HP ProLiant 4500 Disk Subsystem Performance and Scalability - Page 14

ECG025.0997 WHITE PAPER (cont.) ... For instance, let's say you request data stored on your disk drive (refer to Figure 4 for reference). The system first tries to complete the READ request by retrieving the data from the file system cache (memory). If it is not there, the system has to retrieve the data from the hard disk. Figure 5 shows the communications that take place to retrieve data from the disk. Operating system checks to see if the Yes requested data is in memory. No Operating system sends the request through the I/O bus (i.e., PCI bus) to the disk controller. Operating system receives the data, passes it to the requesting application and keeps a copy of the request in its cache. Disk controller keeps a copy of the data in its cache and passes the data through the I/O bus (i.e., PCI bus). Disk controller Yes checks its cache (i.e., Array Accelerator) for the data. No Disk controller sends the request across the SCSI bus to the physical disk drives. Drive transfers the data across the SCSI bus to the disk controller. Drive head(s) seek to the track where the data is located and reads the data. Figure 5: Retrieving data from the hard disk. Now let's examine the same scenario if the requested information were located in file system cache (refer to Figure 5 if necessary). The operating system checks to see if the requested data is in memory (i.e., File system cache). Operating system passes the requested information to the application. As you can conclude from the flowchart example, the more information stored in memory the faster the system can access the requested data. Thus, if you are retrieving data from memory, the speed of the Host Bus will influence system performance. 14