Understanding the Database cache page in NDS iMonitor

Understanding the Database cache page in NDS iMonitor

Database Cache Allows you to configure and monitor the cache segments being used by the database subsystem of eDirectory.  You can get to this page by going to http: //<server ip>:8008/nds/agent?config=CacheCtl.

The information provided on this page helps you determine whether you have an adequate cache size and suggests how much should be allocated. For most applications of the directory, the default amount and settings for the cache are adequate.

The Database Cache page is divided into three sections. The items in the sections are discussed in the following subsections:

Database Information

Database Information DIB Size (KB): The size in KB of the NDS database and associated files on disk. Stream files are part of the associated files but are never cached. These should be subtracted from the DIB size to compute the size of the eDirectory database.

DB Block Size (KB): The size in KB of one database block on disk.

Database Cache Information

Maximum Size (KB):  The maximum size in KB to which the specified cache is allowed to grow

Current Size (KB):  The current size in KB of the specified cache.

Items Cached:  The number of items in the specified cache.

Old Versions Cached:  The number of old versions in the specified cache. Old versions of cache items are kept to maintain the consistency of read transactions in the database. If one thread is in a read transaction and another is in a write transaction, old versions of blocks modified by the writer are maintained on behalf of the reader so that the reader's results are guaranteed to produce a consistent view during the life of its transaction even though modifications are taking place during that time.

Old Versions Size:  The size in KB of the old version items cached.

Database Cache Statistics

Hits:  The number of times an item was successfully accessed from the specified cache.

Hit Looks:  The number of items looked at in the cache before an item was successfully accessed from the specified cache. The Hit Look to Hit Ratio is a measure of cache lookup efficiency. Normally, the ratio should be quite close to 1:1.

Faults:  The number of times an item was not found in the specified cache and had to be obtained in a lower-level cache or from the disk.

Fault Looks:  The number of items looked at in the cache before it was determined that the desired item was not in the specified cache. The Fault Look to Fault Ratio is a measure of cache lookup efficiency. Normally, the ratio should be quite close to 1:1

Requests Services from Cache (%):  This is your cache efficiency.  This is calculated by (Total Hits) / (Total Hits + Total Faults).  Usually this number should be somewhere in the 90's.  If this number is below 90%, you may want to look at how much available memory you have in your server. 

Database Cache Configuration

Database Cache Configuration Two methods, Dynamic Adjust and Hard Limit, are used to specify how much cache to allow the eDirectory database to use. These methods are mutually exclusive.

You can use the Database Information section of this page to determine how much of your data is being cached. The database size divided by the DB block size minus old versions will approximate the maximum number of blocks that could be in the block cache at any given time. The database size can be used to approximate the maximum amount of memory that could possibly be used for cache-if the entire database is in cache.

Because two types of cache are used, each of which will keep old versions, the theoretical maximum cache size would be database size times 4 where database size equals DIB size minus stream files size.

In practice, the maximum usable cache size would approach, but not be less than, 2 times the database size.

On the Database Cache Configuration section of this page, you can view the current cache settings and modify them either temporarily or permanently.

Dynamic Adjust:  Allows the eDirectory database to dynamically adjust the amount of system memory to be used for the cache based on the amount eDirectory determines necessary and the parameters specified below.

Cache Adjust Percentage:  The percentage of available memory allowed to be used for the record and block caches combined.

Cache Size Constraints:  While dynamically adjusting the cache configuration, you should follow the specified constraints. Namely, use no less than the specified amount of memory for the cache and no more than the total amount of available memory minus the specified amount.

Hard Limit:  Allows you to specify the exact amount of system memory to be used for the cache.

Cache Maximum Size: The size in KB of the record and block caches combined.

Block Cache Percentage:  The percentage of the system memory available for caching that should be allocated to the block cache. The remaining percentage is allocated to the record cache.

Cache Adjust Interval:  Interval that only applies when Dynamic Adjust is set. It controls how often the cache size is adjusted based on the specified percentage and constraints.

Cache Cleanup Interval:  Controls how often unused old versions are removed from the cache.

Cache Settings Permanent:  When checked, overwrites any previously saved settings or system defaults with any changes submitted via iMonitor.
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Formerly known as TID# 10082323