因域名解析导致数据库连接延迟分析
一、现状记录
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[oracle@node1 ~]$ /sbin/ifconfig eth2 Link encap:Ethernet HWaddr 00:25:90:04:AB:6B inet addr:192.168.9.140 Bcast:192.168.15.255 Mask:255.255.248.0 inet6 addr: fe80::225:90ff:fe04:ab6b/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:23530402 errors:0 dropped:0 overruns:0 frame:0 TX packets:10959123 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:15308483748 (14.2 GiB) TX bytes:10087987532 (9.3 GiB) –IP地址为192.168.9.140
[oracle@node1 ~]$ more /etc/hosts # Do not remove the following line, or various programs # that require network functionality will fail. 127.0.0.1 ecp-db localhost.localdomain localhost 192.168.9.140 node1.srtcloud.com –域名node1.srtcloud.com对应ip192.168.9.140
[oracle@node1 ~]$ lsnrctl status Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=node1.srtcloud.com)(PORT=1521))) STATUS of the LISTENER ———————— Alias LISTENER Version TNSLSNR for Linux: Version 10.2.0.5.0 – Production Start Date 04-NOV-2011 09:08:51 Uptime 21 days 4 hr. 58 min. 45 sec Trace Level off Security ON: Local OS Authentication SNMP OFF Listener Parameter File /opt/oracle/product/10.2.0/db_1/network/admin/listener.ora Listener Log File /opt/oracle/product/10.2.0/db_1/network/log/listener.log Listening Endpoints Summary… (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=node1.srtcloud.com)(PORT=1521))) Services Summary… Service "ecp" has 2 instance(s). Instance "ecp", status UNKNOWN, has 1 handler(s) for this service… Instance "ecp", status READY, has 1 handler(s) for this service… Service "ecpXDB" has 1 instance(s). Instance "ecp", status READY, has 1 handler(s) for this service… Service "ecp_XPT" has 1 instance(s). Instance "ecp", status READY, has 1 handler(s) for this service… Service "ora11g" has 2 instance(s). Instance "ora11g", status UNKNOWN, has 1 handler(s) for this service… Instance "ora11g", status READY, has 1 handler(s) for this service… Service "ora11gXDB" has 1 instance(s). Instance "ora11g", status READY, has 1 handler(s) for this service… The command completed successfully –说明:ora11g是oracle 11g，ecp是oracle 10g –当前使用域名node1.srtcloud.com监听
[oracle@node1 ~]$ more /opt/oracle/product/10.2.0/db_1/network/admin/tnsnames.ora # tnsnames.ora Network Configuration File: /opt/oracle/product/10.2.0/db_1/network/admin/tnsnames.ora # Generated by Oracle configuration tools.
ECP = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = node1.srtcloud.com)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = ecp) ) )
ORA11G = (DESCRIPTION = (ADDRESS = (PROTOCOL = TCP)(HOST = node1.srtcloud.com)(PORT = 1521)) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = ora11g) ) ) –tns也通过域名访问
[oracle@node1 ~]$ more /etc/resolv.conf nameserver 211.155.235.201 nameserver 211.155.235.188
[oracle@node1 ~]$ more /etc/nsswitch.conf |grep hosts: hosts: files dns –域名解析顺序 |
二、数据库正常工作分析
1、tns工作：客户端通过tns访问数据库，tns配置的是域名访问，所以需要解析，因为此刻解析的顺序是先利用/etc/hosts解析，所以读取hosts文件，获取到ip，然后访问对应数据库，和监听接触。
2、监听工作：监听的是域名，其实也是通过hosts解析成ip的
3、这里能够正常的工作，是因为hosts文件解析了域名
三、模拟数据库访问延迟
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[oracle@node1 ~]$ more /etc/nsswitch.conf |grep hosts: hosts: dns files –先使用dns服务器解析，再使用hosts文件
[oracle@node1 ~]$ more /etc/resolv.conf nameserver 11.1.1.1 –无效的dns服务器
[oracle@node1 ~]$ sqlplus chf/xifenfei@ora11g
SQL*Plus: Release 10.2.0.5.0 – Production on Fri Nov 25 14:44:55 2011
Copyright (c) 1982, 2010, Oracle. All Rights Reserved. –会在这里一个很长的时间等待
[oracle@node1 ~]$ lsnrctl status
LSNRCTL for Linux: Version 10.2.0.5.0 – Production on 25-NOV-2011 14:48:26
Copyright (c) 1991, 2010, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=node1.srtcloud.com)(PORT=1521))) –也会一个长时间的等待
–问题原因:就是因为解析域名的时候，先去访问dns服务器，因为该ip非dns服务器ip，所以会一直等待该ip超时， –然后访问hosts文件获取ip地址（这个就是为什么我们登录或者查看监听状态的时候，会出现如此长的时间的等待） |
其实因为dns延迟的现象有很多种，我这里只是举了一个最简单，比较常见的例子，在处理因dns解析的监听延迟的问题上，可以参考下面几点:
1、如果非特殊情况，尽可能使用ip地址在监听和tns中
2、如果是使用域名，请尽可能使用hosts解析，解析顺序配置为files优先（因为dns服务器有很多不确定，不可控因素）
3、如果一定要使用dns服务器解析，请把稳定的dns服务器配置在第一项，尽可能避免出现dns服务器不可达或者不存在该域名的现象
