年归档：2016

oracle asm disk格式化恢复—格式化为ext4文件系统

发表于 2016 年 6 月 17 日 由 惜分飞

昨天中午接到一位朋友紧急求救电话,大概场景如下,asm data磁盘组一共把个asm disk,但是使用4个lun实现的(也就是说每个lun使用fdisk进行分区),该主机上还有一个lun是用来存放备份的挂载在/xifenfei目录.客户记得他们的/xifenfei目录是/dev/sdh1这个分区,上次主机重启之后,rac工作正常,但是/xifenfei这个目录对应的/dev/sdh1无法挂载上去(提示无法找到超级块).然后我这位朋友上去也不管三七二一直接执行了mkfs.ext4 /dev/sdh1.结果整个asm 磁盘组异常了.最后他通过fdisk命令一看发现完蛋了,以前的/dev/sdh 已经变为了/dev/sdc,而现在的/dev/sdh是以前的asm disk.也就是说,他把asm 磁盘组中的一个disk进行了格式化为ext4文件系统操作.
ext4文件系统大概结构
通过这里大概可以发现在格式化为ext4文件系统并非把所有的磁盘数据全部重写主要就是覆盖一些ext4必要的一些元数据信息,理论上没有覆盖部分的数据依旧可以恢复
ext4

fdisk 分析现在磁盘情况

[root@db3 ~]#  fdisk -l

Disk /dev/sda: 171.8 GB, 171798691840 bytes
255 heads, 63 sectors/track, 20886 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00055b12

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          64      512000   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              64        8223    65536000   82  Linux swap / Solaris
/dev/sda3            8223       20887   101723136   8e  Linux LVM

Disk /dev/sdb: 751.6 GB, 751619276800 bytes
255 heads, 63 sectors/track, 91379 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00b6888f

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1       91379   734001786    5  Extended
/dev/sdb5               1       15000   120487437   83  Linux

Disk /dev/sdd: 1073 MB, 1073741824 bytes
34 heads, 61 sectors/track, 1011 cylinders
Units = cylinders of 2074 * 512 = 1061888 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00083b68

   Device Boot      Start         End      Blocks   Id  System

Disk /dev/sdf: 526.1 GB, 526133493760 bytes
255 heads, 63 sectors/track, 63965 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000c10d8

   Device Boot      Start         End      Blocks   Id  System
/dev/sdf1               1       13054   104856223+  83  Linux
/dev/sdf2           13055       63965   408942607+  83  Linux

Disk /dev/sde: 1073 MB, 1073741824 bytes
34 heads, 61 sectors/track, 1011 cylinders
Units = cylinders of 2074 * 512 = 1061888 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000b120f

   Device Boot      Start         End      Blocks   Id  System

Disk /dev/sdc: 1795.3 GB, 1795296329728 bytes
255 heads, 63 sectors/track, 218265 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x78c377f7

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1               1      218265  1753213581   83  Linux

Disk /dev/sdg: 526.1 GB, 526133493760 bytes
255 heads, 63 sectors/track, 63965 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000ee648

   Device Boot      Start         End      Blocks   Id  System
/dev/sdg1               1       13054   104856223+  83  Linux
/dev/sdg2           13055       63965   408942607+  83  Linux

Disk /dev/sdi: 526.1 GB, 526133493760 bytes
255 heads, 63 sectors/track, 63965 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00027e4b

   Device Boot      Start         End      Blocks   Id  System
/dev/sdi1               1       13054   104856223+  83  Linux
/dev/sdi2           13055       63965   408942607+  83  Linux

Disk /dev/sdh: 526.1 GB, 526133493760 bytes
255 heads, 63 sectors/track, 63965 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000938dc

   Device Boot      Start         End      Blocks   Id  System
/dev/sdh1               1       13054   104856223+  83  Linux
/dev/sdh2           13055       63965   408942607+  83  Linux

Disk /dev/mapper/rootvg-rootfs: 31.7 GB, 31708938240 bytes
255 heads, 63 sectors/track, 3855 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000


Disk /dev/mapper/rootvg-lv01: 32.2 GB, 32212254720 bytes
255 heads, 63 sectors/track, 3916 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

