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Oracle RAC Study之--Cache Fusion
Oracle RAC Study之--Cache Fusion
Concept of cache fusion
Cache Fusion basically is about fusing the memory buffer cache of multiple instance into one single cache. For example if we have 3 instance in a RAC which is using the same datafiles and each instance is having its own memory buffer cache in there own SGA, then cache fusion will make the database behave as if it has a single instance and the total buffer cache is the sum of buffer cache of all the 3 instance. Below figure shows what I mean.
This behavior is possible because of high speed interconnect existing in the cluster between each instance. Each of instance is connected to other instance using a high-speed interconnect. This makes it possible to share the memory between 2 or more servers. Previously only datafile sharing was possible, now because of interconnect, even the cache memory can be shared.
But how this helps? Well, for example if we have a data block in one of the instance and its updating the block and other instance needs the same data block then this data block can be copied from holding instance buffer cache to requesting instance buffer cache using this high-speed interconnect. This high speed interconnect is a private connection made just for sending data blocks and more by instances. External users cannot use this connection. It is this interconnect which makes multiple server behave like a cluster. These servers are bind together using this interconnect.
Moving further, now we know how the cluster is formed and what is the back bone of cluster and what exactly we call “cache fusion”. Next we will see how cache fusion works. But before that we need to discuss few important headings which is very important to understand.
We will discuss following topics before discussing Cache Fusion
（1）Cache Coherency
（2）Multi-Version consistency model
（3）Resource Co-ordination – Synchronization
（4）Global Cache Service (GCS)
（5）Global Enqueue Service
（6）Global Resource Directory
（7）GCS resource modes and roles
（8）Past Images
（9）Block access modes and buffer states
I promise this wont be too heavy. Lets look into the overview of these concepts. I wont be going into the details, just sufficient for you to understand cache fusion.
案例分析：
数据库环境：Oracle 10g RAC
[root@aix211 /]#crs_stat -t
Name Type Target State Host ------------------------------------------------------------ ora....11.lsnr application ONLINE ONLINE aix211 ora.aix211.gsd application ONLINE ONLINE aix211 ora.aix211.ons application ONLINE ONLINE aix211 ora.aix211.vip application ONLINE ONLINE aix211 ora....12.lsnr application ONLINE ONLINE aix212 ora.aix212.gsd application ONLINE ONLINE aix212 ora.aix212.ons application ONLINE ONLINE aix212 ora.aix212.vip application ONLINE ONLINE aix212 ora.prod.db application ONLINE ONLINE aix211 ora....d1.inst application ONLINE ONLINE aix211 ora....d2.inst application ONLINE ONLINE aix212
[oracle@aix211 admin]$sqlplus ‘/as sysdba‘ SQL*Plus: Release 10.2.0.1.0 - Production on Mon Nov 17 15:25:11 2014 Copyright (c) 1982, 2005, Oracle. All rights reserved. Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production With the Partitioning, Real Application Clusters, OLAP and Data Mining options 15:25:11 SYS@ prod1>select instance_name,status from gv$instance; INSTANCE_NAME STATUS ---------------- ------------ prod1 OPEN prod2 OPEN
