Buffer Pool

Disk-Oriented DBMS

什么是buffer pool,buffer pool有什么用(针对Disk-Oriented DBMS)

  • The database is all on disk, and the data in the database files is organized into pages.

  • In order to operate on the data the DBMS needs to bring the data into memory. It does this by having a buffer pool that manages the movement back and forth between disk and memory.

  • The DBMS also have an execution engine that will execute queries.

  • The execution engine will ask the buffer pool for a specific page, and the buffer pool will take care of bringing that page into memory and giving the execution engine a pointer to the page in memory.

  • The buffer pool manager will ensure that the page is there while the execution engine is operating on that memory.

Buffer Pool Organization

buffer pool是如何利用内存的,组织数据的方式

  • Memory region organized as an array of fixed-size pages.An array entry is called a frame.
  • When the DBMS requests a page, an exact copy is placed into one of these frames.

Meta-data maintained by the buffer pool

Page Table


  • In-memory page table (hash table) that keeps track of pages that are currently in memory. It maps page ids to frame locations in the buffer pool.

  • 其实就是为了根据page id快速找到对应的page data,还可以判断page id对应的数据有没有在buffer pool

Dirty Flag

  • Threads set this flag when it modifies a page.
  • This indicates to storage manager that the page must be written back to disk.
  • 其实就是判断page data有没有被修改过,如果修改过在某个时候需要将修改写回到磁盘

Pin Counter

  • This tracks the number of threads that are currently accessing that page (either reading or modifying it).
  • A thread has to increment the counter before they access the page.

  • If a page’s count is greater than zero, then the storage manager is not allowed to evict that page from memory.


  • buffer pool:BufferPoolManagerInstance,本次lab需要实现的部分
  • 磁盘读写相关:DiskManager,已经提供,不需要实现
  • class Page
    • Each Page object contains a block of memory that the DiskManager will use as a location to copy the contents of a physical page that it reads from disk. 将磁盘中内容读到内存中,放到page对象中
    • The BufferPoolManagerInstance will reuse the same Page object to store data as it moves back and forth to disk. This means that the same Page object may contain a different physical page throughout the life of the system. 重复利用page object,所以page object里面的内容不是固定不变的
    • The Page object’s identifer (page_id) keeps track of what physical page it contains. page_id代表了里面的数据对应哪个phusical page
    • page具体内容存放在data_,metadata对应上面提到的两个is_dirty_,pin_count_
  • page table:使用自己实现的ExtendibleHashTable类,maps page_id to frame_id

  • buffer pool满了如何evict page:使用LRU-K算法,对应自己实现的LRUKReplacer

Buffer Pool Manager Instance

  • size_t pool_size_代表了buffer pool中能容纳多少个fixed-size page
  • Page *pages_代表了buffer pool中的page,其大小是固定的pages_ = new Page[pool_size_];代表一开始buffer pool有多少个空的page。其实就是下面这个东西,通过frame_id索引到某个page obejct

  • std::list<frame_id_t> free_list_代表有哪些frame_id对应的page object还可以用来存放从磁盘中读取到的page,首先从这里拿,如果free_list满了,代表buffer page中没有多余的空间了,需要evict page到disk从而reuse page承载新的内容(如果可以)

  • ExtendibleHashTable<page_id_t, frame_id_t> *page_table_,根据pageid可以拿到frame_id,`pages[frame_id]`可以拿到buffer pool中的某个page


  1. NewPageImpFetchPageImp从free_list或者LRUKReplacer拿到一个page后:

    1. 如果page是dirty,需要先将其内容写回到disk,ResetMemory,pin_count需要置为0,is_dirty需要置为false
    2. 需要pin_count++
    3. 将该page_id从page_table中移除
    4. 更新page_id(NewPage是新创建一个,FetchPage是传入page_id)
    5. 将page_id和frame_id对应关系存到page_table
    6. 调用replacer的SetEvictable和RecordAccess


  2. FetchPageImp需要先判断pageid对应的数据有没有已经在page table,如果在代表数据在buffer pool已经有了,不需要从disk中拿,直接返回`pages[frame_id]即可。返回前需要pin_count++`并调用replacer的SetEvictable和RecordAccess。

  3. UnpinPgImp传入的dirty代表是否改变dirty flag,如果是false代表维持当前page的dirty flag,所以这里需要if判断一下才赋值
  4. FlushPgImp记得将对应的page的dirty flag置为false
  5. DeletePgImp需要将对应page_id从page table,replacer中移除。需要Reset Memory和metadata。
  6. 从上面可以看出,很多地方需要reset page,所以这里可以抽出一个方法,避免忘记初始化某个东西。