Product Description
IBM 4748 PCI RAID Disk Unit Controller
- SCSI RAID Controller
IBM 4748/#9748 is an Ultra2 SCSI controller with a 26MB write-cache that provides RAID-5 protection and compression for internal disk units (except for the 35GB disk units) and also supports internal tape units and CD-ROM units. It supports both compression and non-compression modes. The mode of operation is determined by a hardware jumper. #4748/#9748 is shipped in non-compression mode. By moving the hardware jumper the controller will function in compression mode. In addition to providing RAID-5 protection for disks, #4748/#9748 is also designed to work as a high performance controller for disks protected by system mirroring or disks with no protection. In the RAID-5 configuration, disk unit protection is provided at less cost than mirroring, and with greater performance than system checksums. #4748 also supports #4331: 1.6GB Read Cache Device, which provides increased performance. #4331 is supported only when #4748/#9748 is not in compression mode. The #4748/#9748 controller supports a maximum of 18 disk units. A minimum of four disk units of the same capacity are needed for a valid RAID 5 configuration. A maximum of four arrays are allowed per controller, with a maximum of 10 disk units allowed per array. All disk units in an array must be of the same capacity. Parity is spread across four disk units for arrays of four to seven disk units. Parity can be spread across either four or eight disk units for arrays of eight to ten disk units. For systems started with eight to ten disk units in an array, the parity for that array, will be spread across 8 disk units. For systems that are started with fewer than eight disk units in an array and later field upgraded to eight, nine or ten disk units, the RAID function must be stopped and then started before the parity will be spread across eight disk units. The #4748/#9748 controls up to two removable media devices (internal tape or CD-ROM).
