当减少buffer pool大小的时候，为后台线程执行，大概操作的步骤为

1、整体删除AHI
2、从free list和LRU中回收需要的page，如果为脏数据需要刷脏数据
3、删除回收的page的内存
4、启动AHI

第一步删除AHI

 [Note] InnoDB: Requested to resize buffer pool. (new size: 12884901888 bytes)
 [Note] InnoDB: Resizing buffer pool from 68719476736 to 12884901888 (unit=134217728).
 [Note] InnoDB: Disabling adaptive hash index.
 [Note] InnoDB: disabled adaptive hash index.

第二步回收page

整个过程中最后可能出现问题的就是第2步，且加锁比较强，比如free list mutex和LRU list mutex，其主要是为了收集到
能够回收的page，如果这个时候压力较高脏数据较多，free list和LRU中不能及时的收集到可以回收的page，那么将会循环
回收，因为加锁严重必定会影响正常session获取page和刷脏等操作，影响系统的并发。这个状态在Innodb_buffer_pool_resize_status中为
Withdrawing blocks to be shrunken。

[Note] InnoDB: Withdrawing blocks to be shrunken.
[Note] InnoDB: buffer pool 0 : start to withdraw the last 425984 blocks.
[Note] InnoDB: buffer pool 0 : withdrawing blocks. (425982/425984)
[Note] InnoDB: buffer pool 0 : withdrew 288532 blocks from free list. Tried to relocate 97721 pages (425982/425984).
[Note] InnoDB: buffer pool 0 : withdrawing blocks. (425984/425984)
[Note] InnoDB: buffer pool 0 : withdrew 0 blocks from free list. Tried to relocate 2 pages (425984/425984).
[Note] InnoDB: buffer pool 0 : withdrawn target 425984 blocks.
[Note] InnoDB: buffer pool 1 : start to withdraw the last 425984 blocks.
[Note] InnoDB: buffer pool 1 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 1 : withdrew 289275 blocks from free list. Tried to relocate 97801 pages (425983/425984).
[Note] InnoDB: buffer pool 1 : withdrawing blocks. (425984/425984)
[Note] InnoDB: buffer pool 1 : withdrew 0 blocks from free list. Tried to relocate 1 pages (425984/425984).
[Note] InnoDB: buffer pool 1 : withdrawn target 425984 blocks.
[Note] InnoDB: buffer pool 2 : start to withdraw the last 425984 blocks.
[Note] InnoDB: buffer pool 2 : withdrawing blocks. (425982/425984)
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).
[Note] InnoDB: buffer pool 5 : withdrawing blocks. (425983/425984)
[Note] InnoDB: buffer pool 5 : withdrew 0 blocks from free list. Tried to relocate 0 pages (425983/425984).

回收过程中出现ERROR，线程池满了，这是因为回收page，加锁，导致线程被阻塞，不断累加。

 [ERROR] Threadpool could not create additional thread to handle queries, because the number of allowed threads was reached. 
Increasing 'thread_pool_max_threads' parameter can help in this situation.

第三步删除内存

而第3步对应的Innodb_buffer_pool_resize_status状态为Latching whole of buffer pool。

这个步骤会锁定整个buffer pool不允许任何操作但是只是删除内存一般比较快。

[Note] InnoDB: Latching whole of buffer pool.
[Note] InnoDB: buffer pool 0 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 0 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 1 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 1 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 2 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 2 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 3 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 3 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 4 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 4 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 5 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 5 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 6 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 6 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: buffer pool 7 : resizing with chunks 64 to 12.
[Note] InnoDB: buffer pool 7 : 52 chunks (425984 blocks) were freed.
[Note] InnoDB: Resizing hash tables.
[Note] InnoDB: buffer pool 0 : hash tables were resized.
[Note] InnoDB: buffer pool 1 : hash tables were resized.
[Note] InnoDB: buffer pool 2 : hash tables were resized.
[Note] InnoDB: buffer pool 3 : hash tables were resized.
[Note] InnoDB: buffer pool 4 : hash tables were resized.
[Note] InnoDB: buffer pool 5 : hash tables were resized.
[Note] InnoDB: buffer pool 6 : hash tables were resized.
[Note] InnoDB: buffer pool 7 : hash tables were resized.
[Note] InnoDB: page_cleaner: 1000ms intended loop took 5218ms. The settings might not be optimal. (flushed=0 and evicted=0, during the time.)
[Note] InnoDB: Resizing also other hash tables.
[Note] InnoDB: Resized hash tables at lock_sys, adaptive hash index, dictionary.
[Note] InnoDB: Completed to resize buffer pool from 68719476736 to 12884901888.

第四步重启AHI

[Note] InnoDB: Re-enabled adaptive hash index.
[Note] InnoDB: Completed resizing buffer pool at 230616 

整个步骤完成后Innodb_buffer_pool_resize_status状态为Completed resizing buffer pool at ***。
因此建议在低压的情况下进行，也就是数据修改量小的情况下进行。现有资料显示整个resize 减少buffer pool 可能会影响TPS