环境：RHEL5.5 + Oracle 10.2.0.4
此错误一般是因为数据库的初始化参数文件的内存设置不当导致。本例是因为操作系统参数设置问题导致。
1.当前现象：Oracle启动报错ORA-27102
```
[oracle@JYDB1 ~]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on 星期四 7月 30 19:55:10 2015
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
已连接到空闲例程。
SQL> startup
ORA-27102: out of memory
Linux-x86_64 Error: 28: No space left on device
<h1 id="2">2.检查各参数的配置情况</h1>
## 2.1 根据当前的spfile文件创建pfile文件 ##
SQL> create pfile='/tmp/pfile0730.bak' from spfile;
文件已创建。
查看当前的初始化参数配置信息：
[oracle@JYDB1 dbs]$ more /tmp/pfile0730.bak
JYZHAO.__db_cache_size=10938744832
JYZHAO.__java_pool_size=117440512
JYZHAO.__large_pool_size=16777216
JYZHAO.__shared_pool_size=1442840576
JYZHAO.__streams_pool_size=33554432
*.audit_file_dest='/opt/app/oracle//admin/JYZHAO/adump'
*.background_dump_dest='/opt/app/oracle//admin/JYZHAO/bdump'
*.compatible='10.2.0.3.0'
*.control_files='/usr3/oradata1/sysdata/control_file/control01.ctl','/usr3/oradata1/sysdata/control_file/control02.ctl','
/usr3/oradata1/sysdata/control_file/control03.ctl'
*.core_dump_dest='/opt/app/oracle//admin/JYZHAO/cdump'
*.db_block_size=8192
*.db_domain=''
*.db_file_multiblock_read_count=16
*.db_files=2048
*.db_name='JYZHAO'
*.dispatchers='(PROTOCOL=TCP) (SERVICE=JYZHAOXDB)'
*.job_queue_processes=10
*.nls_language='SIMPLIFIED CHINESE'
*.nls_territory='CHINA'
*.open_cursors=300
*.pga_aggregate_target=1572864000
*.processes=600
*.remote_login_passwordfile='EXCLUSIVE'
*.resource_limit=TRUE
*.sessions=665
*.sga_target=12582912000
*.undo_management='AUTO'
*.undo_tablespace='UNDOTBS1'
*.user_dump_dest='/opt/app/oracle//admin/JYZHAO/udump'
可以看到，sga=12G，pga=1.5G，
## 2.2 用pfile文件启动得到相同报错 ##
startup pfile='/tmp/pfile0730.bak'
SQL> startup pfile='/tmp/pfile0730.bak'
ORA-27102: out of memory
Linux-x86_64 Error: 28: No space left on device
SQL> !
## 2.3 检查主机的内存，/dev/shm，ipcs ##
### 2.3.1 内存空闲充足 ###
[oracle@JYDB1 10.2.0]$ free -g
total used free shared buffers cached
Mem: 31 0 30 0 0 0
-/+ buffers/cache: 0 31
Swap: 31 0 31
### 2.3.2 /dev/shm设置为16G，符合当前需求 ###
[oracle@JYDB1 10.2.0]$ df -h /dev/shm
文件系统 容量 已用 可用 已用% 挂载点
tmpfs 16G 0 16G 0% /dev/shm
### 2.3.3 ipcs -a也没有未释放的共享内存 ###
[oracle@JYDB1 10.2.0]$ ipcs -a
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
------ Semaphore Arrays --------
key semid owner perms nsems
------ Message Queues --------
key msqid owner perms used-bytes messages
查到这里我们发现系统的硬件完全可以支持sga=12G，pga=1.5G的配置。
但此时实验了下，大概是只能以sga=6.5G的大小启动数据库。sga再大都会报错：ORA-27102。
## 2.4 检查系统配置文件/etc/sysctl.conf ##
more /etc/sysctl.conf
fs.aio-max-nr = 1048576
fs.file-max = 6815744
kernel.shmall = 2097152
kernel.shmmax = 25769803776
kernel.shmmni = 4096
kernel.sem = 250 32000 100 128
net.ipv4.ip_local_port_range = 9000 65500
net.core.rmem_default = 262144
