ZFS SLOG. Can it be carved out of an SSD volume?

[manxam]

I have a 960GB S3700 that I'm using as a datastore for VMs (iSCSI) but would like to add a SLOG.
Can I recreate the volume on the SSD to use part of it for SLOG and part of it for data?

Thanks!

 

[ttabbal]

It will work, at least, ZFS will let you do it. The downside is you risk throttling the whole thing with other access on the disk. It could well end up worse than not having an SLOG. Particularly with all the VM storage access going to that disk. The only way to know for sure is to try it though. Perhaps your VM activity it low enough it won't hurt things.

 

[manxam]

Thanks for your reply @ttabbal. The VMs are low IOPS but I prefer keeping them on SSD vs spinning rust.

 

[EffrafaxOfWug]

Maybe I'm misunderstanding the term (I'm not a ZFS user) but I thought in order to get any benefit a SLOG was meant to be significantly faster than the underlying storage...? Won't putting a SLOG on the same device as the storage mean that the write path for your data gets longer and more complicated for no gain in speed?

 

[ttabbal]

What the OP is asking about is a device used for VM storage, not the primary storage array. That is likely on spinners. So the SLOG will be on faster storage.

The write path is complicated somewhat by the VM storage being there, but if the SSD is under-utilized, it can work.

Also important to note, an SLOG does nothing for async writes. It only helps on sync writes. If you want to know how much it will help, or not, disable sync writes on the pool. If performance shoots up, SLOG can be useful for you.

 

[manxam]

Thanks for clarifying for me ttabbal; that's exactly right. I have 16x4tB spinning platters that I'd like to use a SLOG with but still want to keep my iSCSI storage on the single SSD.

I'll investigate my async vs sync performance. I have a few NFS mounts also used for ESXi guests stored on spinning platters (my iSCSI is for Hyper-V and is my primary virtualization platform) and I'm under the impression that ESXi forces sync writes to NFS. Figured I had 1/2 of my 960GB SSD remaining so might as well use it for caching.

Thanks for your assistance ttabbal and to EffrafaxOfWug for ensuring I wasn't doing something asinine

 

[whitey]

Certainly goes against best practices so YMMV is my 2cents.

 

[gigatexal]

What SSD is your slog? A fast NVME slog handling the requests from spinning rust should be just fine tho if you can just run a dedicated ssd in raid 1

 

[manxam]

It's an S3700 but I think I'll just abandon this for now reading the posts above.

Thanks for all of your help!