* Edit - adding more backplane swapping details since I just swapped a pair 10 minutes ago also clarifications, and other direct backplane use cases and PDB notes *
Thanks for the mention - all but one of my extra 847s are claimed at this point, PM me if you decide you're interested in the last one
If you can deal with low profile cards and drives on both ends of the chassis, I really like them a lot - the rear of the chassis has an easy to slide out 2U motherboard tray above the rear 12 drives. And of course, they only come with PWS-1K28P-SQ compatible PDBs AFAIK.
If you don't need more than 16 drives, I would highly recommend the SC836 3u units - they are a lot like an SC846, but the SAS2 expander backplanes can be found for cheaper. Plus the control panel is on the main part of the chassis, less damageable than the power switch panel on the left handle of the SC846 and SC847. (I have a couple of extra SC836 with TQ backplanes and a couple with SAS2 EL1 backplanes, which I am going to put the 1280W SQ compatible PDB into)
Although I haven't bothered doing a lot of testing both ways, to sum up my research, which includes others' findings I have seen reported here and in other forums:
- Megaraid cards seem to have to have just the right firmware revision to work with the SAS1 backplanes, but IT mode HBAs are much more happy with them.
- The drives larger than 2TB causing problems in fully populated SAS1 backplanes thing seems to only apply to the 24-bay backplanes, not 16-bay and 12-bay ones.
- The detection problems may only apply to certain drive models, but I have not been able to get enough details from those who know.
I ended up replacing all of my SAS1 backplanes since even though ~160+ per SAS2 backplane is considered a good deal, I think it is worth it for the known reliability and ease of use of SAS2 backplanes over SAS1, even though SAS1 is perfectly serviceable for most situations - SAS1's 12 GBPS to 24 drives is somewhat limiting, but you can still get close to saturating 10G, and 48GBPS from double cabled SAS2 is more than enough bandwidth for spinners. I can confirm all Supermicro 24-bay 4U backplanes are mounted with just 5 screws at the bottom of the case and 4 on the top, plus 2 of the recessed ones on the outside of the case, on the right, and a magnetic screwdriver definitely helps. The front backplane is a little tight against the front fan midplane, be patient and careful and you'll be fine. The rear backplane is super easy to access in contrast since it is under the motherboard tray with plenty of room - 4 screws on top and optional 3 screws on the bottom, which are only used if your backplane has the metal frame (the ones I was taking out did not.) There are notches to fit the bottom of the backplane board into in the bottom of the chassis, which are also compatible with a framed backplane, go supermicro for interchangeability!
I also think the SAS2 expander backplanes are worth it over direct backplanes - they include the 36-port version of the chip in the 24-port Intel/LSI SAS expander, and even the more common $200-250 per backplane is still cheaper than buying 2 Intel expanders and cables to hook them up to a TQ or A backplane, plus the airflow is better airflow, etc.
The one case when I would use a direct backplane is if you are going to use SSDs with adapters in some of the bays, or want to split the backplane to 2 different controllers, a good trick for this in a 24-bay case with just 1 Intel expander is to single cable the expander to the controller for 20 of the drives, and then directly attach the other 4 ports on the controller (assuming an -8i card) to the other 4 bays for SSDs or some other sort of bandwidth-critical drives.
Another direct backplane use case would be to use 2 different controllers with different bays - for a 4U, dual-uplink an Intel expander between an -8i controller and 16 bays, and connect the other 8 bays directly to another controller. In this case, the -TQ backplane gives you more flexibility in how you allocate the bays, I have heard that the -A backplanes connect the groups of 4 drives column by column, whereas I like to fill bays row by row, then I always have groups of 4 drives across per group no matter what size the case instead of split columns in a 4U case.