[20181203]改变文件大小与检查点.txt

--//昨天听别人讲课,提到改变文件大小会触发文件级检查点.我记得以前我做过类似的测试,仅仅改小才会触发,

--//改大不会触发的,我找了半天找到原来我写的链接:http://blog.itpub.net/267265/viewspace-1353454/

--//重复测试看看.

1.环境:

SCOTT@book> @ ver1

PORT_STRING                    VERSION        BANNER

------------------------------ -------------- --------------------------------------------------------------------------------

x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SYS@book> SELECT file#, CHECKPOINT_CHANGE#, CREATION_CHANGE#  , RESETLOGS_CHANGE#,status, CHECKPOINT_COUNT  FROM v$datafile_header where file#=6;

       FILE# CHECKPOINT_CHANGE# CREATION_CHANGE# RESETLOGS_CHANGE# STATUS  CHECKPOINT_COUNT

------------ ------------------ ---------------- ----------------- ------- ----------------

           6        13815328041      13276257767            925702 ONLINE               293

--//当前大小40M.

$ ls -l /mnt/ramdisk/book/tea01.dbf

-rw-r----- 1 oracle oinstall 41951232 2018-12-03 10:30:06 /mnt/ramdisk/book/tea01.dbf

--//40*1024*1024+8192 = 41951232,前面有1个os文件头,占8k.

2.测试:

SYS@book> alter database datafile '/mnt/ramdisk/book/tea01.dbf' resize 41m;

Database altered.

SYS@book> SELECT file#, CHECKPOINT_CHANGE#, CREATION_CHANGE#  , RESETLOGS_CHANGE#,status, CHECKPOINT_COUNT  FROM v$datafile_header where file#=6;

       FILE# CHECKPOINT_CHANGE# CREATION_CHANGE# RESETLOGS_CHANGE# STATUS  CHECKPOINT_COUNT

------------ ------------------ ---------------- ----------------- ------- ----------------

           6        13815328041      13276257767            925702 ONLINE               293

--//可以发现增加没有变化.CHECKPOINT_CHANGE#=13815328041.

SYS@book> alter database datafile '/mnt/ramdisk/book/tea01.dbf' resize 40m;

Database altered.

SYS@book> SELECT file#, CHECKPOINT_CHANGE#, CREATION_CHANGE#  , RESETLOGS_CHANGE#,status, CHECKPOINT_COUNT  FROM v$datafile_header where file#=6;

       FILE# CHECKPOINT_CHANGE# CREATION_CHANGE# RESETLOGS_CHANGE# STATUS  CHECKPOINT_COUNT

------------ ------------------ ---------------- ----------------- ------- ----------------

           6        13815330712      13276257767            925702 ONLINE               294

--//改变文件大小变小后,CHECKPOINT_CHANGE#发生了变化.CHECKPOINT_CHANGE#=13815330712,CHECKPOINT_COUNT也增加1.

--//我之所以重提这个,许多人想当然(也包括自己),只有亲自验证在加上自己的理解,才能更加深刻.