1、在Instance 1上执行select查询
15:25:26 SYS@ prod1>select instance_name from v$instance; INSTANCE_NAME ---------------- prod1 15:26:06 SYS@ prod1>create table t1 as select * from dba_objects; Table created. 做10046分析： 15:27:51 SYS@ prod1>alter session set events ‘10046 trace name context forever,level 12‘; Session altered. 15:28:20 SYS@ prod1>select /*+ first_time */ count(*) from t1; COUNT(*) ---------- 50392 15:28:55 SYS@ prod1>select /*+ second time */ count(*) from t1; COUNT(*) ---------- 50392 15:30:21 SYS@ prod1>alter session set events ‘10046 trace name context off‘; Session altered. 通过trace后，查看分析结果: select /*+ first_time */ count(*) from t1 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 1 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.04 0.08 279 697 0 1 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.04 0.08 279 698 0 1 Misses in library cache during parse: 1 Optimizer mode: ALL_ROWS Parsing user id: SYS Rows Row Source Operation ------- --------------------------------------------------- 1 SORT AGGREGATE (cr=697 pr=279 pw=0 time=81436 us) 50392 TABLE ACCESS FULL T1 (cr=697 pr=279 pw=0 time=1457 us) Elapsed times include waiting on following events: Event waited on Times Max. Wait Total Waited ---------------------------------------- Waited ---------- ------------ SQL*Net message to client 2 0.00 0.00 db file scattered read 42 0.00 0.04 gc cr multi block request 14 0.00 0.01 db file sequential read 8 0.00 0.00 gc cr grant 2-way 6 0.00 0.00 SQL*Net message from client 2 0.00 0.00 ******************************************************************************** select /*+ second time */ count(*) from t1 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 1 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.02 0.01 0 697 0 1 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.02 0.01 0 698 0 1
2、在Instance 2上做同样的操作
15:56:13 SYS@ prod2>select instance_name from v$instance; INSTANCE_NAME ---------------- prod2 15:46:28 SYS@ prod2>alter session set events ‘10046 trace name context forever,level 12‘; Session altered. 15:46:44 SYS@ prod2>select /*+ on_the_other_node */ count(*) from t1; COUNT(*) ---------- 50392 15:47:24 SYS@ prod2>alter session set events ‘10046 trace name context off‘; Session altered. [oracle@aix212@ ~]$ls -lt /u01/app/oracle/admin/prod/udump/|more total 344 -rw-r----- 1 oracle oinstall 3920 Nov 17 15:47 prod2_ora_606458.trc -rw-r----- 1 oracle oinstall 6220 Nov 17 15:39 prod2_ora_393270.trc 查看trace结果： [oracle@aix212@ ~]$tkprof /u01/app/oracle/admin/prod/udump/prod2_ora_606458.trc /home/oracle/trc.txt TKPROF: Release 10.2.0.1.0 - Production on Mon Nov 17 15:48:03 2014 Copyright (c) 1982, 2005, Oracle. All rights reserved. select /*+ on_the_other_node */ count(*) from t1 call count cpu elapsed disk query current rows ------- ------ -------- ---------- ---------- ---------- ---------- ---------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 0 0 0 Fetch 2 0.05 0.35 693 697 0 1 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.05 0.35 693 697 0 1 Misses in library cache during parse: 0 Optimizer mode: ALL_ROWS Parsing user id: