ORA-000845 与 /dev/shm(tempfs)

阿尔萨斯

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MEMORY_TARGET参数在Oracle 11g被引进，主要是用于控制Oracle对于系统内存的使用，首次将SGA与PGA整合到一起实现自动管理。一旦设置了MEMORY_TARGET参数值，Oracle会根据需要自动调整SGA与PGA以合理的分配及使用内存。但如果MEMORY_TARGET设置不当，就容易引起ORA-00845，因为MEMORY_TARGET与/dev/shm(tempfs)息息相关，本文即是对由此引发问题的描述。

1、故障现象
[oracle@linux1 catadb]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Wed Nov 13 21:38:41 2013

Connected to an idle instance.

idle> startup nomount;
ORA-00845: MEMORY_TARGET not supported on this system

2、故障分析
[oracle@linux1 trace]$ oerr ora 00845
00845, 00000, "MEMORY_TARGET not supported on this system"
// *Cause: The MEMORY_TARGET parameter was not supported on this operating system or /dev/shm was not sized correctly on Linux.
// *Action: Refer to documentation for a list of supported operating systems.
Or, size /dev/shm to be at least the SGA_MAX_SIZE on each Oracle instance running on the system.

#从上面的描述可知，/dev/shm的大小应该至少是当前系统每一个实例的SGA_MAX_SIZE，个人认为这个描述不是很准确

#下面查看告警日志
[oracle@linux1 trace]$ more alert_catadb.log
Wed Nov 13 21:38:48 2013
Starting ORACLE instance (normal)
WARNING: You are trying to use the MEMORY_TARGET feature. This feature requires the /dev/shm file system to be mounted for at least
1073741824 bytes. /dev/shm is either not mounted or is mounted with available space less than this size. Please fix this so that
MEMORY_TARGET can work as expected. Current available is 901312512 and used is 161312768 bytes.
Ensure that the mount point is /dev/shm for this directory. memory_target needs larger /dev/shm

#告警日志给出了比较精确的描述，/dev/shm至少是1GB，因为我们的参数文件中定义的MEMORY_TARGET为1GB
#同时也给出了/dev/shm当前已使用的和剩余空间的大小。剩余空间大小901312512不够1GB，因此需要考虑增加dev/shm或减少MEMORY_TARGET的值

#延伸思考
/dev/shm实际上就是对应到tmpfs，也可以称之为临时文件系统(不是块设备)。有点类似于Oracle数据库的临时表空间一样啊，用于加速和优化系统。
我们可以通过df -h来查看到该加载点，该目录并不位于磁盘，而是在内存里，默认情况下，其大小为当前内存的一半，该文件系统会被自动加载。
/dev/shm的分配是按需分配，如我们前面的错误描述，当前分配了1GB，实际使用了154MB，剩余860MB。也就是说这860MB依旧可分配给其他应用程序。
使用tmps文件系统有一些优点，可以实现动态文件系统大小，其次是由于驻留内存，可以闪电加速。重启后被清空，也就是相当于闪存。

#当前系统的/dev/shm
[oracle@linux1 trace]$ df -h | grep shm
tmpfs 1014M 154M 860M 16% /dev/shm
[oracle@linux1 ~]$ ls -hltr /dev/shm
total 154M
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:06 ora_+ASM1_2686978_68
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:06 ora_+ASM1_2686978_39
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:06 ora_+ASM1_2686978_40
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:06 ora_+ASM1_2686978_41
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:06 ora_+ASM1_2686978_42
...............
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:47 ora_+ASM1_2686978_66
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:47 ora_+ASM1_2686978_52
-rw-r----- 1 grid oinstall 4.0M Nov 14 11:47 ora_+ASM1_2686978_67

3、故障解决
从上面的情形可知，我们可以通过增加tempfs文件系统的大小或者是适当减少MEMORY_TARGET的值来解决
a、调整参数memory_target
[oracle@linux1 ~]$ grep memory $ORACLE_HOME/dbs/initcatadb.ora
memory_target=521M
[oracle@linux1 ~]$ export ORACLE_SID=catadb
[oracle@linux1 ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Thu Nov 14 11:51:57 2013

Connected to an idle instance.

idle> startup nomount;
ORACLE instance started.

Total System Global Area 548216832 bytes
Fixed Size 1337804 bytes
Variable Size 331351604 bytes
Database Buffers 209715200 bytes
Redo Buffers 5812224 bytes
idle> shutdown abort;
ORACLE instance shut down.

b、增加tempfs大小
[oracle@linux1 ~]$ grep memory $ORACLE_HOME/dbs/initcatadb.ora
memory_target=1G
#下面的方法用于临时调整tempfs的大小，重启后失效
#即将tempfs的大小调整到1200M，inode数量调整到1000000，此时大约可可存放一百万个小文件
[root@linux1 ~]# mount -o size=1200M -o nr_inodes=1000000 -o noatime,nodiratime -o remount /dev/shm
[root@linux1 ~]# df -h | grep shm
tmpfs 1.2G 154M 1.1G 13% /dev/shm
[root@linux1 ~]# su - oracle
[oracle@linux1 ~]$ export ORACLE_SID=catadb
[oracle@linux1 ~]$ sqlplus / as sysdba

SQL*Plus: Release 11.2.0.1.0 Production on Thu Nov 14 12:04:02 2013

Connected to an idle instance.

idle> startup nomount;
ORACLE instance started.

Total System Global Area 1071333376 bytes
Fixed Size 1341312 bytes
Variable Size 620759168 bytes
Database Buffers 444596224 bytes
Redo Buffers 4636672 bytes

# Author : Leshami
# Blog : http://blog.csdn.net/leshami

#永久增加tempfs的大小，直接修改/etc/fstab
vi /etc/fstab
tmpfs /dev/shm tmpfs defaults,size=2048M 0 0
修改之后remount
mount -o remount /dev/shm

4、Doc ID 1399209.1
Starting with Oracle Database 11g, the Automatic Memory Management feature requires more shared memory (/dev/shm) and file descriptors. The size of the shared memory must be at least the greater of the MEMORY_MAX_TARGET and MEMORY_TARGET parameters for each Oracle instance on the computer. If the MEMORY_MAX_TARGET parameter or the MEMORY_TARGET parameter is set to a nonzero value, and an incorrect size is assigned to the shared memory, it results in an ORA-00845 error at startup.
On Linux systems, if the operating system /dev/shm mount size is too small for the Oracle system global area (SGA) and program global area (PGA), then you encounter the following error:

The cause of this error is an insufficient /dev/shm allocation. The total memory size of the SGA and PGA, which sets the initialization parameter MEMORY_TARGET or MEMORY_MAX_TARGET, cannot be greater than the shared memory file system (/dev/shm) on your operating system.