SSD Pool vs Cache

[NicJak]

Hi everyone

I built my TrueNAS recently with the following specs:

2x 256GB SSD (boot drive)
3x14TB HDD (RAIDZ1; I do have a daily offsite backup (RAIDZ1) of it; so I thought of being able to "afford" only n+1 failure tolerance)
64GB RAM
AMD Ryzen PRO 5650G

I use the system for:
- plex
- some webservices (wiki etc.)
- Developer VM
- CI setup

Now I do have a 4TB SSD that I don't need anymore. I was thinking of three possible options:
1. use it as a SSD cache for the 3x14TB pool
2. sell it and buy 2x2tb for a raid1 separate pool onto which I move responsive services such as websites and dev VM
3. just sell it, it's not worth the performance gain

What are your thoughts on this? Is the question ill posed, or do you lack information for it to propose an answer?

Thank you in advance
NicJak

 

[jenksdrummer]

* SSD cache under ZFS is entirely a read cache.
* SLOG can be a bit of a write cache in certain circumstances; but a SLOG, even when it's pounded on with writes; won't need to be 4TB by any means. I'm seeing mine hitting 4-5GB of usage in testing.

RAM and high clock rate (vs more cores) is the best bet for getting performance.

I have an SSD-only pool and it's plenty fast for sure; But, its not as much faster as you would think; at least when it comes to writes of data.

The nature of ZFS is such that it really leverages what HDDs are really good at; which is sequential writes (as well as sequential reads as a result); and most often in bursts across all the disks. It'll cache up data for about 5 seconds then blast the disks with data to writes using sequential data.

SSDs actually don't do as well a job there; generally speaking...well, except for once you get out of the SATA realm and into SAS or NVME.

 

[jenksdrummer]

Just did a necro thread because it turned up in my similar threads list. LOL