По поводу второго вопроса-бывает когда в промежутке между записями лог был очищен.
По поводу первого-вот объяснение лучше которого не скажешь...
A low Buffer Cache hit rate indicates that SQL Server is finding fewer pages already in memory, and therefore has to perform more disk reads. This is often caused by one of two possibilities:
SQL Server has insufficient memory to work with, or SQL queries are accessing a very large number of pages in a non-sequential manner. The best figure will vary from one application to another, but ideally this ratio should be above 90%.
When this (т.е.меньше 90%) you should:
Ensure SQL Server is configured to use as much physical memory as possible. Check, and if necessary,alter the max server memory (MB) figure in the Configuration.
If you have applications other than SQL Server running on this machine, Total memory shows that SQL Server is not using all the memory it could, then these applications could be taking memory away from SQL Server. Consider increasing your SQL Server Min
Memory parameter setting so that SQL Server gets more memory (Configuration drilldown).
View the largest objects that are in the Buffer Cache. Check for objects that are Pinned and are taking up a large part of the Buffer Cache.Consider adding more physical RAM to the server.Identify inefficient SQL - look for sessions that are active and that are generating a lot of I/Os.Microsoft's SQL Server Profiler tool can help here...