ZFS Log and Cache on the same device/RAID?

[geilemann]

Hi,

i'm currently setting up my new server with Scale.

I want to speed up my write rates a bit so i decided to try Cache with them. Reading is already at max. from 10GBit, Write only about 650mb/s.

So best way to do is, from what I red, to give a 50GB ZFS Log Cache and the rest of the SSD for L2ARC cache, is that correkt?

I have 8x14TB WD Red and 2x2TB NVME (from a chia mining project).

When I try to add Vdev to my pool, I can only add single physical devices. I would like to do RAID 1 Mirror and give a 50GB partition to ZFS Log and the rest of NVME Space for L2ARC.

Is that possible? And how?


Regards

 

[danb35]

So best way to do is, from what I red, to give a 50GB ZFS Log Cache and the rest of the SSD for L2ARC cache, is that correkt?

I don't know what you read, but no, for at least three reasons:

• SLOG only helps sync writes, and sync writes will always be slower than async. SLOG isn't a write cache.
• As you've already found, TrueNAS only allows using whole devices for L2ARC and SLOG. You can use multiple devices if desired, but not part of a device. Sure, you can mess around at the CLI to get around this, but that isn't supported.
• The desirable characteristics of a SLOG device are quite different than the desirable characteristics of a L2ARC device.

 

[jgreco]

As danb35 pointed out, your idea significantly misunderstands SLOG. The SLOG/ZIL is not any sort of cache. You will only be hurting your performance. Please consider reviewing this article on what the SLOG is all about.

Some insights into SLOG/ZIL with ZFS on FreeNAS

What is the ZIL? POSIX provides a facility for the system or an application to make sure that data requested to be written is actually committed to stable storage: a synchronous write request. Upon completion of a sync write request, the underlying filesystem is supposed to guarantee that a...

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