RAID-5 controller with a battery-backed write cache is excellent. Writes appear (to MySQL) to be instantaneous.
When comparing RAID-5 to -10, you need to decide whether you are comparing N drives to N drives or comparing M GB to M GB.
N drives: RAID-10 scatters its reads across N drives; So does RAID-5. Writes hit 2 drives in either case, but the read-modify-write for the parity block may make RAID-5 slightly slower.
M GB: Now you are comparing, say, 6 RAID-10 drives to 4 RAID-5 drives (to get 3 drives' worth of space). So RAID-10 probably wins on both reads and writes.
But... Unless you are saturating the I/O, the RAID controller hides the differences.
RAID-5 or -10 is better than manually putting files on different drives:
- Less risk of running out of disk space on one drive when there is plenty of free space on other drives.
- The I/O is more evenly spread out, hence better throughput. By manually splitting, you are likely to get one drive over-worked while the others are less busy.
One problem with "battery-backed": The controller periodically drains the battery to extend its lifetime. At that point, the write cache is turned off, and writes become miserably slow. (This cycling lasts a few minutes, and happens maybe twice a year.)
Both RAID-5 and RAID-10 can withstand a single-drive failure. -5 cannot handle 2 failures at once; -10 can handle some 2-drive failures. (See also -6.)
Since drive failure is one of many things that can go bad (motherboard, controller, power surge, earthquake, tornado, manual goof, software bug, overtemp due to AC failure, ...), I think it is shortsighted to think only about RAID (or even DRBD) when looking at HA, BCP, etc.
I vote for a single filesystem with N drives and RAID-5 or -10 with BBWC.
Well, that's for spinning drives.
For a mixed HD/SSD system, put "logs" on HD since sequential access is relatively fast; put InnoDB stuff on SSD, where random access is important. If possible, get a brand of SSD that guarantees atomic 16KB writes, then turn off InnoDB's "double write buffering".
For all-SSD, make sure the wear-leveling is good enough so that all drives don't wear out at the same time (thereby rendering RAID useless).