oracle data buffer cache

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data buffer cache管理机制浅析

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内容

1. Oracle如何寻找到需要的 buffer?

2. Oracle如何管理 data buffer cache 里面的块？3. Oracle如何确定哪些块应该写入数据文件，如何写？

4. 和 Data buffer cache相关的一些等待事件

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引入

• Instance最大的内存区域• db_cache_size参数• 分配单位： granule • 提供了 default、 keep、 recyle 三种不不同类型的 cache

• 多种数据块尺寸（ 2、 4、 8、 16 或 32k）的 buffer cache对应不同的 blocksize数据块db_nk_cache_size

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定位 buffer

• Buffer 存放的位置： The hash bucket for a particular block header is

determined based on the modulus of the Data Block Address (DBA) and the value of the _DB_BLOCK_HASH_BUCKETS parameter. For example, hash bucket = MOD(DBA, _DB_BLOCK_HASH_BUCKETS).

• 先在 PGA中构造 buffer discriptor内存结构，同需要的锁定模式一起传入搜索函数， hash 算法找到对应的bucket

搜索函数： kcbget(descriptor,lock_mode)

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Buffer cache 示意图

_db_block_hash_buckets_db_block_hash_latches

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BH 结构

视图： X$BH V$BH

BH (0x0x5cfce8c4) file#: 45 rdba: 0x0b417f8f (45/98191) class 1 ba: 0x0x5c73e000

set: 5 dbwrid: 0 obj: 198268 objn: 198268

hash: [6893c08c,6893c08c] lru: [61ff9348,57fee018]

LRU flags: hot_buffer

ckptq: [NULL] fileq: [NULL]

st: XCURRENT md: NULL rsop: 0x(nil) tch: 2

LRBA: [0x0.0.0] HSCN: [0xffff.ffffffff] HSUB: [255] RRBA: [0x0.0.0]

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Hash chain+BH

CHAIN: 4889 LOC: 0x0x5a6e4100 HEAD: [51fdf58c,56fcc874] BH (0x0x51fdf58c) file#: 1 rdba: 0x00402432 (1/9266) class 1 ba:

0x0x51a1a000 set: 12 dbwrid: 0 obj: 3807 objn: 3807 hash: [56fcc874,5a6e4100] lru: [51fdf8c4,51fdeff4] LRU flags: ckptq: [NULL] fileq: [NULL] st: XCURRENT md: NULL rsop: 0x(nil) tch: 7 LRBA: [0x0.0.0] HSCN: [0xffff.ffffffff] HSUB: [255] RRBA: [0x0.0.0] buffer tsn: 0 rdba: 0x00402432 (1/9266) scn: 0x0000.000071f9 seq: 0x01 flg: 0x04 tail: 0x71f90601 frmt: 0x02 chkval: 0x42ef type: 0x06=trans data

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Hash chain上的搜索

For each buffer in the chain:

- Ignore buffers that do not match RDBA

- Wait for READING buffer and return them

- Skip CR (consistent read) buffers

- If the CUR (current) buffer is held in a compatible mode, then use it

- Otherwise if all other users are CR state objects

– Make it a CR copy and create a new EXLCUR copy of the buffer

– Or wait for the current buffer to be released- If no usable buffers exist in cache, read from disk

搜索的时候，需要持有 cache buffer chain latch

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Working sets

_db_block_lru_latches 缺省值为 DBWR进程的数量 ×8（允许的最大的 buffer pool数量）

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Working sets

• Each list that is shown above will have sublists called the auxiliary write list (AUX) and a MAIN list. For example, the LRU-P list will have a LRUP-AUX and a LRUP-MAIN list.

• LRU-XR, LRU-XO and LRU-P are also called write lists.buffers are linked to these due to a specific write action.

• these lists are candidates for immediate write-outs by the DBWR.

• enable write prioritization capabilities• 一个 BH只能在 LRU或 LRUW上，但能存在多个 write

list上

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working sets

Dump of buffer cache at level 10 (WS) size: 501 wsid: 1 state: 0 (WS_REPL_LIST) main_prev: 5a6ff9bc main_next: 5a6ff9bc aux_prev: 58fffd08 aux_next:

58fa4048curnum: 501 auxnum: 501cold: 5a6ff9bc hbmax: 0 hbufs: 0 (WS_WRITE_LIST) main_prev: 5a6ff9d8 main_next: 5a6ff9d8 aux_prev: 5a6ff9e0 aux_next:

5a6ff9e0curnum: 0 auxnum: 0 (WS_XOBJ_LIST) main_prev: 5a6ff9f4 main_next: 5a6ff9f4 aux_prev: 5a6ff9fc aux_next:

5a6ff9fccurnum: 0 auxnum: 0 (WS_XRNG_LIST) main_prev: 5a6ffa10 main_next: 5a6ffa10 aux_prev: 5a6ffa18 aux_next:

5a6ffa18curnum: 0 auxnum: 0 (WS) fbwanted: 0 (WS) bgotten: 0 sumwrt: 0 sumscan: 0 (WS) numscan: 0 hotscan: 0 dmoves: 0MAIN RPL_LST Queue header (NEXT_DIRECTION)[NULL]MAIN RPL_LST Queue header (PREV_DIRECTION)[NULL]AUXILIARY RPL_LST Queue header (NEXT_DIRECTION)[58fa4048,58fffd08]0x58fa4000=>0x58fa42f0=>0x58fa45e0=>0x58fa48d0=>0x58fa4bc0=>0x58fa4eb0=>0x58fa51a0=>0

x58fa54900x58fa5780=>0x58fa5a70=>0x58fa5d60=>0x58fa6050=>0x58fa6340=>0x58fa6630=>0x58fa6920=>

