IBM 2748 is a SCSI controller with a 26MB write-cache, provides RAID-5 protection and compression for internal disk units, and also supports internal tape and CD-ROM units. #2748 supports both compression and non-compression modes. The mode of operation is determined by a hardware jumper.
The #2748 is shipped with the jumper set in non-compression mode. The #2748 controls Ultra2, Ultra, and Fast Wide SCSI disk units. In addition to RAID-5 protection for disks, #2748 serves as a high- performance controller for disks protected by system mirroring or disks with no protection. In the RAID-5 configuration, disk unit protection costs less than mirroring. #2748 also supports #4331/#6831: 1.6GB Read Cache Device which can increase performance in read intensive applications. #4331/#6831 is supported only when #2748 is not in compression mode.
The#2748 is specified on system orders with OS/400 V4R4 when a PCI disk controller is required. The #2748 controller supports up to a maximum of 15 disk units (depending on system and exp unit configuration). A minimum of four disk units of the same capacity are needed for a valid RAID 5 configuration. A maximum of three arrays are allowed per controller, with a maximum of ten 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. Systems with eight to ten disk units in an array find the parity, for that array, spread across eight disk units. For systems with fewer than eight disk units in an array, and later upgraded to eight, nine, or ten disk units, the RAID function must be stopped and then started before parity will be spread across eight disk units. For best performance use disks, disk controllers, and storage expansion devices capable of Ultra2 SCSI performance. OS/400 V4R4 with PTF MF22587 or Cumulative (cum) PTF package C9229440 or later. Prerequisite: High speed PCI card slot. Note: When dealing with card placement within the #5034/#5035: Migration Tower I and #9329: PCI Integrated Expansion Unit, a #2748 can be ordered if it will replace the existing base disk unit controller. In the case of an existing #2741 in the #5034/#5035: Migration Tower I, the #2748 can be ordered but during installation, the #2741 must be removed from the Migration Tower I and plugged into the #9329 and the #2748 would then be plugged into the Migration Tower I side.