通过asm alert日志确定asm disk信息
这里可以确定asm disk是VOL01-VOL08一共8个盘

SQL> CREATE DISKGROUP DATA EXTERNAL REDUNDANCY  DISK '/dev/oracleasm/disks/VOL01',
'/dev/oracleasm/disks/VOL02',
'/dev/oracleasm/disks/VOL03',
'/dev/oracleasm/disks/VOL04' ATTRIBUTE 'compatible.asm'='11.2.0.0.0','au_size'='1M' /* ASMCA */ 

SQL> ALTER DISKGROUP DATA ADD  DISK '/dev/oracleasm/disks/VOL05' SIZE 399358M ,
'/dev/oracleasm/disks/VOL06' SIZE 399358M ,
'/dev/oracleasm/disks/VOL07' SIZE 399358M ,
'/dev/oracleasm/disks/VOL08' SIZE 399358M /* ASMCA */ 

SQL> ALTER DISKGROUP ALL MOUNT /* asm agent call crs *//* {0:0:2} */ 
NOTE: Diskgroup used for Voting files is:
 	 DATA
Diskgroup with spfile:DATA
Diskgroup used for OCR is:DATA
NOTE: cache registered group DATA number=1 incarn=0x6f3f6eb8
NOTE: cache began mount (not first) of group DATA number=1 incarn=0x6f3f6eb8
NOTE: Assigning number (1,7) to disk (/dev/oracleasm/disks/VOL08)
NOTE: Assigning number (1,6) to disk (/dev/oracleasm/disks/VOL07)
NOTE: Assigning number (1,5) to disk (/dev/oracleasm/disks/VOL06)
NOTE: Assigning number (1,4) to disk (/dev/oracleasm/disks/VOL05)
NOTE: Assigning number (1,3) to disk (/dev/oracleasm/disks/VOL04)
NOTE: Assigning number (1,1) to disk (/dev/oracleasm/disks/VOL02)
NOTE: Assigning number (1,2) to disk (/dev/oracleasm/disks/VOL03)
NOTE: Assigning number (1,0) to disk (/dev/oracleasm/disks/VOL01)
GMON querying group 1 at 2 for pid 23, osid 32026
NOTE: cache opening disk 0 of grp 1: DATA_0000 path:/dev/oracleasm/disks/VOL01
NOTE: cache opening disk 1 of grp 1: DATA_0001 path:/dev/oracleasm/disks/VOL02
NOTE: cache opening disk 2 of grp 1: DATA_0002 path:/dev/oracleasm/disks/VOL03
NOTE: cache opening disk 3 of grp 1: DATA_0003 path:/dev/oracleasm/disks/VOL04
NOTE: cache opening disk 4 of grp 1: DATA_0004 path:/dev/oracleasm/disks/VOL05
NOTE: cache opening disk 5 of grp 1: DATA_0005 path:/dev/oracleasm/disks/VOL06
NOTE: cache opening disk 6 of grp 1: DATA_0006 path:/dev/oracleasm/disks/VOL07
NOTE: cache opening disk 7 of grp 1: DATA_0007 path:/dev/oracleasm/disks/VOL08
NOTE: cache mounting (not first) external redundancy group 1/0x6F3F6EB8 (DATA)

