标签：edit count nts HERE 进程 product 管理 数据库 font
ORACLE 将buffer cache中所有的buffer通过一个内部的Hash算法运算之后，将这些buffer放到不同的 Hash Bucket中。每一个Hash Bucket中都有一个Hash Chain List（也叫：cache buffers chain），通过这个list，将这个Bucket中的block串联起来。
对应每个Bucket，只存在一个Chain，当用户试图搜索Cache Buffer Chain时，必须首先获得Cache Buffer Chain Latch
Hash Chain List可以通过基表x$bh查看 ??
Buffer Header 数据可以从数据库的字典表X$BH中查询得到 ??
SQL> set linesize 50 SQL> desc x$bh Name Null? Type ----------------------- -------- ---------------- ADDR RAW(8) ---block在buffercache中的address INDX NUMBER INST_ID NUMBER CON_ID NUMBER HLADDR RAW(8) --（Hash Chain Latch Address）latch:cache buffers chains 的地址 BLSIZ NUMBER NXT_HASH RAW(8) ---指向同一个Hash Chain List的下一个block地址 PRV_HASH RAW(8) ---指向同一个HashChain List的上一个block地址 NXT_REPL RAW(8) ---指向LRU list中的下一个block地址 PRV_REPL RAW(8) ---指向LRUlist中的上一个block地址 ...... TS# NUMBER FILE# NUMBER ...... TCH NUMBER --表征一个Buffer的访问次数，Buffer访问次数越多，说明该Buffer越“抢手”，也就越可能存在热点块竞争的问题 ......
查询可以获得当前数据库最繁忙的Buffer：
SQL> set linesize 200 SQL> select * from (select addr,ts#,file#,dbarfil,dbablk,tch from x$bh order by tch desc) where rownum<11; ADDR TS# FILE# DBARFIL DBABLK TCH ---------------- ---------- ---------- ---------- ---------- ---------- 00007F8DD59D2718 0 1 1 51224 255 00007F8DD59D2128 0 1 1 51217 255 00007F8DD59D2718 0 1 1 3917 255 00007F8DD59D2128 0 1 1 48233 255 00007F8DD59D5920 0 1 1 21927 255 00007F8DD59D57A0 0 1 1 3915 255 00007F8DD59D2128 0 1 1 20846 255 00007F8DD59D2128 0 1 1 13657 255 00007F8DD59D2718 0 1 1 51200 255 00007F8DD59D22A8 0 1 1 48234 255 10 rows selected.
查询得到这些热点Buffer都来自哪些对象：
SQL> col OWNER for A20 SQL> select e.owner,e.segment_name,e.segment_type from dba_extents e, (select * from (select addr,ts#,file#,dbarfil,dbablk,tch from x$bh order by tch desc) where rownum<11) b where e.relative_fno=b.dbarfil and e.block_id<=b.dbablk and e.block_id+ 2 3 e.blocks>b.dbablk; OWNER SEGMENT_NAME SEGMENT_TYPE -------------------- -------------------------------------------------------------------------------------------------------------------------------- ------------------ SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER SYS C_FILE#_BLOCK# CLUSTER 10 rows selected.
每个Buffer对应x$bh中一条记录，Hash Chain List就是由x$bh中的NXT_HASH，PRV_HASH 这2个指针构成了一个双向链表,通过NXT_HASH，PRV_HASH这2个指针，那么在同一个Hash Chain List的block就串联起来了。
SQL> select BANNER_FULL from v$version; BANNER_FULL ---------------------------------------------------------------------------------------------------------------------------------------------------------------- Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production Version 19.3.0.0.0
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ FROM x$ksppi x,x$ksppcv y WHERE x.inst_id =USERENV (‘Instance‘) AND y.inst_id =USERENV (‘Instance‘) AND x.indx = y.indx AND x.ksppinm LIKE‘%_db_block_hash%‘; NAME VALUE DESCRIB ----------------------------------------------------------------------- _db_block_hash_buckets 262144 Number of database block hash buckets _db_block_hash_latches 2048 Number of database block hash latches SQL> select count(distinct hladdr) from x$bh ; COUNT(DISTINCTHLADDR) --------------------- 2048 SQL> select count(*) from v$latch_children a,v$latchname b where a.latch#=b.latch# and b.name=‘cache buffers chains‘; COUNT(*) ---------- 2048
验证
SQL> select * from(select hladdr,count(*) from x$bh group by hladdr) where rownum<=5; HLADDR COUNT(*) ---------------- ---------- 00000000A80CE028 25 00000000A80CE0F0 31 00000000A80CE1B8 40 00000000A80CE280 18 00000000A80CE348 17 SQL> select hladdr,obj,dbarfil,dbablk,nxt_hash,prv_hash from x$bh where hladdr=‘00000000A80CE028‘ order by obj; HLADDR OBJ DBARFIL DBABLK NXT_HASH PRV_HASH ---------------- ---------- ---------- ---------- ---------------- ---------------- 00000000A80CE028 2 1 12226 00000000A80CEC68 00000000A80CEC68 00000000A80CE028 64 1 114311 00000000A80CE828 00000000A80CE828 00000000A80CE028 119 1 1280 00000000D4F9A130 00000000CEFC4598 00000000A80CE028 119 1 1280 00000000D1FC9E30 00000000A80CEBB8 00000000A80CE028 119 1 1280 00000000D8F84740 00000000D1FC9E30 00000000A80CE028 119 1 1280 00000000A80CEBB8 00000000D4F9A130 00000000A80CE028 378 1 25756 00000000A80CEA38 00000000A80CEA38 00000000A80CE028 385 1 21575 00000000A80CE878 00000000A80CE878 00000000A80CE028 568 1 3864 00000000D9F57B78 00000000D2FC53A8 00000000A80CE028 568 1 3864 00000000D5F995F0 00000000CFFC4700 00000000A80CE028 568 1 3864 00000000D2FC53A8 00000000A80CE8E8 00000000A80CE028 568 1 3864 00000000A80CE8E8 00000000D5F995F0 00000000A80CE028 720 1 115908 00000000A80CECC8 00000000A80CECC8 00000000A80CE028 780 1 14810 00000000E4FB9CD8 00000000A80CE998 00000000A80CE028 20324 1 47648 00000000D0FDF550 00000000A80CEB98 00000000A80CE028 20324 1 47648 00000000D3FAFB20 00000000CDFB50E8 00000000A80CE028 20324 1 47648 00000000D6FEF6A8 00000000D0FDF550 00000000A80CE028 20324 1 47648 00000000A80CEB98 00000000D3FAFB20 00000000A80CE028 73154 3 64962 00000000A80CEBC8 00000000A80CEBC8 00000000A80CE028 73610 3 78492 00000000A80CE998 00000000D6F864C8 00000000A80CE028 74505 3 173812 00000000A80CE678 00000000A80CE678 00000000A80CE028 74952 3 254615 00000000A80CECE8 00000000A80CECE8 00000000A80CE028 74978 3 253018 00000000A80CE848 00000000A80CE848 00000000A80CE028 75239 3 224361 00000000A80CE808 00000000A80CE808 00000000A80CE028 4294967295 4 1752 00000000A80CEAF8 00000000A80CEAF8 25 rows selected.
[ORACLE] oracle Buffer Cache 之Hash Bucket与Hash Chain List（cache bufferschain）
标签：edit count nts HERE 进程 product 管理 数据库 font
原文地址：https://www.cnblogs.com/tingxin/p/12831375.html
