物理Data Guard中哪個程式處理Redo GAP

  在Oracle Data Guard中，Redo Gap的產生是由於一些網路或者其他問題導致redo的傳輸中斷。當故障消除後，這些沒有傳輸過去的redo檔案會由一些程式發現，並且將它們傳輸到備庫。

術語：

ARC:歸檔程式
MRP:Media Recovery Process，在備庫上負責應用redo
RFS:Remote File Server ，在備庫上接收傳送過來的redo檔案
FAL:Fetch Archive Log

測試目的：由於網路問題發生了gap後，確定哪個程式負責處理gap。

測試環境：Oracle 11.2.0.2 on Linux 5.

測試過程：

1.確保當前主庫和備庫是同步的：
Primary:
MAX(SEQUENCE#)
--------------
           86

Standby:
MAX(SEQUENCE#)
--------------
           86

2. 模擬網路中斷，導致gap:
在主庫將網路卡停掉： #ifconfig eth0 down

將主庫執行數次switch logfile:
SQL>alter system switch logfile;
SQL>alter system switch logfile;
...

Primary:
MAX(SEQUENCE#)
--------------
           96

這時主庫alert log報出了與備庫連線不通的錯誤：
TNS-00513: Destination host unreachable
   nt secondary err code: 101
   nt OS err code: 0
Error 12543 received logging on to the standby
FAL[server, ARCp]: Error 12543 creating remote archivelog file 'STANDBY'
FAL[server, ARCp]: FAL archive failed, see trace file.
ARCH: FAL archive failed. Archiver continuing
ORACLE Instance orcl - Archival Error. Archiver continuing.



3.將主庫的這些歸檔臨時換個目錄，保證這些歸檔無法傳到備庫：
mv *.arc ../

4. 啟動主庫的網路卡：
#ifconfig eth0 up

5.這時，主庫的ARC並沒有把缺少的日誌傳到備庫。最終備庫的MRP發現了gap並把gap fetching.

備庫alert log:
Thu Mar 29 19:58:49 2012
Media Recovery Waiting for thread 1 sequence 87 (in transit) <====  網路中斷時，等待87
...
Thu Mar 29 20:08:45 2012
...
Media Recovery Waiting for thread 1 sequence 94
Thu Mar 29 20:11:01 2012
RFS[61]: Assigned to RFS process 13643
RFS[61]: Opened log for thread 1 sequence 97 dbid 1285401128 branch 757620395
Archived Log entry 80 added for thread 1 sequence 97 rlc 757620395 ID 0x4c9d8928 dest 2:
Thu Mar 29 20:11:02 2012
RFS[62]: Assigned to RFS process 13645
RFS[62]: Selected log 4 for thread 1 sequence 98 dbid 1285401128 branch 757620395
Thu Mar 29 20:11:02 2012
Primary database is in MAXIMUM PERFORMANCE mode
Re-archiving standby log 4 thread 1 sequence 98
Thu Mar 29 20:11:02 2012
Archived Log entry 81 added for thread 1 sequence 98 ID 0x4c9d8928 dest 1:
RFS[63]: Assigned to RFS process 13647
RFS[63]: Selected log 4 for thread 1 sequence 99 dbid 1285401128 branch 757620395
Thu Mar 29 20:11:05 2012
Fetching gap sequence in thread 1, gap sequence 94-96 <===========取gap
...

6.透過MRP的trace，可以確定是MRP 作了fetching gap:

MRP trace:

*** 2012-03-29 20:08:45.375 4265 krsh.c
Media Recovery Waiting for thread 1 sequence 94

*** 2012-03-29 20:11:05.543
*** 2012-03-29 20:11:05.543 4265 krsh.c
Fetching gap sequence in thread 1, gap sequence 94-96 <==========MRP取gap.
Redo shipping client performing standby login
*** 2012-03-29 20:11:05.593 4595 krsu.c
Logged on to standby successfully
Client logon and security negotiation successful!

7.將移走的歸檔日誌移回之後，備庫的RFS接收到了這些日誌, MRP 將這些日誌進行了apply.

Thu Mar 29 20:12:06 2012
RFS[64]: Assigned to RFS process 13649
RFS[64]: Opened log for thread 1 sequence 94 dbid 1285401128 branch 757620395
Archived Log entry 82 added for thread 1 sequence 94 rlc 757620395 ID 0x4c9d8928 dest 2:
Thu Mar 29 20:12:06 2012
RFS[65]: Assigned to RFS process 13651
RFS[65]: Opened log for thread 1 sequence 95 dbid 1285401128 branch 757620395
Thu Mar 29 20:12:06 2012
RFS[66]: Assigned to RFS process 13653
RFS[66]: Opened log for thread 1 sequence 96 dbid 1285401128 branch 757620395
Archived Log entry 83 added for thread 1 sequence 95 rlc 757620395 ID 0x4c9d8928 dest 2:
Archived Log entry 84 added for thread 1 sequence 96 rlc 757620395 ID 0x4c9d8928 dest 2:
Thu Mar 29 20:12:16 2012
Media Recovery Log /home/oracle/arch1/standby/1_94_757620395.arc
Media Recovery Log /home/oracle/arch1/standby/1_95_757620395.arc
Media Recovery Log /home/oracle/arch1/standby/1_96_757620395.arc
Media Recovery Log /home/oracle/arch1/standby/1_97_757620395.arc
Media Recovery Log /home/oracle/arch1/standby/1_98_757620395.arc


測試結論：
透過這個例子說明，對於這種gap的處理，主庫的ARC程式對於以前產生的gap檔案，並沒有進行處理。是備庫的MRP程式在apply log的時候發現了gap,將這些檔案透過FAL程式取回。

注：在11g,理論上主庫的ARC程式和備庫的RFS、MRP程式在某些情況都有可能處理gap.

8. 為了進一步確定是MRP透過FAL取了gap檔案，我將主庫的密碼修改了一下，結果MRP的trace中報錯：FAL[client, MRP0]，說明是透過FAL取的。

 *** 2012-03-29 21:18:15.964 4265 krsh.c
Error 1031 received logging on to the standby
*** 2012-03-29 21:18:15.964 4265 krsh.c
FAL[client, MRP0]: Error 1031 connecting to PRIMARY for fetching gap sequence