查询asm对应的disk信息
这部分信息,可以确定VOL03被格式化,而且应该就是朋友反馈的被他格式化成ext4的/dev/sdh1
asm1

[root@db3 disks]# ls
VOL01  VOL02  VOL04  VOL05  VOL06  VOL07  VOL08
[root@db3 disks]# oracleasm querydisk -p 'VOL01'
Disk "VOL01" is a valid ASM disk
/dev/sdf1: LABEL="VOL01" TYPE="oracleasm" 
[root@db3 disks]# oracleasm querydisk -p 'VOL02'
Disk "VOL02" is a valid ASM disk
/dev/sdg1: LABEL="VOL02" TYPE="oracleasm" 
[root@db3 disks]# oracleasm querydisk -p 'VOL03'
Disk "VOL03" does not exist or is not instantiated
[root@db3 disks]# oracleasm querydisk -p 'VOL04'
Disk "VOL04" is a valid ASM disk
/dev/sdi1: LABEL="VOL04" TYPE="oracleasm" 
[root@db3 disks]# oracleasm querydisk -p 'VOL05'
Disk "VOL05" is a valid ASM disk
/dev/sdf2: LABEL="VOL05" TYPE="oracleasm" 
[root@db3 disks]# oracleasm querydisk -p 'VOL06'
Disk "VOL06" is a valid ASM disk
/dev/sdg2: LABEL="VOL06" TYPE="oracleasm" 
[root@db3 disks]# oracleasm querydisk -p 'VOL07'
Disk "VOL07" is a valid ASM disk
/dev/sdh2: LABEL="VOL07" TYPE="oracleasm" 
[root@db3 disks]# oracleasm querydisk -p 'VOL08'
Disk "VOL08" is a valid ASM disk
/dev/sdi2: LABEL="VOL08" TYPE="oracleasm"

通过kfed分析被格式化成ext4的磁盘信息

[grid@db3 ~]$ kfed read /dev/sdh1
kfbh.endian:                          0 ; 0x000: 0x00
kfbh.hard:                            0 ; 0x001: 0x00
kfbh.type:                            0 ; 0x002: KFBTYP_INVALID
kfbh.datfmt:                          0 ; 0x003: 0x00
kfbh.block.blk:                       0 ; 0x004: blk=0
kfbh.block.obj:                       0 ; 0x008: file=0
kfbh.check:                   810307429 ; 0x00c: 0x304c4f65
kfbh.fcn.base:                        0 ; 0x010: 0x00000000
kfbh.fcn.wrap:                        0 ; 0x014: 0x00000000
kfbh.spare1:                          0 ; 0x018: 0x00000000
kfbh.spare2:                          0 ; 0x01c: 0x00000000
000000000 00000000 00000000 00000000 304C4F65  [............eOL0]
000000010 00000000 00000000 00000000 00000000  [................]
000000020 4C43524F 44524C43 00000000 00000000  [ORCLCLRD........]
000000030 00000000 00000000 00000000 00000000  [................]
  Repeat 252 times
KFED-00322: Invalid content encountered during block traversal: [kfbtTraverseBlock][Invalid OSM block type][][0]


[grid@db3 ~]$ kfed read /dev/sdh1 aun=1
kfbh.endian:                          0 ; 0x000: 0x00
kfbh.hard:                          129 ; 0x001: 0x81
kfbh.type:                            0 ; 0x002: KFBTYP_INVALID
kfbh.datfmt:                          0 ; 0x003: 0x00
kfbh.block.blk:                   98560 ; 0x004: blk=98560
kfbh.block.obj:                  164096 ; 0x008: file=164096
kfbh.check:                      229632 ; 0x00c: 0x00038100
kfbh.fcn.base:                   295168 ; 0x010: 0x00048100
kfbh.fcn.wrap:                   819456 ; 0x014: 0x000c8100
kfbh.spare1:                     884992 ; 0x018: 0x000d8100
kfbh.spare2:                    1605888 ; 0x01c: 0x00188100
000100000 00008100 00018100 00028100 00038100  [................]
000100010 00048100 000C8100 000D8100 00188100  [................]
000100020 00288100 003E8100 00798100 00AB8100  [..(...>...y.....]
000100030 01388100 016C8100 00000000 00000000  [..8...l.........]
000100040 00000000 00000000 00000000 00000000  [................]
  Repeat 251 times
KFED-00322: Invalid content encountered during block traversal: [kfbtTraverseBlock][Invalid OSM block type][][0]


