Why using software raid when there are plenty hardware raid controllers with dedicated processors for checksum calculations, dedicated memory, cache and bbu? Well there is one major advantage of software raid solutions – hardware independence.
Recently we had a simultaneously crash of two disks in raid5 on 3ware 9550-sxu controller. That`s a real bad luck :-> Of course we recovered data from backups, but before that we tried to rebuild array. We discovered that one dik had crashed, but the other had just few bad sectors. As an experiment we cloned that disk without these few sectors and tried to rebuild the array. Unfortunatelly controller didn`t recognize that disk as a member of the array. It turned out that 3ware controller appends serial numbers of all members of array at the end of disks with a checksum (we tried to replace serial number of cloned disk with no luck).
So how would it look on software raid? Much easier. Linux for example writes only array identificator (UUID) so disk cloning is possible. Furthermore disks can be transfered to another hardware. With 3ware controller in case of its failure we would have to find another 3ware controller (probably the same model).
So a lesson has been taught – don`t rely on hardware raid, use raid6 when possible and keep your backups safe because they may save your ass someday