onstat -h command: Print buffer header hash chain information

Use the onstat -h command to display information about the buffer header hash chains (sometimes called "hash buckets") that are used to access pages in each buffer pool.

Figure 1: Syntax:

Example output

The output is displayed in the form of a numeric histogram of chain lengths, with summary information for each buffer pool. All numeric values in the output are decimal. Shorter hash chains enable requested buffers to be located more quickly by the server, because on average it will need to check fewer buffer headers on a target chain to find the target buffer.

The page size of the buffer pool in bytes is shown as a header to the output for each buffer pool. The histogram and summary information are then presented for that buffer pool.

Figure 2: onstat -h command output


Buffer pool page size: 2048

buffer hash chain length histogram
   # of chains         of len
          3423              0
          4546              1
           223              2
          8192   total chains
          4992   hashed buffs
          5000    total buffs

Buffer pool page size: 4096

buffer hash chain length histogram
   # of chains         of len
           707              0
           315              1
             2              2
          1024   total chains
           319   hashed buffs
          1000    total buffs

Output description

Histogram Information on Hash Chains

The histogram information has a row for each buffer hash chain length that presently exists in the system. Each row has two columns:

# of chains: Number of hash chains of the given length
of len: Length of these chains

Summary Information Per Buffer Pool

total chains: Number of hash chains that exist for this buffer pool

hashed buffs: Number of buffer headers currently hashed into the hash chains for this buffer pool

total buffs: Total number of buffers in this buffer pool