因域名解析致使數據庫鏈接延遲分析

1、現狀記錄

[oracle@node1 ~]$ /sbin/ifconfig eth1      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地址爲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對應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 --說明:ora11g是oracle 11g，ecp是oracle 10g --當前使用域名node1.srtcloud.com監聽   [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也經過域名訪問   [oracle@node1 ~]$ more /etc/resolv .conf nameserver 211.155.235.201 nameserver 211.155.235.188 --當前有效的dns服務器   [oracle@node1 ~]$ more /etc/nsswitch .conf | grep hosts: hosts:     files dns --域名解析順序

2、數據庫正常工做分析
一、tns工做：客戶端經過tns訪問數據庫，tns配置的是域名訪問，因此須要解析，由於此刻解析的順序是先利用/etc/hosts解析，因此讀取hosts文件，獲取到ip，而後訪問對應數據庫，和監聽接觸。
二、監聽工做：監聽的是域名，其實也是經過hosts解析成ip的
三、這裏可以正常的工做，是由於hosts文件解析了域名

3、模擬數據庫訪問延遲

[oracle@node1 ~]$ more /etc/nsswitch .conf | grep hosts: hosts:     dns files --先使用dns服務器解析，再使用hosts文件   [oracle@node1 ~]$ more /etc/resolv .conf nameserver 11.1.1.1 --無效的dns服務器   [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. --會在這裏一個很長的時間等待   [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))) --也會一個長時間的等待   --問題緣由:就是由於解析域名的時候，先去訪問dns服務器，由於該ip非dns服務器ip，因此會一直等待該ip超時， --而後訪問hosts文件獲取ip地址（這個就是爲何咱們登陸或者查看監聽狀態的時候，會出現如此長的時間的等待）

其實由於dns延遲的現象有不少種，我這裏只是舉了一個最簡單，比較常見的例子，在處理因dns解析的監聽延遲的問題上，能夠參考下面幾點:
一、若是非特殊狀況，儘量使用ip地址在監聽和tns中
二、若是是使用域名，請儘量使用hosts解析，解析順序配置爲files優先（由於dns服務器有不少不肯定，不可控因素）
三、若是必定要使用dns服務器解析，請把穩定的dns服務器配置在第一項，儘量避免出現dns服務器不可達或者不存在該域名的現象

轉：http://www.xifenfei.com/2011/11/%e5%9b%a0%e5%9f%9f%e5%90%8d%e8%a7%a3%e6%9e%90%e5%af%bc%e8%87%b4%e6%95%b0%e6%8d%ae%e5%ba%93%e8%bf%9e%e6%8e%a5%e5%bb%b6%e8%bf%9f%e5%88%86%e6%9e%90.html