Optane SSD size for SLOG / L2ARC

TrueNAS General
1

Hi,

i’m updating one of my truenas homelab servers (going from a I7-4000k series to a xeon gold.)

So, new motherboard, CPU, ram (128GB). Adding some NVME and 6x 6TB spinning disks (more in the future).

Primary use will be file storage and SAN for VMs

I would like to build the 6x ?TB in raidz2 and add a slog and or l2arc to it. Reading posts suggest that optane ssds are the way to go.

the question is : What size do I need ? Browsing on ebay and the 16GB are a good price but is that enough ? the prices go up and availability goes down when looking at bigger sized drive.

What formula do I need to properly size the SLOG and L2ARC (do I need a optane for l2arc?).

Thanks for info on this :smiley:

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What formula do I need to properly size the SLOG and L2ARC (do I need a optane for l2arc?).

By default, you want 5 seconds of your fastest transfer rate for your SLOG. You didn’t specify your network speed, so I’ll assume 1G, which translates to ~120 MiB/s * 5s = 600 MiB. I don’t know enough about L2ARC, so someone else will have to cover that, but personally, I’d always opt for more RAM over extra L2ARC, but that’s just my personal preference.

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See ZFS Write Cache section for SLOG.

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Network speed is 10GB.

I do have 128GB of ram. hope that is enough to remove the need for a L2ARC drive

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What makes you think you need one? SLOGs are only useful when you have lots of sync writes, which most workloads don’t use.

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This pool will host a VMFS for vmware. vmware does do sync writes. So that would help in performace.

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Then in that case, your pool should consist of mirrors, not RAIDZ2.

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General answers

SLOG

Rule 0: You probably do not need a SLOG.
Rule 1: A SLOG should have PLP, high endurance and low write latency.
Rule 2: Size should hold two transaction groups, so 2*5 s by default. (Larger does not hurt, save for the wallet.)

16 GB M10 Optane fall somewhat short on endurance and throughput for SLOG; however they make nice boot devices. Although the size of a M10 would be sufficient, the real entry point for an Optane SLOG is a 100 GB DC P4800X.

L2ARC

Rule 0: You probably do not need a L2ARC.
Rule 1: Bump RAM first (64 GB minimum). Then run arc_summary and refer to Rule 0.
Rule 2: Between 5*RAM and 10*RAM. No more!

Mind the gentle Dragons!

For the adventurous homelaber, a single Optane 900p may be partitioned into SLOG and L2ARC.

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