Is slog/l2arc recommended with very fast SSD zpool?

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beezel

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I know enough about zfs/freenas to get myself into trouble. I am very aware of SSD zils/l2arcs and getting the right type of ssd for the job, and how much it can improve our VMware env. We run several SSD ZILs on our various pools and they are a must-have for sync=always, like our ESX env does.

We've just put some very fast PCIe SSDs from Intel (750) into production, and I'm wondering if it's worth dedicating the hardware (and more importantly, the SATA channels) at doing zil/l2arc.

Has anyone tested it both ways extensively? Our lab setup isn't quite nice enough to really figure it out, and I don't want/cant keep our production box down long enough to get some meaningful data. We don't have enough PCIe slots, our sata SSDs are consumer grade, and we don't have 10gbE in our lab.

In theory, SSD zil/l2arc speeds up platter-based vdevs considerably. Does it help at all when your vdev is Faster Than Piss? In our case (due to limited PCIe lanes) putting in a zil/l2arc would actually occur on our SATA backplane and probably be SLOWER than the 750s.

I am just not sure if the method of caching and writing would benefit us still, or it would be a wash, or a performance loss.
 

depasseg

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For a SLOG, the latency is probably the biggest driver. If I had to decide between an SSD vdev on PCIe or a dedicated SLOG on SATA, I'd stick with the vdev and skip the slog.

As for L2ARC, I don't see a benefit here either to use a dedicated Sata SSD.
 

Stux

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No point when your pool is "faster than piss". You'd just slow it down.
 

mav@

iXsystems
iXsystems
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For fast SSD-only pool I can think only about using some NVDIMMs in role of SLOG device. Those should be efficient by even more reducing latency of synchronous writes. comparing to SSDs. But that hardware is still very new and in significant flux, that makes it difficult to aim it for production.
 

brando56894

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The only way they would probably be useful was if they were NVME but since you said SATA I'd say don't waste your time.
 
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