SOLVED Zil drive question

[Fred974]

Hi,

I am building a new TrueNAS server that will have a raidz2 pool of 6x 400GB Sas SSD disks.
Question is: Do I still benefit of having a zil drive on an ssd pool?

The server is mainly for an xcp-ng NFS Share storage.

Thank you

 

[Patrick M. Hausen]

If xcp-ng uses synchronous writes over NFS, then probably yes.

 

[jgreco]

See

https://www.truenas.com/community/threads/some-insights-into-slog-zil-with-zfs-on-freenas.13633/

 

[Fred974]

@jgreco thank you very much for this great write up. Can I add a SLOG device even tough my pool as already be create?
My understanding is that it need to be set at pool creation time

 

[jgreco]

You can add or remove a SLOG device at any time.

However, warning, warning, be careful, it has happened to people that they accidentally add a device to their pool as a new vdev instead of as a SLOG. This is kinda catastrophic, so be sure you are reading the manual, maybe see if you can find a recent YouTube video, etc., to make sure you understand the correct steps to add the SLOG device as a SLOG device.

 

[Fred974]

@jgreco Thank you. How big the SLOG need to be?
Can I use a 16GB SATADOM or will that be too small?

 

[Fred974]

Also, I have ssd pool and spining disk pool.
Is it 1 slog per pool?

 

[c77dk]

@jgreco Thank you. How big the SLOG need to be?
Can I use a 16GB SATADOM or will that be too small?

It should be big enough, but since SLOG are writes only (untill system crash) you want something with good write endurance, which is seldom seen in SATADOM

 

[jgreco]

A SATADOM is not useful as a SLOG device. In order for a SLOG to operate correctly, you need a device that guarantees correct operation, such as a SSD with power loss protection, an Optane memory device, or something else like that. As @c77dk says, it also needs to be something with significant write endurance.

Adding a random SSD as SLOG is not useful, since most SSD's will not handle power loss. There isn't much of a point in adding a SLOG device unless you're going to do it correctly.

 

[Fred974]

Thank you all. This was very informative.