ISCSI issue with vmware

[kdragon75]

Most VM environment end up requiring synchronous writes (send write, wait for confirmation its on disk, send next) as opposed to async (send as fast as we can). ZFS uses the ZIL (ZFS Intent Log, defaults to being stored on the pool) to store sync writes and a SLOG is a Separate LOG device. The SLOG is typically SSD as they and low latency writes (ideal for sync writes).

I want to say I am seeking a log device for my purposes

So, yes you want a SLOG. Though if your combine pool is faster than the SLOG device, you will see no gain. It is therefore most common to see NVMe or 3D X-Point based SLOGs.

On the other hand you have the L2ARC. This is the Level/Layer 2 Adaptive/Adjustable Replacement Cache. This is much more difficult to spec and tune correctly. L2ARC is generally not recommended until you have added as much ARC (RAM) as is reasonable. Even they there are a number of factors that requires careful consideration in its implementation.

 

[kdragon75]

I must also say that the SLOG will never make a pool faster than running the pool with sync=disabled (forces sync to behave like async). The SLOG is not tiered storage or cache. Even with the SLOG and sync writes, the writes sit in RAM and get flushed to the pool with each transaction group. This means the SLOG only gets read when the system crashes during writes. It will then read the outstanding transaction group and write it out to the pool as (in your case) the VMs already thought happened.

 

[HoneyBadger]

Its been my understanding that a pool with that many drives was not advisable. Could be old thinking or my own misconceptions.

Terminology mistake I think - a vdev with that many drives certainly would be questioned, but a pool would not. Unless there's a need to have physical separation of the data, you'll likely get better performance results by merging the two into one larger pool (more vdevs, more free space) and only need a single log device.

As far as log/cache drive, Its because I forget which one is more beneficial for vm environments. I want to say I am seeking a log device for my purposes and while I type this am noticing your sig, that will be my next stop. Thanks.

A log device is indeed what you're looking for and @kdragon75 has addressed the major points. Optane is the most popular choice for a fast SLOG these days.