagine the latency, IOPS, or raw throughput of that SSD are better than the pool itself. But I could be wrong.
For Qeustion 2: Maybe an improvement in latency? Throughput and IOPs would be better from the pool than that drive I would think.
 
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