#### 15.8.3.3 Making the Buffer Pool Scan Resistant

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 Rather than using a strict [LRU](glossary.html#glos_lru) algorithm, `InnoDB` uses a technique to minimize the amount of data that is brought into the [buffer pool](glossary.html#glos_buffer_pool) and never accessed again. The goal is to make sure that frequently accessed (“hot”) pages remain in the buffer pool, even as [read-ahead](glossary.html#glos_read_ahead) and [full table scans](glossary.html#glos_full_table_scan) bring in new blocks that might or might not be accessed afterward.

 Newly read blocks are inserted into the middle of the LRU list. All newly read pages are inserted at a location that by default is `3/8` from the tail of the LRU list. The pages are moved to the front of the list (the most-recently used end) when they are accessed in the buffer pool for the first time. Thus, pages that are never accessed never make it to the front portion of the LRU list, and “age out” sooner than with a strict LRU approach. This arrangement divides the LRU list into two segments, where the pages downstream of the insertion point are considered “old” and are desirable victims for LRU eviction.

 For an explanation of the inner workings of the `InnoDB` buffer pool and specifics about the LRU algorithm, see [Section 15.5.1, “Buffer Pool”](innodb-buffer-pool.html).

 You can control the insertion point in the LRU list and choose whether `InnoDB` applies the same optimization to blocks brought into the buffer pool by table or index scans. The configuration parameter [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) controls the percentage of “old” blocks in the LRU list. The default value of [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) is `37`, corresponding to the original fixed ratio of 3/8. The value range is `5` (new pages in the buffer pool age out very quickly) to `95` (only 5% of the buffer pool is reserved for hot pages, making the algorithm close to the familiar LRU strategy).

 The optimization that keeps the buffer pool from being churned by read-ahead can avoid similar problems due to table or index scans. In these scans, a data page is typically accessed a few times in quick succession and is never touched again. The configuration parameter [`innodb_old_blocks_time`](innodb-parameters.html#sysvar_innodb_old_blocks_time) specifies the time window (in milliseconds) after the first access to a page during which it can be accessed without being moved to the front (most-recently used end) of the LRU list. The default value of [`innodb_old_blocks_time`](innodb-parameters.html#sysvar_innodb_old_blocks_time) is `1000`. Increasing this value makes more and more blocks likely to age out faster from the buffer pool.

 Both [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) and [`innodb_old_blocks_time`](innodb-parameters.html#sysvar_innodb_old_blocks_time) can be specified in the MySQL option file (`my.cnf` or `my.ini`) or changed at runtime with the [`SET GLOBAL`](set-variable.html) statement. Changing the value at runtime requires privileges sufficient to set global system variables. See [Section 5.1.9.1, “System Variable Privileges”](system-variable-privileges.html).

 To help you gauge the effect of setting these parameters, the `SHOW ENGINE INNODB STATUS` command reports buffer pool statistics. For details, see [Monitoring the Buffer Pool Using the InnoDB Standard Monitor](innodb-buffer-pool.html#innodb-buffer-pool-monitoring).

 Because the effects of these parameters can vary widely based on your hardware configuration, your data, and the details of your workload, always benchmark to verify the effectiveness before changing these settings in any performance-critical or production environment.

 In mixed workloads where most of the activity is OLTP type with periodic batch reporting queries which result in large scans, setting the value of [`innodb_old_blocks_time`](innodb-parameters.html#sysvar_innodb_old_blocks_time) during the batch runs can help keep the working set of the normal workload in the buffer pool.

 When scanning large tables that cannot fit entirely in the buffer pool, setting [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) to a small value keeps the data that is only read once from consuming a significant portion of the buffer pool. For example, setting `innodb_old_blocks_pct=5` restricts this data that is only read once to 5% of the buffer pool.

 When scanning small tables that do fit into memory, there is less overhead for moving pages around within the buffer pool, so you can leave [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) at its default value, or even higher, such as `innodb_old_blocks_pct=50`.

 The effect of the [`innodb_old_blocks_time`](innodb-parameters.html#sysvar_innodb_old_blocks_time) parameter is harder to predict than the [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) parameter, is relatively small, and varies more with the workload. To arrive at an optimal value, conduct your own benchmarks if the performance improvement from adjusting [`innodb_old_blocks_pct`](innodb-parameters.html#sysvar_innodb_old_blocks_pct) is not sufficient.

