ZFS: Partitioning drives = write cache disabled?

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TheBloke

Active Member
Feb 23, 2017
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Brighton, UK
Hi all

I'm hoping to get a high-performance PCI-E SSD to add to my home Solaris 11.3 file server/NAS. Specifically, a 1.2TB LSI Nytro Warp drive. (Forum thread here.) This device is an LSI 2308 with 6 x 200GB SSDs, which by default are configured as a hardware RAID-0. But I believe it is also possible to access the individual SSDs, so my plan is to use one for ZIL and the other 5 will be used together in a striped config.

But I want to use the 5 drives for multiple things. At the least, I will want to use them for L2ARC read cache on my main data pool (27 x 2TB drives in 3 x RAIDZ2 VDEVs) and my boot rpool. I will also want some ISCSI volumes on the Warp drive, but these can be created under rpool.

In order to get the full performance of the SSDs for each task, I want to give each task a 5 x device stripe. So I will need to partition the SSDs and then add 5 x slices to L2ARC, and 5 x more slices for rpool.

I know how to slice the disk up and give individual slices to ZFS. But I have read in several places that in Solaris, giving a partition to ZFS results in the write cache on that device being disabled? Apparently this is because of the co-existence of ZFS with old UFS?

I can't find any 100% confirmation if this is still the case in Solaris 11.3, and/or if there's any way to avoid this? Or if it's even a problem with an SSD/PCI-E device?

So my question: will I experience any major performance or other problems if I slice these SSDs into two or three parts each, and then give these to L2ARC and at least one pool?

(I did wonder if it was possible to do this in the LSI firmware instead, creating multiple separate volumes so that Solaris ZFS saw them as whole disks. But from what I can see in the manuals, I do not believe the LSI IR firmware supports creating multiple smaller volumes on the same disks? All my LSI cards are on IT firmware so I can't test this right now.)

Thanks