Compaq ProLiant 6000 Performance of Microsoft Exchange Server 5.0 on Compaq Pr - Page 17

WHITE PAPER (cont.) NOTE: The SMART-2 cache can be tuned for WRITE or READ performance, although READ tuning will be beneficial only in applications that perform sequential READs. NOTE: There has been no measured benefit to adding additional system RAM and increasing IS Buffers beyond 20,000. Although adding additional buffers has been seen to relieve disk I/O, current architecture limits in Exchange Server prevent taking advantage of these additional resources. ECG052.0897 1...7 • The disk subsystem becomes a bottleneck when the disk queue length exceeds from three to five I/Os outstanding. Memory Subsystem System Memory plays a crucial role in the performance of Windows NT and NT applications. It is very important to have enough system memory in an Exchange Server computer. For best performance, both Compaq and Microsoft recommend designing your system for NO operating system paging. In other words, the server should have enough physical memory to accommodate the application requirements in addition to enough free memory for Windows NT. Excessive paging results in drastic performance degradation due to unnecessary disk I/O activity. The key memory resource area in Microsoft Exchange Server is IS Buffers. IS Buffers are the 4K pages that Exchange Server uses to cache information from the information stores. Rather than retrieving information from disk and incurring disk I/O, Exchange Server uses IS Buffers for better performance. Exchange Server does not use standard NT caching services. During installation of Exchange Server, the Performance Optimizer is run to tune the Exchange Server based on the available hardware resources and performance characteristics. When tuning IS Buffers, PerfWiz (as it is called) can select a maximum of 20,000 IS Buffers by default. Therefore, Exchange Server can utilize a maximum of 80 MB of system memory for IS Buffers (20K * 4K per buffer). The 20,000-buffer limit is imposed by current Exchange architecture and will be addressed in the next release of Exchange Server. Effect of Increasing IS Buffers Aside from the amount of memory required for running Windows NT and Exchange Server, system memory and disk performance are interrelated. This is because the IS buffers are allocated from system memory. A small IS buffer pool will constrain disk I/O, whereas a large IS buffer pool will tend to reduce the disk I/O demands on the disk subsystem. The following points should be considered: • Adding extra IS Buffers produces significant benefit, especially at higher user levels. This is because the extra IS buffers relieve the disk subsystem of a certain amount of I/O load. • Most of the benefit from extra IS Buffers will be from READs. All WRITEs have to make it to the disk sooner or later, so the net number of WRITEs does not change greatly. However, significant reductions in READ I/O are observed. • Depending on the cost of RAM versus the cost of extra disks, adding RAM may be a costeffective alternative to adding disks to a RAID set in order to increase performance. Effect of Increasing IS Buffers Too Much It is possible to manually set the IS Buffers using the PerfWiz "-v" option. If the IS buffers are increased by too much without adding extra RAM, you will cause the system to become memory constrained and start paging. This will defeat the entire purpose because response time will degrade as a result. IS Buffers should be increased (up to a maximum of 20,000 buffers) based on free memory available in the system, not simply as a result of adding more RAM. Be careful not to take away memory from system processes. There should be at least 10 to 15 MB of free memory in the system at all times. It is always safe to run Exchange Optimizer to get a good recommendation for setting the number of IS Buffers based on system memory.