link:http://www.eygle.com/case/sga2.htm案例描述:這是一個大型生產系統問題出現時系統累計大量用戶進程用戶請求得不到及時響應，新的進程不斷嘗試建立連接 連接數很快被用完數據庫版本:9.2.0.3操作系統:Solaris81.檢查alert文件日志中記錄如下錯誤信息，說明磁盤異步IO出現問題:WARNING: aiowait timed out 2 timesTue Aug 26 15:33:32 2003WARNING: aiowait timed out 2 timesTue Aug 26 15:33:34 2003WARNING: aiowait timed out 2 timesTue Aug 26 15:33:36 2003WARNING: aiowait timed out 2 timesTue Aug 26 15:33:38 2003WARNING: aiowait timed out 2 timesTue Aug 26 15:33:43 2003WARNING: aiowait timed out 1 timesTue Aug 26 15:33:46 2003WARNING: aiowait timed out 1 timesTue Aug 26 15:33:49 2003WARNING: aiowait timed out 1 timesTue Aug 26 15:33:51 2003WARNING: aiowait timed out 1 timesTue Aug 26 15:33:52 2003WARNING: aiowait timed out 1 timesTue Aug 26 15:33:53 2003WARNING: aiowait timed out 1 times.............我們知道在SUN的某些版本上異步IO存在問題而異步IO缺省是打開的SQL> show parameter disk_aNAME TYPE VALUE------------------------------------ ----------- ------------------------------disk_asynch_io boolean TRUE針對此問題，我們停用了數據庫的異步IO寫入。2.共享內存問題alert文件中還記錄了以下錯誤信息: Tue Aug 26 21:37:40 2003 WARNING: EINVAL creating segment of size 0x0000000190400000 fix shm parameters in /etc/system or equivalent該信息說明內核參數設置過小或者和SGA不匹配我們檢查system配置文件$ cat /etc/system.......................set shmsys:shminfo_shmmax=4096000000 set shmsys:shminfo_shmmin=1set shmsys:shminfo_shmmni=200set shmsys:shminfo_shmseg=200set semsys:seminfo_semmap=1024set semsys:seminfo_semmni=2048set semsys:seminfo_semmns=2048set semsys:seminfo_semmnu=2048set semsys:seminfo_semume=200set semsys:seminfo_semmsl=2048我們發現最大共享內存設置僅有4G3.檢查SGA設置SQL*Plus: Release 9.2.0.3.0 - ProdUCtion on 星期二 8月 26 21:46:35 2003Copyright (c) 1982, 2002, Oracle Corporation. All rights reserved.Connected to:Oracle9i Enterprise Edition Release 9.2.0.3.0 - 64bit ProductionWith the Partitioning, OLAP and Oracle Data Mining optionsJServer Release 9.2.0.3.0 - ProductionSQL> show sgaTotal System Global Area 6695660272 bytesFixed Size 740080 bytesVariable Size 2399141888 bytesDatabase Buffers 4294967296 bytesRedo Buffers 811008 bytes我們發現SGA設置接近7G，這也就是步驟2中錯誤提示出現的原因4.交換區問題我們用top工具檢查系統運行狀況# /usr/local/bin/toplast pid: 16899; load averages: 0.82, 0.81, 0.83 21:49:051230 processes:1228 sleeping, 1 running, 1 on cpuCPU states: 50.1% idle, 7.4% user, 8.6% kernel, 33.9% iowait, 0.0% swapMemory: 8192M real, 118M free, 12G swap in use, 11G swap freePID USERNAME THR PRI NICE SIZE RES STATE TIME CPU COMMAND15751 oracle 11 44 0 6456M 6408M sleep 0:02 0.49% oracle15725 oracle 11 58 0 6458M 6410M sleep 0:02 0.46% oracle251 