Which pool would a slog device benefit most?

gregg098

Member
Apr 5, 2018
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I have a Dell T30 with two ZFS pools. The first is a mirrored pool (rpool) of two S3700's created on install. This has the host OS and also my VMs/containers.

The other pool consists of four 8TB hard drives setup basically as RAID10 in mirrored pairs.

For KVMs, I run Windows 10 and Debian 9. Debian runs Docker with a bunch of Usenet stuff, UNMS, my test lab for Home Automation (use a dedicated NUC for the actual home automation setup), etc. I also play around with different Linux builds (Mint, Ubuntu, etc.) periodically. For containers, I have one for Plex and another that is just a file server. Nothing crazy for work load. Plenty of RAM as well.

On the hard drives, I just store bulk files. Mainly my media for Plex and backups from other computers.

I managed to snag a 16GB Optane NVMe module and a PCI to NVMe adapter for really cheap. I understand I don't really need it, but I got a good deal on it. If I install this, which pool would benefit the most with a SLOG device?

The main pool, rpool, has pretty fast drives already. Pveperf shows about 4000+ fsyncs/sec
The hard drive pool is obviously slower and shows about 300 fsyncs/sec

I don't really do a lot of synchronous writes (the fact that my hard drives are an NFS destination for a few machines as a backup is probably it), but would like to utilize the Optane module. Which pool would show more benefit?
 

sb-jw

Active Member
Jan 23, 2018
551
49
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If you have constant (sequentia) Streams of big data (eg an ISO File) normally a Cache wouldn't help you here. If you have more smaller files and a bit more random (eg you work with Photoshop directly on the device) then a cache would help you.

Keep in mind, you can destroy any cache device very fast with sequential streams.
 

gregg098

Member
Apr 5, 2018
32
2
8
38
If you have constant (sequentia) Streams of big data (eg an ISO File) normally a Cache wouldn't help you here. If you have more smaller files and a bit more random (eg you work with Photoshop directly on the device) then a cache would help you.

Keep in mind, you can destroy any cache device very fast with sequential streams.
Thanks for the reply. I'm not adding a cache device though, just a small SLOG. I don't think L2ARC would benefit me at all, less than adding a SLOG. As for wear out, Optane should hold out well in the long term (in my usage anyways).

I did add it in last night and have tested it with both pools. I get roughly 10,000 fsyncs/sec with pveperf on either pool. These aren't real world test though. Reading into it more, I think it would be beneficial to stay on the hard drive datapool since I server files over NFS from there. Also, the rpool consists of Intel S3700's which are pretty fast as is.

Open to any thoughts from anyone on this though.
 

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