ORA-000845 与 /dev/shm(tempfs)

    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

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

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

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

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

Copyright (c) 1982, 2009, Oracle.  All rights reserved.

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.