[grid@db3 ~]$ kfed read /dev/sdh1 aun=1 blkn=254
kfbh.endian:                        254 ; 0x000: 0xfe
kfbh.hard:                          129 ; 0x001: 0x81
kfbh.type:                            0 ; 0x002: KFBTYP_INVALID
kfbh.datfmt:                          0 ; 0x003: 0x00
kfbh.block.blk:                   98814 ; 0x004: blk=98814
kfbh.block.obj:                  164350 ; 0x008: file=164350
kfbh.check:                      229886 ; 0x00c: 0x000381fe
kfbh.fcn.base:                   295422 ; 0x010: 0x000481fe
kfbh.fcn.wrap:                   819710 ; 0x014: 0x000c81fe
kfbh.spare1:                     885246 ; 0x018: 0x000d81fe
kfbh.spare2:                    1606142 ; 0x01c: 0x001881fe
0001FE000 000081FE 000181FE 000281FE 000381FE  [................]
0001FE010 000481FE 000C81FE 000D81FE 001881FE  [................]
0001FE020 002881FE 003E81FE 007981FE 00AB81FE  [..(...>...y.....]
0001FE030 013881FE 016C81FE 00000000 00000000  [..8...l.........]
0001FE040 00000000 00000000 00000000 00000000  [................]
  Repeat 251 times
KFED-00322: Invalid content encountered during block traversal: [kfbtTraverseBlock][Invalid OSM block type][][0]


[grid@db3 ~]$ kfed read /dev/sdh1 aun=9
kfbh.endian:                          0 ; 0x000: 0x00
kfbh.hard:                            0 ; 0x001: 0x00
kfbh.type:                            0 ; 0x002: KFBTYP_INVALID
kfbh.datfmt:                          0 ; 0x003: 0x00
kfbh.block.blk:                       0 ; 0x004: blk=0
kfbh.block.obj:                       0 ; 0x008: file=0
kfbh.check:                           0 ; 0x00c: 0x00000000
kfbh.fcn.base:                        0 ; 0x010: 0x00000000
kfbh.fcn.wrap:                        0 ; 0x014: 0x00000000
kfbh.spare1:                          0 ; 0x018: 0x00000000
kfbh.spare2:                          0 ; 0x01c: 0x00000000
000900000 00000000 00000000 00000000 00000000  [................]
  Repeat 255 times
KFED-00322: Invalid content encountered during block traversal: [kfbtTraverseBlock][Invalid OSM block type][][0]

从这里可以看出来该asm的元数据信息大量被破坏(asm 备份的磁盘头也损坏),直接修复的可能性基本上为0,现在为了最大限度的恢复数据,通过底层扫描asm au结合数据库block信息,进行重组出来数据文件(这次的恢复有点难度,因为该asm磁盘组中有两个数据库,需要通过结合asm file+datafile来区分文件)
恢复出来两套库的数据文件信息

/orabak/xifenfei/oradb/1.dbf
/orabak/xifenfei/oradb/2.dbf
/orabak/xifenfei/oradb/3.dbf
/orabak/xifenfei/oradb/4.dbf
/orabak/xifenfei/oradb/5.dbf
/orabak/xifenfei/oradb/6.dbf
/orabak/xifenfei/oradb/7.dbf
/orabak/xifenfei/oradb/8.dbf
/orabak/xifenfei/oradb/9.dbf
/orabak/xifenfei/oradb/10.dbf
/orabak/xifenfei/oradb/11.dbf
/orabak/xifenfei/oradb/12.dbf
/orabak/xifenfei/oradb/13.dbf
/orabak/xifenfei/oradb/14.dbf
/orabak/xifenfei/oradb/15.dbf
/orabak/xifenfei/oradb/16.dbf
/orabak/xifenfei/oradb/17.dbf
/orabak/xifenfei/oradb/18.dbf
/orabak/xifenfei/oradb/19.dbf

/orabak/xifenfei/xgdb/1.dbf
/orabak/xifenfei/xgdb/2.dbf
/orabak/xifenfei/xgdb/3.dbf
/orabak/xifenfei/xgdb/4.dbf
/orabak/xifenfei/xgdb/5.dbf
/orabak/xifenfei/xgdb/6.dbf
/orabak/xifenfei/xgdb/7.dbf
/orabak/xifenfei/xgdb/8.dbf
/orabak/xifenfei/xgdb/9.dbf
/orabak/xifenfei/xgdb/10.dbf
/orabak/xifenfei/xgdb/11.dbf
/orabak/xifenfei/xgdb/12.dbf
/orabak/xifenfei/xgdb/13.dbf
/orabak/xifenfei/xgdb/14.dbf
/orabak/xifenfei/xgdb/15.dbf
/orabak/xifenfei/xgdb/16.dbf

