File cache not only speeds access to file data, it is used to cache portions of the NDS® database. If you want to tune your NetWare server in general, or NDS in particular, tune the file cache.
File cache buffers are organized into a linked list. Most recently used (MRU) buffers are at the head of the list and least recently used (LRU) buffers are at the tail of the list. The length of time the oldest buffer has been in the list is shown as the LRU Sitting Time statistic in MONITOR's Cache Utilization Statistics screen (MONITOR > Available Options > Disk Cache Utilization.)
In tuning file cache, the goal is to determine how much memory the server needs so that recently accessed data can always be retrieved from cache. Monitoring the LRU Sitting Time statistic is key to tuning file cache.
The LRU Sitting Time statistic (MONITOR > Available Options > Disk Cache Utilization), is updated and displayed once per second. The statistic is calculated by taking the difference between the current time and the time stamp of the last block in the cache list. (The time stamp is the time at which the last block entered the list.) The result is displayed in HH:MM:SS.0 format. The LRU Sitting Time measures the length of time it takes for a cache buffer at the head of the list to make its way down to the tail, where it becomes the least recently used (LRU) buffer. Every cache buffer in the list is being reused within that period of time.
In configurations with excessive cache, the LRU Sitting Time can be very high, even many hours. In configurations with too little cache, the LRU Sitting Time can be measured in seconds. The time will vary widely, depending on circumstances.
On inactive servers, such as those sitting unused overnight or those in lab environments with long periods of idle time, the LRU Sitting Time statistic is incremented by one second every second. This is because the LRU Sitting Time indicates the age of the LRU cache buffer. The LRU Sitting Time statistic is useless under these circumstances, except to confirm the obvious, that new data is not being written to the server's cache. This statistic is most useful during peak workloads when the server's cache is servicing the greatest number of users and the broadest possible data set.
Follow these steps to determine how much file cache your server needs.
For example, if your server's original low watermark was 7 minutes, you will need to add enough memory to increase the LRU Sitting Time to an average of 12 minutes during peak workloads. This added memory increases the likelihood that repeatedly used data will still be cached when the next user request is received.
On the other hand, if the server's original low watermark was 18 minutes, the server has more than adequate cache. You can leave the excess memory in the server for future growth or remove it and use it somewhere else.
You can control when the server alerts you to serious decreases in the number of available file cache buffers. Use the following server parameters, found in MONITOR > Available Options > Server Parameters > File Caching: