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
4194304 total chains
2999950 hashed buffs
3000000 total buffs
|# of chains
||A count of the number of hash chains for which the length for members of that chain is the same.
||The length of the chains.
||The number of hash chains (buckets) created.
||The number of buffer headers hashed into the hash buckets.
||The number of buffers in the buffer pool.
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.