L2Arc not utilized

[BabaJega]

Hello all,

I had 2x 500gb samsung 960 evo NVME ssds collecting dust and thought why not use them as l2arc for my vm storage ssd pool (used via iscsi for a hyperv cluster).
Said and done the l2arc is in service since 2 days and its doing nothing (I think?).

under the "reports --> zfs" tab I can see that after adding an l2arc the utilization shown in the first chart is next to non-existent.
It says "max l2arc 973MB" and l2arc hit ratio is at 0.06 mean and 1.37 max. Am I looking at the wrong chart? What can I do to utilize the l2arc?
Or should I just get rid of the l2arc?


The system:

Xeon Silver 4210 10c 20t
192 GB DDR4 Reg ECC
Intel X540 SFP+ dual port 10gbps nic
10x 4tb HDD (mirrored vdevs)
6x 500gb samsung 860 evo SATA ssd (mirrored vdevs) + 2x 500gb samsung 960 evo NVME ssd as l2arc

Thanks in advance for your help!

EDIT: ARC hit ratio looks good as far as I can tell with a mean of 96.06.
EDIT 2: completed the ssd definitions

 

[sretalla]

vm storage ssd pool

This type of storage isn't usually suited to L2ARC... more to SLOG.

To get good performance, you really need a specialist SLOG device like the PCIe cards from Intel, but you may see some improvement from using NVME or even some minor change with those EVO drives. (if the pool disks were HDDs)

Since you're already on SSD for the pool itself, only the Intel cards or NVME will bring any real performance gain.

Or should I just get rid of the l2arc?

Yes. Even if you saw a good hit ratio, with the same model of SSD as L2ARC as your pool SSDs, you're not helping anything there.

 

[BabaJega]

This type of storage isn't usually suited to L2ARC... more to SLOG.

To get good performance, you really need a specialist SLOG device like the PCIe cards from Intel, but you may see some improvement from using NVME or even some minor change with those EVO drives. (if the pool disks were HDDs)

Since you're already on SSD for the pool itself, only the Intel cards or NVME will bring any real performance gain.


Yes. Even if you saw a good hit ratio, with the same model of SSD as L2ARC as your pool SSDs, you're not helping anything there.

Im sorry, my post was misleading and I corrected it already.
The Vm storage pool contains 6x SATA samsung 860 ssds and the l2arc contains 2x NVME samsung 960 ssds.
The NVME ssds should be much faster than the SATA ssds not only in terms of throughput but also in terms of iops (right?).
Thats why I thought I use them as L2ARC

 

[sretalla]

For your use case, L2ARC is waste. You have plenty of RAM, so your ARC probably never even fills.

As I mentioned, NVME may bring some minor advantages (i.e. IOPS) to your situation as SLOG.

Take care to look around the forum for advice on SLOG before deciding to use those... SLOG is likely not to use more than 30GB at peak, so a lot of wasted high-performance SSD in going there.

 

[BabaJega]

For your use case, L2ARC is waste. You have plenty of RAM, so your ARC probably never even fills.

As I mentioned, NVME may bring some minor advantages (i.e. IOPS) to your situation as SLOG.

Take care to look around the forum for advice on SLOG before deciding to use those... SLOG is likely not to use more than 30GB at peak, so a lot of wasted high-performance SSD in going there.

Thanks but I dont think I want to use a SLOG because I always set sync=disabled for my vmstorage zvols. I run daily backups via veeam for all my vms and those backups get backuped to a dedicated backup target server once every two weeks so I really dont care if my VMs dont use sync writes.

I guess I will just remove the drives then and maybe put them in an extra pool or something.

 

[sretalla]

I really don't care if my VMs don't use sync writes

You seem to understand the risks, so it's 100% your call.

I guess I will just remove the drives then and maybe put them in an extra pool or something.

I agree this will deliver you the best value for that hardware.

 

[BabaJega]

You seem to understand the risks, so it's 100% your call.


I agree this will deliver you the best value for that hardware.

Thanks for sharing your opinion! I dont know how this usually works but as far as I am concerned this thread can be closed now. I got my answers.