SYS Rows Row Source Operation ------- --------------------------------------------------- 1 SORT AGGREGATE (cr=697 pr=693 pw=0 time=356121 us) 50392 TABLE ACCESS FULL T1 (cr=697 pr=693 pw=0 time=12651 us) Elapsed times include waiting on following events: Event waited on Times Max. Wait Total Waited ---------------------------------------- Waited ---------- ------------ SQL*Net message to client 2 0.00 0.00 gc cr grant 2-way 1 0.00 0.00 db file sequential read 1 0.00 0.00 gc cr multi block request 46 0.00 0.03 db file scattered read 52 0.05 0.27 SQL*Net message from client 2 0.00 0.00 ******************************************************************************** 可以看到，在Instance 2上出现了大量‘gc cr multi block request‘事件，说明大部分数据块是从node 1（Instance 1）上传递过来的。 但也在node 2上有一部分磁盘I/O（disk=693）,说明一部分数据从本地读取。 通过trace源文件，查看： select /*+ on_the_other_node */ count(*) from t1 END OF STMT PARSE #1:c=0,e=339,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,tim=26779237777 BINDS #1: EXEC #1:c=0,e=135,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,tim=26779237991 WAIT #1: nam=‘SQL*Net message to client‘ ela= 3 driver id=1650815232 #bytes=1 p3=0 obj#=-1 tim=26779238042 WAIT #1: nam=‘gc cr grant 2-way‘ ela= 670 p1=1 p2=60945 p3=4 obj#=52686 tim=26779239211 WAIT #1: nam=‘db file sequential read‘ ela= 5910 file#=1 block#=60945 blocks=1 obj#=52686 tim=26779245166 WAIT #1: nam=‘gc cr multi block request‘ ela= 898 file#=1 block#=60952 class#=1 obj#=52686 tim=26779246461 WAIT #1: nam=‘db file scattered read‘ ela= 4083 file#=1 block#=60946 blocks=7 obj#=52686 tim=26779250635 WAIT #1: nam=‘gc cr multi block request‘ ela= 720 file#=1 block#=60960 class#=1 obj#=52686 tim=26779251880 WAIT #1: nam=‘gc cr multi block request‘ ela= 182 file#=1 block#=60960 class#=1 obj#=52686 tim=26779252115 WAIT #1: nam=‘db file scattered read‘ ela= 4897 file#=1 block#=60953 blocks=8 obj#=52686 tim=26779257069 WAIT #1: nam=‘gc cr multi block request‘ ela= 880 file#=1 block#=60968 class#=1 obj#=52686 tim=26779258422 WAIT #1: nam=‘db file scattered read‘ ela= 7796 file#=1 block#=60961 blocks=8 obj#=52686 tim=26779266300 WAIT #1: nam=‘gc cr multi block request‘ ela= 877 file#=1 block#=60976 class#=1 obj#=52686 tim=26779267650 WAIT #1: nam=‘db file scattered read‘ ela= 8525 file#=1 block#=60969 blocks=8 obj#=52686 tim=26779276258 WAIT #1: nam=‘gc cr multi block request‘ ela= 948 file#=1 block#=60984 class#=1 obj#=52686 tim=26779277666 WAIT #1: nam=‘gc cr multi block request‘ ela= 78 file#=1 block#=60984 class#=1 obj#=52686 tim=26779277795 WAIT #1: nam=‘db file scattered read‘ ela= 4645 file#=1 block#=60977 blocks=8 obj#=52686 tim=26779282501 WAIT #1: nam=‘gc cr multi block request‘ ela= 757 file#=1 block#=60992 class#=1 obj#=52686 tim=26779283757 WAIT #1: nam=‘db file scattered read‘ ela= 7600 file#=1 block#=60985 blocks=8 obj#=52686 tim=26779291441 WAIT #1: nam=‘gc cr multi block request‘ ela= 862 file#=1 block#=61000 class#=1 obj#=52686 tim=26779292778 WAIT #1: nam=‘db file scattered read‘ ela= 4774 file#=1 block#=60993 blocks=8 obj#=52686 tim=26779297636 WAIT #1: nam=‘gc cr multi block request‘ ela= 873 file#=1 block#=61008 class#=1 obj#=52686 tim=26779298953 WAIT #1: nam=‘db file scattered read‘ ela= 4615 file#=1 block#=61001 blocks=8 obj#=52686 tim=26779303648 