#### 23.6.14.30 The ndbinfo diskpagebuffer Table

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

 The `diskpagebuffer` 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 whether [`DiskPageBufferMemory`](mysql-cluster-ndbd-definition.html#ndbparam-ndbd-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`](mysql-cluster-ndbd-definition.html#ndbparam-ndbd-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`](mysql-cluster-ndbd-definition.html#ndbparam-ndbd-diskpagebuffermemory) needs to be increased in the `config.ini` file.

Note

 A change in [`DiskPageBufferMemory`](mysql-cluster-ndbd-definition.html#ndbparam-ndbd-diskpagebuffermemory) 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 the [`threadblocks`](mysql-cluster-ndbinfo-threadblocks.html) table. Using this information, you can obtain information about disk page buffer metrics relating to individual threads; an example query using `LIMIT 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)
```