然后使用工具拷贝出来redo信息,最后通过重建控制文件.其中一套库完美直接open,另外一套库system中的c_obj$被覆盖,不过使用一个多月以前的备份的system文件强制打开库成功,数据基本上完美导出,实现完美恢复.由于在格式化为ext4的时候,会在磁盘中部分位置写入一些
数据文件恢复参考:asm disk header 彻底损坏恢复
另外有一次win平台类似恢复经历:asm disk格式化为ntfs恢复
如果您遇到此类情况,无法解决请联系我们，提供专业ORACLE数据库恢复技术支持
Phone:17813235971    Q Q:107644445QQ咨询惜分飞    E-Mail:dba@xifenfei.com

发表在 Oracle ASM, 非常规恢复 | 标签为 , , , , , | 评论关闭

oracle dul 11 正式发布

发表于 2016 年 6 月 13 日 由 惜分飞

千呼万唤oracle dul 11终于发布了,迫不及待的下载来测试,现在版本号为dul 11.2.0.0.1,目前只发布了for linux,其他版本估计要等等.期待该版本有引进新功能
oracle dul 11发布
dul-11.2
oracle-dul-11

dul 11 测试恢复oracle 12c版本

[root@ora1221 dul11]# ./dul

Data UnLoader: 11.2.0.0.1 - Internal Only - on Mon Jun 13 00:04:50 2016
with 64-bit io functions

Copyright (c) 1994 2016 Bernard van Duijnen All rights reserved.

 Strictly Oracle Internal Use Only


DUL: Warning: Recreating file "dul.log"
Found db_id = 1414800805
Found db_name = ORCL

DUL> bootstrap;
Probing file = 1, block = 520
. unloading table                BOOTSTRAP$
DUL: Warning: block number is non zero but marked deferred trying to process it anyhow
      60 rows unloaded
DUL: Warning: Dictionary cache DC_BOOTSTRAP is empty
Reading BOOTSTRAP.dat 60 entries loaded
Parsing Bootstrap$ contents
DUL: Warning: Recreating file "dict.ddl"
Generating dict.ddl for version 12
 OBJ$: segobjno 18, file 1 block 240
 TAB$: segobjno 2, tabno 1, file 1  block 144
 COL$: segobjno 2, tabno 5, file 1  block 144
 USER$: segobjno 10, tabno 1, file 1  block 208
Running generated file "@dict.ddl" to unload the dictionary tables
. unloading table                      OBJ$   72547 rows unloaded
. unloading table                      TAB$    2137 rows unloaded
. unloading table                      COL$  115645 rows unloaded
. unloading table                     USER$     129 rows unloaded
Reading USER.dat 129 entries loaded
Reading OBJ.dat
DUL: FATAL Error: File OBJ.dat, line 22094: identifier too long

dul 11 恢复oracle 12c 报错分析
看报错OBJ.dat报错数据,从定义上判断是长度过长

[root@ora1221 dul11]# vi OBJ.dat 
  22091 "22435" "" "0" "JVMFCB" "" "9" ""
  22092 "22436" "" "0" "JVMFCB" "" "11" ""
  22093 "22437" "56417" "0" "JAVA$CBOBJ$" "" "2" ""
----  22094 "22439" "" "0" "sun/util/xml/PlatformXmlPropertiesProvider$Resolver" "" "29" ""
  22095 "22440" "" "0" "sun/util/xml/PlatformXmlPropertiesProvider$EH" "" "29" ""
  22096 "22441" "" "0" "sun/util/xml/PlatformXmlPropertiesProvider$1" "" "29" ""
  22097 "22442" "" "0" "sun/util/resources/LocaleNamesBundle" "" "29" ""

对象名长度分析

[oracle@ora1221 ~]$ ss

SQL*Plus: Release 12.2.0.0.3 Production on Fri Aug 7 02:50:35 2015

Copyright (c) 1982, 2016, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.0.3 - 64bit Production

SQL> select length(object_name) from dba_objects 
  2  where object_name='sun/util/xml/PlatformXmlPropertiesProvider$Resolver';