WAIT #1: nam=‘gc cr multi block request‘ ela= 870 file#=1 block#=61016 class#=1 obj#=52686 tim=26779304979 WAIT #1: nam=‘db file scattered read‘ ela= 2009 file#=1 block#=61009 blocks=8 obj#=52686 tim=26779307065 WAIT #1: nam=‘gc cr multi block request‘ ela= 741 file#=1 block#=61024 class#=1 obj#=52686 tim=26779308335 WAIT #1: nam=‘db file scattered read‘ ela= 2562 file#=1 block#=61017 blocks=8 obj#=52686 tim=26779310984 WAIT #1: nam=‘gc cr multi block request‘ ela= 871 file#=1 block#=61032 class#=1 obj#=52686 tim=26779312325 WAIT #1: nam=‘db file scattered read‘ ela= 2115 file#=1 block#=61025 blocks=8 obj#=52686 tim=26779314521 WAIT #1: nam=‘gc cr multi block request‘ ela= 857 file#=1 block#=61040 class#=1 obj#=52686 tim=26779315819 WAIT #1: nam=‘db file scattered read‘ ela= 2060 file#=1 block#=61033 blocks=8 obj#=52686 tim=26779317956 WAIT #1: nam=‘gc cr multi block request‘ ela= 845 file#=1 block#=61048 class#=1 obj#=52686 tim=26779319241 WAIT #1: nam=‘db file scattered read‘ ela= 2770 file#=1 block#=61041 blocks=8 obj#=52686 tim=26779322090 WAIT #1: nam=‘gc cr multi block request‘ ela= 586 file#=1 block#=61055 class#=1 obj#=52686 tim=26779323145 WAIT #1: nam=‘gc cr multi block request‘ ela= 94 file#=1 block#=61054 class#=1 obj#=52686 tim=26779323281 WAIT #1: nam=‘db file scattered read‘ ela= 9974 file#=1 block#=61049 blocks=8 obj#=52686 tim=26779333323 WAIT #1: nam=‘db file scattered read‘ ela= 5632 file#=1 block#=61057 blocks=8 obj#=52686 tim=26779339348 WAIT #1: nam=‘db file scattered read‘ ela= 2081 file#=1 block#=61065 blocks=8 obj#=52686 tim=26779341785 WAIT #1: nam=‘gc cr multi block request‘ ela= 1438 file#=1 block#=61208 class#=1 obj#=52686 tim=26779343792 WAIT #1: nam=‘db file scattered read‘ ela= 4347 file#=1 block#=61193 blocks=16 obj#=52686 tim=26779348250 WAIT #1: nam=‘gc cr multi block request‘ ela= 715 file#=1 block#=61224 class#=1 obj#=52686 tim=26779349686 WAIT #1: nam=‘gc cr multi block request‘ ela= 407 file#=1 block#=61224 class#=1 obj#=52686 tim=26779350147 WAIT #1: nam=‘db file scattered read‘ ela= 3475 file#=1 block#=61209 blocks=16 obj#=52686 tim=26779353704 WAIT #1: nam=‘gc cr multi block request‘ ela= 1351 file#=1 block#=61240 class#=1 obj#=52686 tim=26779355740 WAIT #1: nam=‘db file scattered read‘ ela= 3359 file#=1 block#=61225 blocks=16 obj#=52686 tim=26779359216 WAIT #1: nam=‘gc cr multi block request‘ ela= 713 file#=1 block#=61256 class#=1 obj#=52686 tim=26779360637 WAIT #1: nam=‘gc cr multi block request‘ ela= 403 file#=1 block#=61256 class#=1 obj#=52686 tim=26779361094 WAIT #1: nam=‘db file scattered read‘ ela= 2935 file#=1 block#=61241 blocks=16 obj#=52686 tim=26779364111 WAIT #1: nam=‘gc cr multi block request‘ ela= 1307 file#=1 block#=61272 class#=1 obj#=52686 tim=26779366106 WAIT #1: nam=‘db file scattered read‘ ela= 8885 file#=1 block#=61257 blocks=16 obj#=52686 tim=26779375104 WAIT #1: nam=‘gc cr multi block request‘ ela= 754 file#=1 block#=61288 class#=1 obj#=52686 tim=26779376579 WAIT #1: nam=‘gc cr multi block request‘ ela= 370 file#=1 block#=61288 class#=1 obj#=52686 tim=26779377005 WAIT #1: nam=‘db file scattered read‘ ela= 3442 file#=1 block#=61273 blocks=16 obj#=52686 tim=26779380530 WAIT #1: nam=‘gc cr multi block request‘ ela= 1323 file#=1 block#=61304 class#=1 obj#=52686 