Cache Drives

[HANDLEric]

Hello,

i'm in the process of finalizing my FreeNAS build that I plan to use for my VMWare lab, I want to make sure I have a well rounded platform that covers my needs and I was wondering if I would benefit from adding a cache drive to my pool.

Here is my current build:

PowerEdge R710

• (2) Intel Xeon E5620
• 288GB RAM
• PERC H310 (Running IT mode for OS drives)
• (2) 300GB 10k RPM drives (OS volume)
• QLogic QLE8242 dual port 10G converged network adapter
• Intel DC P3700 (Used for log)
• LSI SAS9200 (Running IT mode for external arrays)
• PowerVault MD1200
  • (6) Seagate IronWolf Pro 4TB (6GB/s SATA)

I was thinking about adding one/maybe two Seagate IronWolf SSD drives into the host box as cache drives for the pool, would this help my overall performance for a VMWare environment?

 

[PhiloEpisteme]

I assume by adding a cache drive you mean adding an L2ARC device? If so, before you consider adding L2ARC devices you should check your ARC hit ratio; if that ratio is low then yes, it may help performance.

 

[HANDLEric]

Okay, so what does this ratio represent? - According to the performance graph i'm completely railed.

 

[PhiloEpisteme]

That shows that your ARC his ratio is nearly at 100% constantly. This is not surprising considering the size of your ARC. I don't think an L2ARC is going to help you here.

 

[HANDLEric]

I'm not sure how L2ARC actually falls into the mix, would it only be used after ARC is fully exhausted or would it try to load balance the two? Would I take a performance hit if I installed L2ARC?

 

[PhiloEpisteme]

The L2ARC is an ARC cache used after the primary ARC cache is full. The index for the L2ARC is stored in memory though; and because your ARC lives in memory adding an L2ARC when none is required may harm performance because it eats a bit into your RAM and thus decreases the speed of your ARC.

In terms of performance you should add more ram if you're seeing low ARC hit percentages or you're using a lot of VMs which eat your ram. After your ram has been maxed out on your board you should consider adding an L2ARC device of the appropriate size; going too big may eat too much into your ARC and actually harm performance. After that if money is no issue you go to SSD-backed pools rather than spinning rust.

 

[HANDLEric]

Okay so based on that last statement I think you're saying that I should potentially evaluate adding L2ARC as all of the system RAM has been exhausted and i'm still seeing 100%, assumingly from the virtual environment.

I was looking at a 480GB Seagate IronWolf SSD 110 - do you think this is too big?

 

[PhiloEpisteme]

Okay so based on that last statement I think you're saying that I should potentially evaluate adding L2ARC as all of the system RAM has been exhausted and i'm still seeing 100%, assumingly from the virtual environment.

No, I'm saying you shouldn't add an L2ARC because your ARC hit percentage is 100% :) That means the system is doing a good job of hitting the cache and you don't need more cache. :)