ZIL - Is it worth the effort?

[George777]

I feel like I've read as much as I can on the SLOG, but there is also possibly conflicting information that is likely from old versions.

I understand that SLOG is not worth the effort if you're system is not doing synchronous writes.
I also understand that there was a risk to using a SLOG such that if the drive went bad, your entire pool would no longer import because it broke the import process. I believe I have read this is no longer the case in zfs version 5000 and that if a SLOG is lost, it only affects the transactions not committed off the SLOG before losing the SLOG.
I also read that it is desirable to have a mirrored SLOG, but that seemed to me to be a way to reduce risk of losing your pool if the previous issue still was possible.
There is a tradeoff between size of SLOG and size of RAM. If you're capped at 32GB ram, you shouldn't have a SLOG larger than 128GB.

My questions:
* I had read somewhere you can run a command to see if you're system will benefit from a SLOG. Does this command help discover the frequency of synchronous writes?
* Is it no longer a risk that if the SLOG is lost, you lose your pool? I've avoided installing a SLOG for this reason even though I use VMs that perform synchronous writes.
* Am I correct that it may not be as worth the cost to run a mirrored SLOG?
* Since many of the faster SSDs are not coming in smaller sizes these days, could I partition off 64GB from at 256GB SSD and use that as my SLOG? Could I use the remaining 192GB for other storage?
* If the SSD has enough bandwidth, and the previous answer is yes, could I put a 64GB SLOG and a 192GB L2ARC on a 256GB SSD? I'm thinking something like a Samsung Pro 256GB.
* What are the reasons not to combine the SLOG and L2ARC?

Edit: Thank you. Was using the wrong term.

 

[m0nkey_]

Do you have a ZIL in your pool?

Everyone who has ZFS has a ZIL.

What you're describing is a SLOG (Secondary log) device.

 

[Ericloewe]

Every single ZFS pool has a ZIL.

The question you should be asking is whether SLOG is useful in your situation.

Is it no longer a risk that if the ZIL is lost, you lose your pool?

Hasn't been for many years.

Since many of the faster SSDs are not coming in smaller sizes these days, could I partition off 64GB from at 256GB SSD and use that as my ZIL? Could I use the remaining 192GB for other storage?

You could. You sure as hell should not.

* If the SSD has enough bandwidth, and the previous answer is yes, could I put a 64GB ZIL and a 192GB L2ARC on a 256GB SSD?

You could. You sure as hell should not.

I'm thinking something like a Samsung Pro 256GB.

That is a horrible SLOG device. SLOG must have power loss protection, otherwise it's basically the same as forcing sync=off, but with lower performance and higher cost.

* What are the reasons not to combine the ZIL and L2ARC?

• ZFS is not designed to share SLOG and L2ARC
• They're completely different workloads suited to completely different SSDs
• SLOG requires extremely low latency to be useful

 

[Cpuroast]

There is a tradeoff between size of ZIL and size of RAM. If you're capped at 32GB ram, you shouldn't have a ZIL larger than 128GB.

You are probably thinking of L2ARC, which does require RAM to track, the larger your L2ARC grows, the more memory it takes to track, the more memory it eats away from the faster ARC(L1ARC) in RAM.

However, the contents of a SLOG doesn't grow a lot, during normal operation a SLOG only gets written a few GB of data and then purged when the data it contained gets committed from RAM to the actual pool.

As in it only contains a TXG or so of sync writes, so size of it doesn't matter at all, increased capacity for a NAND based SLOG, just increases wear levelling area and therefore overall SSD endurance.

It's essentially an "in case of emergency, break glass" type thing.
It's a dump bucket, that never gets read unless a power outage occurs, and yes PLP has to be present on the SLOG device, if not it negates all data integrity benefits a SLOG is supposed to provide for sync writes.