tim=26779382541 WAIT #1: nam=‘db file scattered read‘ ela= 2887 file#=1 block#=61289 blocks=16 obj#=52686 tim=26779385540 WAIT #1: nam=‘gc cr multi block request‘ ela= 746 file#=1 block#=61311 class#=1 obj#=52686 tim=26779386964 WAIT #1: nam=‘db file scattered read‘ ela= 3422 file#=1 block#=61305 blocks=16 obj#=52686 tim=26779390477 WAIT #1: nam=‘db file scattered read‘ ela= 3364 file#=1 block#=61321 blocks=16 obj#=52686 tim=26779394422 WAIT #1: nam=‘db file scattered read‘ ela= 3418 file#=1 block#=61337 blocks=16 obj#=52686 tim=26779398408 WAIT #1: nam=‘db file scattered read‘ ela= 7254 file#=1 block#=61353 blocks=16 obj#=52686 tim=26779406214 WAIT #1: nam=‘db file scattered read‘ ela= 3403 file#=1 block#=61369 blocks=16 obj#=52686 tim=26779410176 WAIT #1: nam=‘db file scattered read‘ ela= 7044 file#=1 block#=61385 blocks=16 obj#=52686 tim=26779417779 WAIT #1: nam=‘db file scattered read‘ ela= 7282 file#=1 block#=61401 blocks=16 obj#=52686 tim=26779425628 WAIT #1: nam=‘db file scattered read‘ ela= 3874 file#=1 block#=61417 blocks=16 obj#=52686 tim=26779430050 WAIT #1: nam=‘gc cr multi block request‘ ela= 967 file#=1 block#=61448 class#=1 obj#=52686 tim=26779431766 WAIT #1: nam=‘db file scattered read‘ ela= 3433 file#=1 block#=61433 blocks=16 obj#=52686 tim=26779435300 WAIT #1: nam=‘gc cr multi block request‘ ela= 1315 file#=1 block#=61464 class#=1 obj#=52686 tim=26779437308 WAIT #1: nam=‘db file scattered read‘ ela= 3388 file#=1 block#=61449 blocks=16 obj#=52686 tim=26779440806 WAIT #1: nam=‘gc cr multi block request‘ ela= 727 file#=1 block#=61480 class#=1 obj#=52686 tim=26779442228 WAIT #1: nam=‘gc cr multi block request‘ ela= 419 file#=1 block#=61480 class#=1 obj#=52686 tim=26779442701 WAIT #1: nam=‘db file scattered read‘ ela= 5163 file#=1 block#=61465 blocks=16 obj#=52686 tim=26779447957 WAIT #1: nam=‘gc cr multi block request‘ ela= 1319 file#=1 block#=61496 class#=1 obj#=52686 tim=26779449987 WAIT #1: nam=‘db file scattered read‘ ela= 2761 file#=1 block#=61481 blocks=16 obj#=52686 tim=26779452861 WAIT #1: nam=‘gc cr multi block request‘ ela= 715 file#=1 block#=61512 class#=1 obj#=52686 tim=26779454292 WAIT #1: nam=‘gc cr multi block request‘ ela= 405 file#=1 block#=61512 class#=1 obj#=52686 tim=26779454753 WAIT #1: nam=‘db file scattered read‘ ela= 3434 file#=1 block#=61497 blocks=16 obj#=52686 tim=26779458272 WAIT #1: nam=‘gc cr multi block request‘ ela= 1308 file#=1 block#=61528 class#=1 obj#=52686 tim=26779460269 WAIT #1: nam=‘db file scattered read‘ ela= 2761 file#=1 block#=61513 blocks=16 obj#=52686 tim=26779463140 WAIT #1: nam=‘gc cr multi block request‘ ela= 727 file#=1 block#=61544 class#=1 obj#=52686 tim=26779464570 WAIT #1: nam=‘gc cr multi block request‘ ela= 402 file#=1 block#=61544 class#=1 obj#=52686 tim=26779465029 WAIT #1: nam=‘db file scattered read‘ ela= 3372 file#=1 block#=61529 blocks=16 obj#=52686 tim=26779468487 WAIT #1: nam=‘gc cr multi block request‘ ela= 1337 file#=1 block#=61560 class#=1 obj#=52686 tim=26779470506 WAIT #1: nam=‘db file scattered read‘ ela= 2804 file#=1 block#=61545 blocks=16 obj#=52686 tim=26779473420 WAIT #1: nam=‘gc cr multi block request‘ ela= 741 file#=1 block#=61567 class#=1 obj#=52686 tim=26779474836 WAIT #1: nam=‘db file scattered read‘ ela= 7761 file#=1 block#=61561 