LENGTH(OBJECT_NAME)
-------------------
                 51

SQL> select max(length(object_name)) from dba_objects;

MAX(LENGTH(OBJECT_NAME))
------------------------
                     123

SQL> desc dba_objects
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OWNER                                              VARCHAR2(128)
 OBJECT_NAME                                        VARCHAR2(128)
 SUBOBJECT_NAME                                     VARCHAR2(128)
 OBJECT_ID                                          NUMBER
 DATA_OBJECT_ID                                     NUMBER
 OBJECT_TYPE                                        VARCHAR2(23)
 CREATED                                            DATE
 LAST_DDL_TIME                                      DATE
 TIMESTAMP                                          VARCHAR2(19)
 STATUS                                             VARCHAR2(7)
 TEMPORARY                                          VARCHAR2(1)
 GENERATED                                          VARCHAR2(1)
 SECONDARY                                          VARCHAR2(1)
 NAMESPACE                                          NUMBER
 EDITION_NAME                                       VARCHAR2(128)
 SHARING                                            VARCHAR2(18)
 EDITIONABLE                                        VARCHAR2(1)
 ORACLE_MAINTAINED                                  VARCHAR2(1)
 APPLICATION                                        VARCHAR2(1)
 DEFAULT_COLLATION                                  VARCHAR2(100)
 DUPLICATED                                         VARCHAR2(1)
 SHARDED                                            VARCHAR2(1)
 CREATED_APPID                                      NUMBER
 CREATED_VSNID                                      NUMBER
 MODIFIED_APPID                                     NUMBER
 MODIFIED_VSNID                                     NUMBER

在oracle 12c中对象名长度已经变为128,但是dul依旧不能支持列长度，既然作者已经在init.dul的Compatible参数曾增加了对12c的支持,那对这样明显的bug应该在不久的将来即可修复。当然这个问题,可以通过一些人工技巧绕过去

发表在 非常规恢复 | 标签为 , , | 评论关闭

_use_single_log_writer和_max_outstanding_log_writes

发表于 2016 年 6 月 7 日 由 惜分飞

SCALABLE LGWR是12cR1中引入的一个令人激动的特性， 这是由于在OLTP环境中LGWR写日志往往成为系统的主要性能瓶颈， 如果LGWR进程能像DBWR(DBW0~DBWn)那样多进程（LGNN）写出redo到LOGFILE那么就可能大幅释放OLTP的并发能力，增长Transcation系统的单位时间事务处理能力。这里在12.2版本中进行测试，确定_use_single_log_writer和_max_outstanding_log_writes参数对于SCALABLE LGWR特性的影响
数据库版本

SQL> select * from v$version;

BANNER                                                                                         CON_ID
------------------------------------------------------------------------------------------ ----------
Oracle Database 12c Enterprise Edition Release 12.2.0.0.3 - 64bit Production                        0
PL/SQL Release 12.2.0.0.3 - Production                                                              0
CORE    12.2.0.0.3      Production                                                                  0
TNS for Linux: Version 12.2.0.0.3 - Production                                                      0
NLSRTL Version 12.2.0.0.3 - Production                                                              0

_use_single_log_writer和_max_outstanding_log_writes默认值

SQL> /
Enter value for param: _use_single_log_writer
old   6:    and upper(a.ksppinm) LIKE upper('%&param%')
new   6:    and upper(a.ksppinm) LIKE upper('%_use_single_log_writer%')

NAME                                                 VALUE                    DESCRIPTION
---------------------------------------------------- ------------------------ -------------------------------------------
_use_single_log_writer                               ADAPTIVE                 Use a single process for redo log writing

SQL> /
Enter value for param: _max_outstanding_log_writes
old   6:    and upper(a.ksppinm) LIKE upper('%&param%')
new   6:    and upper(a.ksppinm) LIKE upper('%_max_outstanding_log_writes%')

