ML350 G5 - upgrade hot swap drives to 6 Gb/s?

That's a lot to address. I'd keep the existing Smart Array p400 controller in place for now, provided it has a working battery-backed cache unit. Otherwise, your LSI will work, but it's not ideal.

The on-disk RAID formats are different, so this means a full rebuild. You won't be able to preserve your data if you do what you're proposing.

As for 3G versus 6G, it does not make a difference unless you're dealing with SAS expanders or solid-state drives. You won't see any difference in performance in your case.

I tend to think that HP controllers run better in their servers. I'm also not a fan of LSI's management software (MegaCLI is awful). Using LSI HBA cards for other solutions makes sense, though. Your best bet in this case is to make sure you have an HP P400 controller with BBWC cache (here's why). If you don't have the cache, your path of least resistance is to obtain one.

That way, you can be sure that your backplane, alerts, LED indicators and integration with system monitoring all work properly.

As for rails, check: http://www.racksolutions.com/hp/racking-hp/ml350

Ugh - now I remember why I prefer white box servers for one-off or small business vs. the "big boys" like HP, Dell, etc...

Building your own is almost never cost effective. By the time you factor in your time spent building the machine, figuring out what is supported, warranty, future support, downtime if something goes wrong, downtime hassling with vendor warranties (or keeping spare parts)... TCO is not just CapEx.

And anyone have a good source for rails for this thing? The HP prices are ridiculous - I could build another server for what they want for rails!

Try eBay and other outlets. You can usually find them cheap. Much less than HP would charge directly.

I haven't found much about the differences between 3 and 6 Gb/s SAS interface from a physical layer.

The only difference is the speed of the data; and physically speaking the cables are a higher quality to support the higher speeds. The connectors used in the original spec call for 10GHz minimum.

I have a nice 8 port LSI MegaRAID SAS controller sitting here with battery backup and everything - can I just swap out the controllers and expect everything to work?

That would depend on what controller you're using now. If you're using the SmartArray controller, no. You would need to rebuild the array.

Unless you're using new 15k drives or SSDs you will not be saturating a 3Gb bus, so you wouldn't see any performance gains from swapping to 6Gb. Also the LSI card would not be warrantied by HP.

four HP 300GB 15K SAS drives - and of course, they have all now failed

What on Earth did you do to those drives? I've never seen drives fail that quickly. I've got dozens of such drives and only one has failed in 4 years.

Please check for hard drive firmware upgrade as well as RAID controller firmware upgrade, there was a notification about false hard drive pre-failure depending on drive type. I have never seen that many drives in one server fail or pre-fail unless something else is an issue. We have drives running for 6 years without an issue. If you are supporting HP hardware please know that you can sign up for their alerts here https://h41183.www4.hp.com/user-prefs-alerts.php