Maxmize ARC RAM Usage

[cengjingcaihua]

I have a system running TrueNAS Core 13.0 U3.1 with 256GB of RAM. This server is only used for file serving and I want to let ARC use as much memory as possible. With the default setting there's always 11GB free and ARC size stays at 228GB. I have tried setting vfs.zfs.arc.lotsfree_percent to 1, vfs.zfs.arc.sys_free to 1073741824. I know reserving the right amount of free memory is crucial to the system performance. What tunables can I use to lower the bar that system collects memory from ARC?

 

[sretalla]

I'm not sure that trying to hammer a square peg into the round hole is the right answer... the system is managing it and under CORE it will already use as much as possible (with safeguards against going too far).

I suggest looking into arc_summary before feeling like you're leaving performance on the table.

 

[cengjingcaihua]

The reason I believe there's still room to improve performance is because this system was migrated from a 32GB system. The free RAM on the old one was always around 1GB and workload and use cases haven't changed. I am also trying to understand how zfs determines when to collect memory from ARC.

 

[sretalla]

Then you are indeed interested in looking at arc_summary

If your working set (the data used regularly) on the system was bigger than 32GB, then of course you would use all available RAM.

arc_summary will probably show you that is no longer the case with 256GB