What SCSI harddisk is used in server?

What exactly is your question? Servers do have harddisks for data storage. Each server has different numbers of slots and that's how many disks can be on that server. However, some servers are scalable--meaning slots can be added and mapped to the server. Also, if the harddisks are 16GB--you can swap them out for 36GB (dependent on the server's configuration limits). Databases map to these harddisks and that's where the data is stored. Ideally, disks are mirrored or DR (data replication) so if there is a disk failure, a production environment does not become unavailable. The mirror automatically becomes the primary. Fix the bad disk, then switch the paths back. No down time--no data loss. Without a mirror, you have to do a recovery--either as good as your last backup (if no database archive logs) or you can use the last backup and the archive logs to roll forward to that point. Bad thing is with doing a recovery--you do have production down time (at least for that particular object). And companies now equate down time in $$$$.