17.5.10.8 The ndbinfo diskpagebuffer Table

The diskpagebuffer table provides statistics about disk page buffer usage by MySQL Cluster Disk Data tables.

The following table provides information about the columns in the diskpagebuffer table. For each column, the table shows the name, data type, and a brief description. Additional information can be found in the notes following the table.

Column NameTypeDescription
node_idintegerThe data node ID
block_instanceintegerBlock instance
pages_writtenintegerNumber of pages written to disk.
pages_written_lcpintegerNumber of pages written by local checkpoints.
pages_readintegerNumber of pages read from disk
log_waitsintegerNumber of page writes waiting for log to be written to disk
page_requests_direct_returnintegerNumber of requests for pages that were available in buffer
page_requests_wait_queueintegerNumber of requests that had to wait for pages to become available in buffer
page_requests_wait_iointegerNumber of requests that had to be read from pages on disk (pages were unavailable in buffer)

You can use this table with MySQL Cluster Disk Data tables to determine whether DiskPageBufferMemory is sufficiently large to allow data to be read from the buffer rather from disk; minimizing disk seeks can help improve performance of such tables.

You can determine the proportion of reads from DiskPageBufferMemory to the total number of reads using a query such as this one, which obtains this ratio as a percentage:

SELECT  
  node_id, 
  100 * page_requests_direct_return / 
    (page_requests_direct_return + page_requests_wait_io) 
      AS hit_ratio 
FROM ndbinfo.diskpagebuffer;

The result from this query should be similar to what is shown here, with one row for each data node in the cluster (in this example, the cluster has 4 data nodes):

+---------+-----------+
| node_id | hit_ratio |
+---------+-----------+
|       5 |   97.6744 |
|       6 |   97.6879 |
|       7 |   98.1776 |
|       8 |   98.1343 |
+---------+-----------+
4 rows in set (0.00 sec)

hit_ratio values approaching 100% indicate that only a very small number of reads are being made from disk rather than from the buffer, which means that Disk Data read performance is approaching an optimum level. If any of these values are less than 95%, this is a strong indicator that the setting for DiskPageBufferMemory needs to be increased in the config.ini file.

Note

A change in DiskPageBufferMemory requires a rolling restart of all of the cluster's data nodes before it takes effect.