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onstat -h Buffer Hash Chain Length Histogram

The onstat -h command provides information on the buffer header hash chain used to access pages in the buffer pool.

Buffer pool page size: 2048

buffer hash chain length histogram
   # of chains         of len
       3021028              0
        616286              1
        238274              2
        141138              3
         17567              4
         15102              5
         12672              6
         17334              7
         18243              8
         21599              9
         29354             10
         41973             11
          3568             12
           164             13
             2             14
       4194304   total chains
       2999950   hashed buffs
       3000000    total buffs

Output Description

Heading Description Format See Also
# of chains A count of the number of hash chains for which the length for members of that chain is the same. Dec onstat -b
of len The length of the chains. Dec  
total chains The number of hash chains (buckets) created. Dec  
hashed buffs The number of buffer headers hashed into the hash buckets. Dec  
total buffs The number of buffers in the buffer pool. Dec  


In some earlier versions, the onstat -h command does not show up in the normal onstat command usage.

Monitoring and Tuning

Tuning to the hash chain is performed indirectly by tuning the number of buffers. As pages are read into the buffer pool, the page number is hashed to a value for one of the hash buckets. Since there are only a limited number of hash buckets, multiple page numbers will hash to the same value and thus the same hash bucket, creating a chain. Although the onstat -h command doesn't show the length of each individual hash bucket, it does show the total number of hash buckets with a particular length.

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