If there's one thing that's true of all busy Exchange servers, it's that they generate massive amounts of disk I/O. There's a joke around here that Exchange is the world's biggest hard disk diagno...
Updated Jul 01, 2019
Version 2.0
Blog Post
Thanks, Mike Smith, for answering the question about setting log buffers in Exchange 2000/3, and my apologies to Donna for referencing an obsolete KB article. An updated article on setting log buffers for Exchange 2000/3 can be found here:
http://support.microsoft.com/?id=328466
Something else to keep in mind when setting the log buffers is that Exchange rounds down whatever you use as a value to the nearest value evenly divisible by 128, with a minimum of 128. Therefore, if you set 500 as the buffer size, the actual number of buffers will be 384. If you set 512, the number of buffers will be 512.
Why then does Exchange set the number to 500 by default? It's a bug, but not a very important one. The functional difference between 384 and 512 buffers is usually negligible.
There have been recommendations made in the past to set the log buffers to 9000. This recommendation does not apply to Exchange 2000 SP3 or later, though there's not much harm in it--you're just wasting some memory.
Each log buffer is 512 bytes in size and buffers one log sector. The maximum value you can set for the buffers is 10240, which will buffer an entire log file (each log file has 10240 sectors in it: 10240 x 512 = 5 MB, which is the file size of a log file).
As a general rule, when playing with the log buffers, start with 512. If your log stalls (mostly) go away from this setting, leave it there. If not, you can increase the buffers in multiples of 128 until the log stalls are relieved. If you get to the max of 10240, and you're still having log stalls, then the next likely suspect for why this is still happening is a disk I/O bottleneck.
FYI, if you have restored from online backup, and you are replaying a large number of transaction log files (thousands of them), you may increase the speed of replay by temporarily changing both the msExchEseParamLogBuffers (to 10240) and msExchEseParamCacheSizeMax (to 307200). Don't forget to set them back to their previous values after you're finished, especially the Cache. If you leave the ESE cache that large (307200 x 4K = 1.2 GB), you may start running out of virtual address space. For more information about memory and buffer tuning for Exchange see this article:
http://support.microsoft.com/?id=815372
http://support.microsoft.com/?id=328466
Something else to keep in mind when setting the log buffers is that Exchange rounds down whatever you use as a value to the nearest value evenly divisible by 128, with a minimum of 128. Therefore, if you set 500 as the buffer size, the actual number of buffers will be 384. If you set 512, the number of buffers will be 512.
Why then does Exchange set the number to 500 by default? It's a bug, but not a very important one. The functional difference between 384 and 512 buffers is usually negligible.
There have been recommendations made in the past to set the log buffers to 9000. This recommendation does not apply to Exchange 2000 SP3 or later, though there's not much harm in it--you're just wasting some memory.
Each log buffer is 512 bytes in size and buffers one log sector. The maximum value you can set for the buffers is 10240, which will buffer an entire log file (each log file has 10240 sectors in it: 10240 x 512 = 5 MB, which is the file size of a log file).
As a general rule, when playing with the log buffers, start with 512. If your log stalls (mostly) go away from this setting, leave it there. If not, you can increase the buffers in multiples of 128 until the log stalls are relieved. If you get to the max of 10240, and you're still having log stalls, then the next likely suspect for why this is still happening is a disk I/O bottleneck.
FYI, if you have restored from online backup, and you are replaying a large number of transaction log files (thousands of them), you may increase the speed of replay by temporarily changing both the msExchEseParamLogBuffers (to 10240) and msExchEseParamCacheSizeMax (to 307200). Don't forget to set them back to their previous values after you're finished, especially the Cache. If you leave the ESE cache that large (307200 x 4K = 1.2 GB), you may start running out of virtual address space. For more information about memory and buffer tuning for Exchange see this article:
http://support.microsoft.com/?id=815372