0x58fa6c100x58fa6f00=>0x58fa71f0=>0x58fa74e0=>0x58fa77d0=>0x58fa7ac0=>0x58fa7db0=>0x58fa80a0=>0

x58fa8390

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确定可重用 buffer的过程

• 8i:从 list的尾端开始 scan，将冷端的buffer head所指向的内容牺牲掉

• 9i:当查询所需要的块需要从磁盘读进来，挂在 lru链上时，

1. 从 list的尾端开始 scan，先扫描辅 list，再扫描主 list

2. lru算法 +touch count数3. 热块往主 list上移动，从中插入主 list。辅 list上空了之后，执行相同的算法在 Lru中找出可牺牲的块，换到辅list上

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LRU算法

IF ( touch count of scanned buffer >_db_aging_hot_criteria ) THENGive buffer another chance (do not select as a victim)IF (_db_aging_stay_count >= _db_aging_hot_criteria) THENHalve the buffer's touch countELSESet the buffer's touch count to _db_aging_stay_countEND IFELSE Select buffer as a victimEND IF

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LRUW

• LRUW list:

The LRU-W (write) list is used to hold buffers that aged out of the LRU but need to be written to disk before they can be reused.

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block 更改的过程

• Update block的过程：假如我要修改 BH2 指向的块的内容 1)oracle会将 BH2从辅助 LRU链表上摘下，同时插入主 LRU链表的中间，也就是插入 BH1和 BH4中间，同时增加 BH2的 touch的数量。2) 将该 BH2的标记设置为钉住（ ping）。（ latch保护）3) 更新 BH2对应的内存数据块的内容。4) 更新完以后，取消钉住的标记。5) 将 BH2从主 LRU链表转移到主 LRUW链表上。6) 如果这个时候又有进程发出更新 BH2所对应的内存数据块的内容，则 BH2再次被钉住，更新，取消钉住。7) DBWR启动以后，在扫描主 LRUW链表时会将 BH2转移到辅助LRUW链表上。

8) DBWR将辅助 LRUW链表上的 BH2对应的数据块写入数据文件。9) 确认成功写入数据文件以后，将 BH2从辅助 LRUW链表上转移到辅助 LRU链表上

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checkpoint

Checkpoints（ checkpoint queue latch）• To ensure that the data blocks that have their redo

generated up to a certain point in the redo log (RBA) are written to the disk

• Checkpoint structure includes:

– Checkpoint SCN

– Checkpoint RBA

– Thread that allocated the checkpoint

– Enabled thread bitmap

– Timestamp

关于 DDL:

– The Oracle server ensures that the DDL is successfully mini-checkpointed before the DROP (which depends on the SCN and seq# of the data blocks within the object).

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CKPTQ & FQ

Pre-Oracle8 DBWR scanned the entire cache to find buffers with checkpoint bit set.

CKPTQ and FQs eliminate this scan.

– When the buffer is first modified, it is inserted into the CKPTQ in RBA order

– The buffer is also inserted into the appropriate FQ

When a checkpoint is initiated, DBWR writes all buffers on the queue until the checkpoint RBA is less than the head of the CKPTQ RBA.

全量检查点发生条件：发出命令： alter system checkpoint；除了 shutdown abort以外的正常关闭数据库。

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DBWR

触发条件：1.Lru链上扫描以查找可以覆盖的 buffer header时，如果已经扫描的 buffer header的数量到达一定的限度（由隐藏参数： _db_block_max_scan_pct决定，我的库中是 40 ）

2.当 DBWR在主 LRUW链表上查找已经更新完而正在等待被写入数据文件的 buffer header时，如果找到的 buffer header的数量超过一定限度（由隐藏参数：_db_writer_scan_depth_pct 决定 我的库中是 25 ）

3. 如果主 LRUW链表和辅助 LRUW链表上的脏数据块的总数超过一定限度，。该限度由隐藏参数： _db_large_dirty_queue（我的库是 25）决定。

4. 完全检查点时触发 DBWR。5.将表空间设置为离线（ offline）状态时触发 DBWR。6. 发出命令： alter tablespace … begin backup，从而将表空间设置为热备份状态时触发DBWR。7.将表空间设置为只读状态时，触发 DBWR。8.删除对象时（比如删除某个表）会触发 DBWR。

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写数据

DBWR会将要写的脏数据块所对应的 buffer header拷贝到一个名为批量写（ write batch）的结构中。每个 working set所对应的 DBWR进程都可以向该结构里拷贝 buffer header。当 write batch的 buffer header的个数达到一定限额时，才会发生实际的 I/O

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等待事件

buffer busy waits

P1=file# p2=block_id p3= reason code

Reason code 130和 220是最常见

Free buffer waits

If a session spends a lot of time on the free buffer waits event, it is usually due to one or a combination of the following five reasons:

•Inefficient SQL statements

•Not enough DBWR processes

•Slow I/O subsystem

•Delayed block cleanouts.

•Small buffer cache

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等待事件

Latch free

– Cache buffers chain

热点块问题

可以通过 v$session_wait的 p1raw字段来判断 latch free等待事件是否是由于出现了热点块。如果 p1raw保持一致，那么说明 session在等待同一个 latch 地址，系统存在热点块。

– Cache buffers lru chains– Checkpoint queue latch