Do-it-yourself RAID tips

When planning a new RAID, you would want some factors straight.
 
Protection against drive failures.
Keep in mind that RAID is not a substitute for a proper regular backup. RAID can not save you from user errors or natural disasters like flooding, earthquake, tornado, or whatever is common in your area.

Anyway, some RAID types will continue working even if one of the array's drives stops working.
These levels include RAID1, RAID10, RAID 4, RAID5, RAID 6, and exotics like RAID5E, RAID 5 EE, and RAID DP. It is discussed that RAID5 with a large number of member disks is unreliable, using calculations based on vendor-specified URE values, which can be shown to be incorrect. 

Capacity.
The RAID array size limited by

• the size of a single hard drive,
• maximum number of disks the RAID controller would handle,
• and the overhead needed for redundancy, if you choose to use a fault-tolerant array.

Should you need simple array size calculations done for you, have a look at this free RAID Calculator.

Performance requirements.
Of all the fault-tolerant RAID types, those not using parity, namely RAID1 and RAID10 are the preferred for random writes. If the RAID is almost always used for reads (for example a static HTTP server), or as a write-only storage (a backup), then RAID levels using parity, namely RAID5, RAID6, and variations thereof are OK.
If fast random writes are needed, use RAID 1+0.

To quickly learn about performance, price, and redundancy for various array types, look at the "RAID triangle".
Keep in mind that RAID doesn't decrease random access time. For good access times, use Solid State Drive.