If you’re building a ZFS system, you’ve probably come across the Dell PERC H730 Mini. It’s great since it fits in the integrated slot and doesn’t occupy one of the PCIe expansion slots.
This H730 Mini has an “HBA mode” to passthrough the drives to the OS for use with Ceph or ZFS. Sounds perfect, right? Not so fast. Literally!
I’ve been trying to figure out what’s actually going on with these cards after this problem reared it’s head with some SAS SSDs (which should be pushing some serious performance numbers) and I’m pretty sure Dell is telling a white lie with their HBA mode.
Note: this is speculation based on what I’ve observed and have read online.
The PERC H730 Mini does not support true HBA/IT mode and here’s why; the LSI SAS3108 chipset is physically INCAPABLE of IT mode – meaning whichever way you look at it, it’s probably bad. So when you put the controller into “HBA” or “non-RAID” mode, 1 of 2 things are likely happening:
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It wraps each physical disk into a single-disk RAID 0 virtual drive and presents this virtual disk to the OS.
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It ’emulates’ the drive. This might sound ok, but it still means that every bit is going through the RAID firmware and stack, increasing latency and adding a throttle point. I’m not sure it even emulates everything (like SMART or TRIM).
Long story short, I’m just not sure I’m getting the performance I could be getting. I hope to do more testing on this in future, but I’m just putting it out there for now! Consider that you may be better getting a IT mode dedicated card.