iSCSI disk arrangement - is there a better way

NugentS

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I have a pool of SSD's called in an imaginative manner SSDPool.
It consists of 6 SSD's in 3 mirrored vdev's and an Intel Optane slog
Its purpose in life is to work as iSCSI storage only

Note that sizes are approximated for ease of writing

Each disk is 1TB so the raw storage capacity is approx 6TB
Given that its mirrored pairs then the raw capacity is 3TB

The iSCSI zvol itself is 2.17 TiB of 2.47 TiB available so in round terms I have about 2TB of available space from 6TB of raw disk (although 3TB might be fairer). Creating the zvol would only let me use 80% (and I may not have used quite all of that)

I am only using 450 GiB of the 2TB - so I do have easily enough space. It just doesn't seem very efficient
 

HoneyBadger

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It just doesn't seem very efficient

Mirrors by their nature will only give you 50% of the raw space.

If your iSCSI clients can send UNMAP commands, I'd highly recommend creating sparse ZVOLs and seeing how much of a win you get from compression - that 2T of "usable space" on the pool could be much higher if you're storing compressible data.
 

NugentS

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Using VMWare 6.7
I get a 1.41x compression according to FreeNAS
Not sure how to set a sparse ZVol (or have I done it already?)

VMWare seems to think I have used 341GB of 2TB available

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HoneyBadger

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Using VMWare 6.7
I get a 1.41x compression according to FreeNAS
Not sure how to set a sparse ZVol (or have I done it already?)
You have to set "sparse" at creation time, it's under the Advanced settings when you make a new ZVOL. You can create a new ZVOL and then svMotion (if you have a license) or offline move (if you don't)

Won't impact compression but if your 1.41x ratio holds, that means your "2TB LUN" when full would only consume about 1.4TB on the pool - leaving you with the potential of squeezing a little more space if you really need it, or a bit more unused NAND for wear-leveling.
 
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