InnoDB Buffer Pool MYSQL
InnoDB Buffer Pool MYSQL ,
At the head, a sublist of “new” (or “young”) blocks that have been recently used.
At the tail, a sublist of “old” blocks that are less recently used.
As a result of the algorithm, the new sublist contains blocks that are heavily used by queries. The old sublist contains less-used blocks, and candidates for eviction are taken from this sublist.
The LRU algorithm operates as follows by default:
3/8 of the buffer pool is devoted to the old sublist.
The midpoint of the list is the boundary where the tail of the new sublist meets the head of the old sublist.
When InnoDB reads a block into the buffer pool, it initially inserts it at the midpoint (the head of the old sublist). A block can be read in as a result of two types of read requests: Because it is required (for example, to satisfy query execution), or as part of read-ahead performed in anticipation that it will be required.
The first access to a block in the old sublist makes it “young”, causing it to move to the head of the buffer pool (the head of the new sublist). If the block was read in because it was required, the first access occurs immediately and the block is made young. If the block was read in due to read-ahead, the first access does not occur immediately (and might not occur at all before the block is evicted).
As long as no accesses occur for a block in the pool, it “ages” by moving toward the tail of the list. Blocks in both the new and old sublists age as other blocks are made new. Blocks in the old sublist also age as blocks are inserted at the midpoint. Eventually, a block that remains unused for long enough reaches the tail of the old sublist and is evicted.