Thediskpagebuffer
table provides statistics about disk page buffer usage by NDB Cluster Disk Data tables.
Thediskpagebuffer
table contains the following columns:
node_id
The data node ID
block_instance
Block instance
pages_written
Number of pages written to disk.
pages_written_lcp
Number of pages written by local checkpoints.
pages_read
Number of pages read from disk
log_waits
Number of page writes waiting for log to be written to disk
page_requests_direct_return
Number of requests for pages that were available in buffer
page_requests_wait_queue
Number of requests that had to wait for pages to become available in buffer
page_requests_wait_io
Number of requests that had to be read from pages on disk (pages were unavailable in buffer)
Notes
You can use this table with NDB Cluster Disk Data tables to determine whetherDiskPageBufferMemory
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 fromDiskPageBufferMemory
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 forDiskPageBufferMemory
needs to be increased in theconfig.ini
file.
A change inDiskPageBufferMemory
requires a rolling restart of all of the cluster's data nodes before it takes effect.
block_instance
refers to an instance of a kernel block. Together with the block name, this number can be used to look up a given instance in thethreadblocks
table. Using this information, you can obtain information about disk page buffer metrics relating to individual threads; an example query usingLIMIT 1
to limit the output to a single thread is shown here:
mysql> SELECT > node_id, thr_no, block_name, thread_name, pages_written, > pages_written_lcp, pages_read, log_waits, > page_requests_direct_return, page_requests_wait_queue, > page_requests_wait_io > FROM ndbinfo.diskpagebuffer > INNER JOIN ndbinfo.threadblocks USING (node_id, block_instance) > INNER JOIN ndbinfo.threads USING (node_id, thr_no) > WHERE block_name = 'PGMAN' LIMIT 1\G *************************** 1. row *************************** node_id: 1 thr_no: 1 block_name: PGMAN thread_name: rep pages_written: 0 pages_written_lcp: 0 pages_read: 1 log_waits: 0 page_requests_direct_return: 4 page_requests_wait_queue: 0 page_requests_wait_io: 1 1 row in set (0.01 sec)