The sum of all I/O activity for all database files. If this is high:

1. Examine the current physical structure of databases on the server to see if it is possible to reduce I/O load by redistributing the database files to distinct disks.
2. Examine queries and database design to determine if they can be tuned to reduce I/O.

The rate at which SQL Server finds the data blocks it needs in memory rather than having to read from disk for this instance. By itself, the buffer cache hit ratio is not very meaningful except for servers with undersized memory settings. Tuning queries and performing index optimization is the best way to increase buffer cache hit ratios. To see the current metrics for the buffer cache, run the following query:

select * from master..sysperfinfo where object_name like 'Buffer Manager'

The percentage of time when SQL Server looks for an execution plan in the procedure cache and finds it for this instance. If this is low, try to write more reusable code or consider increasing the size of the procedure cache. To see current metrics for the procedure cache, run the following query:

select * from master..sysperfinfo where object_name like '%Plan Cache%';

The number of compilations performed by SQL Server per second. Compilations are a natural part of SQL Server operations but do utilize CPU and other resources. Compare this to the Batch Requests/sec metric to understand if this metric is too high. Minimizing compilations will help overall performance. For more information, see the following Microsoft Knowledgebase article: http://support.microsoft.com/kb/243588.