Software && Hardware RAID

Software RAID
It uses fully software components for implementation. In Linux systems these are special kernel modules. The raid is managed using the mdadm utility. Software RAID has its advantages and disadvantages. On the one hand, it costs nothing (unlike hardware RAID controllers). On the other hand, software RAID uses CPU resources, and at times of peak load on the disk system, the CPU may spend a significant part of its power on servicing RAID devices.
 
Hardware RAID.
A separate board, equipped with its own processor that performs all the necessary calculations and, usually, cache memory. If a RAID controller has a cache memory on board, it can be used for intermediate storage of data being written or read. This allows I/O operations to be managed more efficiently.
In order not to lose the data in the cache during a power failure, hardware controllers are equipped with their own battery (BBU - Battery Backup Unit), which allows storing data in memory for up to 3 days. And as soon as the server starts working, the controller first of all drops the contents of the cache to disk.
If you have a battery, you can activate WriteBack mode: when writing to disk, the controller will report the success of the operation at the moment when the data gets into the cache, and then in the "background" mode will dump them to disks. Therefore, other processes will spend less time waiting for confirmation.
 
The main advantages of apparte RAID:

• WriteBack mode can be activated
• slightly higher performance due to WriteBack and its own processor
• high rebuild speed

 
Software RAID is recommended for SSD drives.