blocks=16 obj#=52686 tim=26779482689 WAIT #1: nam=‘db file scattered read‘ ela= 3177 file#=1 block#=61577 blocks=16 obj#=52686 tim=26779486442 WAIT #1: nam=‘db file scattered read‘ ela= 3289 file#=1 block#=61593 blocks=16 obj#=52686 tim=26779490280 WAIT #1: nam=‘db file scattered read‘ ela= 2727 file#=1 block#=61609 blocks=16 obj#=52686 tim=26779493546 WAIT #1: nam=‘db file scattered read‘ ela= 2709 file#=1 block#=61625 blocks=16 obj#=52686 tim=26779496825 WAIT #1: nam=‘db file scattered read‘ ela= 3334 file#=1 block#=61641 blocks=16 obj#=52686 tim=26779500720 WAIT #1: nam=‘db file scattered read‘ ela= 3257 file#=1 block#=61657 blocks=16 obj#=52686 tim=26779504537 WAIT #1: nam=‘db file scattered read‘ ela= 2735 file#=1 block#=61673 blocks=16 obj#=52686 tim=26779507829 WAIT #1: nam=‘gc cr multi block request‘ ela= 954 file#=1 block#=61704 class#=1 obj#=52686 tim=26779509463 WAIT #1: nam=‘db file scattered read‘ ela= 8587 file#=1 block#=61689 blocks=16 obj#=52686 tim=26779518146 WAIT #1: nam=‘gc cr multi block request‘ ela= 1386 file#=1 block#=61720 class#=1 obj#=52686 tim=26779520274 WAIT #1: nam=‘db file scattered read‘ ela= 57988 file#=1 block#=61705 blocks=16 obj#=52686 tim=26779578384 WAIT #1: nam=‘gc cr multi block request‘ ela= 907 file#=1 block#=61736 class#=1 obj#=52686 tim=26779580432 WAIT #1: nam=‘gc cr multi block request‘ ela= 323 file#=1 block#=61736 class#=1 obj#=52686 tim=26779580830 WAIT #1: nam=‘db file scattered read‘ ela= 3431 file#=1 block#=61721 blocks=16 obj#=52686 tim=26779584371 WAIT #1: nam=‘gc cr multi block request‘ ela= 2595 file#=1 block#=61752 class#=1 obj#=52686 tim=26779587917 WAIT #1: nam=‘db file scattered read‘ ela= 3418 file#=1 block#=61737 blocks=16 obj#=52686 tim=26779591482 WAIT #1: nam=‘gc cr multi block request‘ ela= 757 file#=1 block#=61757 class#=1 obj#=52686 tim=26779592898 WAIT #1: nam=‘db file scattered read‘ ela= 928 file#=1 block#=61753 blocks=5 obj#=52686 tim=26779593962 FETCH #1:c=50000,e=356115,p=693,cr=697,cu=0,mis=0,r=1,dep=0,og=1,tim=26779594203 WAIT #1: nam=‘SQL*Net message from client‘ ela= 485 driver id=1650815232 #bytes=1 p3=0 obj#=52686 tim=26779594899 FETCH #1:c=0,e=2,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=0,tim=26779594986 WAIT #1: nam=‘SQL*Net message to client‘ ela= 2 driver id=1650815232 #bytes=1 p3=0 obj#=52686 tim=26779595018 WAIT #1: nam=‘SQL*Net message from client‘ ela= 490 driver id=1650815232 #bytes=1 p3=0 obj#=52686 tim=26779595585 STAT #1 id=1 cnt=1 pid=0 pos=1 obj=0 op=‘SORT AGGREGATE (cr=697 pr=693 pw=0 time=356121 us)‘ STAT #1 id=2 cnt=50392 pid=1 pos=1 obj=52686 op=‘TABLE ACCESS FULL T1 (cr=697 pr=693 pw=0 time=12651 us)‘ WAIT #0: nam=‘SQL*Net message to client‘ ela= 2 driver id=1650815232 #bytes=1 p3=0 obj#=52686 tim=26779595906 *** 2014-11-17 15:53:32.816 WAIT #0: nam=‘SQL*Net message from client‘ ela= 22469814 driver id=1650815232 #bytes=1 p3=0 obj#=52686 tim=26802065757 =====================
这个例子说明，Oracle Cache Fusion技术，主要用于保证同一个数据块的版本一致。比如，一个实例内存中的数据块被修改了，其他的Instance就不能再从磁盘上读取这个数据块，必须使用当前的这个数据块，这样才能保证数据块版本的一致性。
而对于各个实例都是通过只读方式访问数据块，则可以考虑直接从磁盘上读取一部分数据，在有些时候，这样的效率反而会更高一些。
本文出自 “天涯客的blog” 博客，请务必保留此出处http://tiany.blog.51cto.com/513694/1577600
Oracle RAC Study之--Cache Fusion
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