1. use raid-1 period. trust me here.
2. the samsung 840 pro must use 30% OP aka 192gb out of 256 or 384 out of 512 (dont use the 128)
3. don't worry about trim.
4. ENSURE DRIVE WRITE CACHE is enabled!
the 840 pro is seriously buggy compared to the old 830. Even with the LSI controller.
Best bet is to use the software raid (B120i) HPVSA, second best is the LSI2308 (B320i) in IT mode or HPVSA
The P420i works fine but the newest "HP SMARTPATH" basically bypasses all the caches to get speed up. It is the equivalent of LSI's cut through switching option (but is dangerous).
So if you don't have HP SMARTPATH option , then you must set read to 0% and write to 100% and enable drive write cache.
Raid-1 works. Raid-10/5/60 just doesn't scale at all except linear.
Fact is SATA -> SATA is fastest (B120i), then dumb software raid (B320i) then P420i or LSI.
Beware the LSI megaraid product has serious issues with Samsung 840 Pro and Intel S3x00 as well!
PM me if you want to learn more!
I have P420/1gb FBWC running in DL180 G6 (and Se1120/p4300g2) - it is a great controller for esxi and the HP SMARTCACHE but its generally too heavy for ssd except with the new smartpath under the right conditions.
I use the 840 PRO in DL380G7/DL360 G7/DL320 G6 with LSI megaraid (9260/9261/9266/9271) solid! with 30% OP.
So software raid my friend , or raid-1.
Maximum performance from the 840 pro on HP gear was using the LSI 9266 (no battery) - I run two of those with 4 to 8 SSD each.
Best esxi performance is with raid-1 smaller strip size !! spanned with extents to make a larger volume.
P420i will disable DWC which kills the ssd literally, and will create strip far too big! P/E of 840 pro is 4K so strip size bigger thank 8K is wasteful! Strip = 1 drive, stripe=all drives.
So yeah you spent a lot of money on a controller that is not well suited for those drives.
Did you know folks sell the sandisk or HP branded 400gb SAS ssd for $1/gb on ebay? :) SAS gets rid of a ton of issues.
Also never use a sas-expander with SSD. Ensure it is 1 SAS port to 4 ssd maximum. SAS expander will cause tons of issues with SATA SSD to SAS controller.