HP StorageWorks 4000 Compaq RAID Array 4000 Overview - Page 7

Compaq RAID Array 4000 Overview 7 Once system power is restored, the posted-write data will automatically be written to disk. Should the base controller board fail, the small write cache board can be removed and installed on a replacement base controller board, and the posted-write data will automatically be written to disk. The 16-MB read-ahead cache integrated on the RA4000 Controller uses an intelligent read-ahead algorithm that anticipates data needs and reduces latency. The algorithm detects sequential read activity on single or multiple I/O threads and predicts that sequential read requests will follow. The algorithm then reads data from the disk drives before the data is actually requested. When the read request occurs, the data is serviced out of the high-speed cache memory at microsecond speeds rather than from the disk drive at millisecond speeds. When the firmware detects non-sequential read activity, the read-ahead feature is automatically disabled. This adaptive read-ahead scheme provides excellent performance for sequential small block read requests. At the same time, it avoids degradation of random read performance. The ECC technology used on the Array Accelerator provides greater data protection than mirroring and is much more cost effective. The ECC scheme generates 16 bits of check data for every 64 bits of user data. This check data can be used to detect and correct errors. ECC can be used to detect and correct DRAM bit errors, DRAM chip failures, and memory bus errors on the Array Accelerator connector. Compaq StorageWorks Fibre Channel Host Adapter The Fibre Channel Host Adapter is available in either a PCI version or an EISA6 version. The Fibre Channel Host Adapter/P is a PCI bus master device. The Fibre Channel Host Adapter/E is for use in a server equipped only with an EISA expansion bus or with a server that has a majority of EISA bus slots. It takes advantage of the EISA architecture by performing 32 bit bus master burst transfers. Both the PCI and EISA versions of the Fibre Channel Host Adapter require installation of a gigabit interface converter (GBIC) into the I/O port before the multi-mode fiber cable is connected. In single adapter environments, the server can be connected directly to the RA4000 or multiple RA4000s using a storage hub. There are no redundant paths to ensure data availability in this configuration should a component fail. Figure 1 through Figure 4 depict examples of some possible RA4000 configurations. For more detailed information, refer to the appropriate RA4000 manuals. Using custom-length, 50-micron multi-mode cables, interconnect distances between components can be up to 500 meters. Therefore, RA4000s can be located up to 1 kilometer from the Fibre Channel Host Adapter. Using custom-length 9-micron single-mode cables, interconnect distances between components can be up to 10 kilometers. Therefore, RA4000s can be located up to 20 kilometers from the Fibre Channel Host Adapter. 6 Extended Industry Standard Architecture. ECG0205699A