root 12 48 0 7096K 1944K sleep 126:00 0.45% picld16540 oracle 11 58 0 6458M 6411M sleep 0:01 0.45% oracle16766 root 1 43 0 3744K 2248K cpu/1 0:01 0.41% top16408 oracle 11 58 0 6457M 6410M sleep 0:01 0.34% oracle15989 oracle 11 58 0 6458M 6409M sleep 0:01 0.34% oracle15919 oracle 11 58 0 6457M 6409M sleep 0:02 0.30% oracle16404 oracle 11 58 0 6457M 6409M sleep 0:00 0.28% oracle16327 oracle 11 55 0 6457M 6410M sleep 0:00 0.27% oracle14870 oracle 11 58 0 6457M 6412M sleep 0:05 0.24% oracle16851 oracle 11 35 0 6457M 6411M sleep 0:00 0.22% oracle16467 oracle 11 58 0 6457M 6409M sleep 0:00 0.21% oracle16163 oracle 11 58 0 6457M 6408M sleep 0:03 0.21% oracle15159 oracle 11 58 0 6457M 6408M sleep 0:05 0.21% oracleMemory: 8192M real, 118M free, 12G swap in use, 11G swap free我們發現系統僅有8G RAM,物理內存僅有118M可用現在SWAP區使用了12G我們初步作出以下判定:SGA設置過大(將近7G)導致運行時產生大量交換大量SWAP交換進而引發磁盤問題這也就應該是我們第一步看到WARNING: aiowait timed out 1 times的原因大量交換導致數據庫性能急劇下降進而導致用戶請求得不到快速響應，堵塞、累積，直至數據庫失去響應5.解決方案此問題主要是由于SGA設置不當引起，我們馬上縮小了SGA設置:SQL> show sgaTotal System Global Area 3591870848 bytesFixed Size 735616 bytesVariable Size 1442840576 bytesDatabase Buffers 2147483648 bytesRedo Buffers 811008 bytes此時，數據庫減少了交換,達到了穩定運行,用戶請求可以得到快速響應。 問題解決完成.6.系統狀態調整后系統運行狀況:$ top last pid: 12745; load averages: 0.46, 0.79, 0.65 22:22:49228 processes: 227 sleeping, 1 on cpuCPU states: 92.3% idle, 5.0% user, 1.6% kernel, 1.1% iowait, 0.0% swapMemory: 8192M real, 3817M free, 4015M swap in use, 15G swap free PID USERNAME THR PRI NICE SIZE RES STATE TIME CPU COMMAND 12610 oracle 1 51 0 3511M 22M sleep 0:04 1.96% oracle 12595 oracle 1 48 0 3511M 22M sleep 0:03 0.92% oracle 12630 oracle 1 38 0 3511M 21M sleep 0:01 0.84% oracle 12614 oracle 1 46 0 3511M 22M sleep 0:01 0.64% oracle 12620 oracle 1 58 0 3511M 22M sleep 0:01 0.53% oracle 12709 oracle 1 48 0 3511M 21M sleep 0:00 0.45% oracle 265 root 11 38 0 7032K 1920K sleep 3:16 0.42% picld 12729 oracle 1 0 0 3511M 20M sleep 0:00 0.26% oracle 12741 oracle 1 58 0 2768K 1760K cpu/3 0:00 0.19% top 12745 oracle 1 44 0 3506M 16M sleep 0:00 0.17% oracle 12711 oracle 1 48 0 3506M 16M sleep 0:00 0.11% oracle 12738 oracle 1 43 0 3506M 16M sleep 0:00 0.06% oracle 7606 oracle 1 45 0 17M 6928K sleep 0:07 0.05% tnslsnr 12721 oracle 1 34 0 3506M 16M sleep 0:00 0.05% oracle 12723 oracle 1 53 0 3506M 16M sleep 0:00 0.05% oracle 該系統調整完以后，一直穩定運行至今.一點總結:這個案例和前面我提到的另外一個極其相似同樣都是SGA設置不當引起的數據庫問題本身并不復雜這一類問題應該在數據庫規劃和建設階段就避免掉.其時，該問題對我更像是個心理測試當所有老板都站在你背后的時候，你能否冷靜快速的找到并解決問題.關于SUN上的aiowait timed out 有很多總情況及誘因我后面還有相應的案例說明 .-Eygle