NAME                                                 VALUE                    DESCRIPTION
---------------------------------------------------- ------------------------ ----------------------------------------------
_max_outstanding_log_writes                          2                        Maximum number of outstanding redo log writes

lg进程数量
这里可以看出来，有一个lgwr进程，两个lg进程和_max_outstanding_log_writes参数配置匹配

[oracle@ora1221 ~]$ ps -ef|grep ora_lg
oracle    49790      1  0 10:32 ?        00:00:00 ora_lgwr_orcl12c2
oracle    49794      1  0 10:32 ?        00:00:00 ora_lg00_orcl12c2
oracle    49798      1  0 10:32 ?        00:00:00 ora_lg01_orcl12c2

修改_max_outstanding_log_writes参数
通过修改_max_outstanding_log_writes参数为4，发现lg进程数量也变为了4，证明_max_outstanding_log_writes进程决定lg进程数量

SQL> alter system set "_max_outstanding_log_writes"=4 ;     
alter system set "_max_outstanding_log_writes"=4
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified


SQL> alter system set "_max_outstanding_log_writes"=4 scope=spfile;

System altered.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup
ORACLE instance started.

Total System Global Area 2516582400 bytes
Fixed Size                  8260048 bytes
Variable Size             671090224 bytes
Database Buffers         1828716544 bytes
Redo Buffers                8515584 bytes
Database mounted.
Database opened.
SQL> exit
Disconnected from Oracle Database 12c Enterprise Edition Release 12.2.0.0.3 - 64bit Production
[oracle@ora1221 ~]$ ps -ef|grep lg
oracle    72339      1  0 13:45 ?        00:00:00 ora_lgwr_orcl12c2
oracle    72343      1  0 13:45 ?        00:00:00 ora_lg00_orcl12c2
oracle    72347      1  0 13:45 ?        00:00:00 ora_lg01_orcl12c2
oracle    72351      1  0 13:45 ?        00:00:00 ora_lg02_orcl12c2
oracle    72359      1  0 13:45 ?        00:00:00 ora_lg03_orcl12c2

修改_use_single_log_writer参数
通过测试_use_single_log_writer参数修改，我们可以确定_use_single_log_writer修改为true后，数据库恢复到12c之前的lgwr管理方式

[oracle@ora1221 ~]$ ss

SQL*Plus: Release 12.2.0.0.3 Production on Thu Aug 6 13:45:33 2015

Copyright (c) 1982, 2016, Oracle.  All rights reserved.


Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.0.3 - 64bit Production

SQL> alter system set "_use_single_log_writer"=1 ;     
alter system set "_use_single_log_writer"=1
                 *
ERROR at line 1:
ORA-02095: specified initialization parameter cannot be modified


SQL>  alter system set "_use_single_log_writer"=1 scope=spfile;
 alter system set "_use_single_log_writer"=1 scope=spfile
*
ERROR at line 1:
ORA-00096: invalid value 1 for parameter _use_single_log_writer, must be from
among ADAPTIVE, FALSE, TRUE


SQL> alter system set "_use_single_log_writer"=TRUE scope=spfile;

System altered.

SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> exit
Disconnected from Oracle Database 12c Enterprise Edition Release 12.2.0.0.3 - 64bit Production
[oracle@ora1221 ~]$ ps -ef|grep lg
oracle    72702  71510  0 13:46 pts/0    00:00:00 grep lg
[oracle@ora1221 ~]$ ss

SQL*Plus: Release 12.2.0.0.3 Production on Thu Aug 6 13:46:50 2015

Copyright (c) 1982, 2016, Oracle.  All rights reserved.

Connected to an idle instance.

SQL> startup
ORACLE instance started.

Total System Global Area 2516582400 bytes
Fixed Size                  8260048 bytes
Variable Size             671090224 bytes
Database Buffers         1828716544 bytes
Redo Buffers                8515584 bytes
Database mounted.
Database opened.
SQL> exit   
Disconnected from Oracle Database 12c Enterprise Edition Release 12.2.0.0.3 - 64bit Production
[oracle@ora1221 ~]$ ps -ef|grep lg
oracle    72754      1  0 13:46 ?        00:00:00 ora_lgwr_orcl12c2
oracle    73008  71510  0 13:47 pts/0    00:00:00 grep lg

从这里可以确定_use_single_log_writer确定是否启用SCALABLE LGWR(多个lg子进程),_max_outstanding_log_writes确定lg进程个数

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