net.core.rmem_max = 4194304
net.core.wmem_default = 262144
net.core.wmem_max = 1048586
可以看到是按照11g 安装文档建议配置项配置的，其中kernel.shmmax是根据主机内存的75%计算来的。其他参数没有改变。
因为这里的环境是Oracle 10g，所以我们还是按照10g官档的建议，修改为10g版本的安装文档配置项：
kernel.shmall = 2097152
kernel.shmmax = 25769803776
kernel.shmmni = 4096
kernel.sem = 250 32000 100 128
fs.file-max = 65536
net.ipv4.ip_local_port_range = 1024 65000
net.core.rmem_default = 262144
net.core.rmem_max = 262144
net.core.wmem_default = 262144
net.core.wmem_max = 262144
sysctl -p 生效配置后，此时尝试启动数据库，结果很不幸，依旧报错ORA-27102。
<h1 id="3">3.定位解决问题</h1>
Google了一下，发现还有可能是kernel.shmall这个参数导致的。
查看当前的shmmax和shmall配置
[root@JYDB1 ~]# cat /proc/sys/kernel/shmmax
25769803776
[root@JYDB1 ~]# cat /proc/sys/kernel/shmall
2097152
注释掉新增的kernel.shmall参数的配置（原/etc/sysctl.conf中有kernel.shmall的配置值为4294967296）
kernel.shmall = 2097152
kernel.shmmax = 25769803776
kernel.shmmni = 4096
kernel.sem = 250 32000 100 128
fs.file-max = 65536
net.ipv4.ip_local_port_range = 1024 65000
net.core.rmem_default = 262144
net.core.rmem_max = 262144
net.core.wmem_default = 262144
net.core.wmem_max = 262144
再次查看当前的shmmax和shmall配置
[root@JYDB1 ~]# cat /proc/sys/kernel/shmmax
25769803776
[root@JYDB1 ~]# cat /proc/sys/kernel/shmall
4294967296
此时重启数据库就可以sga=12G，pga=1.5G正常启动，那么之前的报错就是因为kernel.shmall参数的配置项过小导致了。
<h1 id="4">4.延伸总结</h1>
Oracle官档中，无论是10g还是11g，kernel.shmall参数的配置建议就是kernel.shmall = 2097152，那么在这里的环境有什么问题呢？
**下面我们来弄清楚kernel.shmall这个参数的意义**
[root@JYDB1 ~]# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5.5 (Tikanga)
在RedHat 5的[文档](https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html/Tuning_and_Optimizing_Red_Hat_Enterprise_Linux_for_Oracle_9i_and_10g_Databases/sect-Oracle_9i_and_10g_Tuning_Guide-Setting_Shared_Memory-Setting_SHMALL_Parameter.html)中，关于kernel.shmall这个参数有这样一段描述：
> **7.3. Setting SHMALL Parameter**
> This parameter sets the total amount of shared memory pages that can be used system wide. Hence, SHMALL should always be at least ceil(shmmax/PAGE_SIZE).
> The default size for SHMALL in Red Hat Enterprise Linux 2.1, 3, 4 and 5 is 2097152 which is also Oracle's recommended minimum setting for 9i and 10g on x86 and x86-64 platforms. In most cases this setting should be sufficient since it means that the total amount of shared memory available on the system is 2097152\*4096 bytes (shmall\*PAGE_SIZE) which is 8 GB. PAGE_SIZE is usually 4096 bytes unless you use Chapter 14, Large Memory Optimization, Big Pages, and Huge Pages which supports the configuration of larger memory pages.
**查看系统默认的PAGE_SIZE值：**
[root@JYDB1 ~]# getconf PAGESIZE
4096
计算shmmax/PAGE_SIZE的值`shmmax/PAGE_SIZE=25769803776/4096=6291456`,
这样问题根本原因找到了，在当前环境，kernel.shmall的值至少应该被设置为6291456。
**总结：**Oracle部署过程中，还要对建议的系统参数值有一些深入的了解。有些官档建议的参数可能不符合实际情况，这时候可以结合系统主机的文档来综合定位问题。