Wednesday, 19 December 2012

SWITCH LOGFILE vs ARCHIVELOG CURRENT

 Oracle has two ways to force a logfile switch, ALTER SYSTEM SWITCH LOGFILE and ALTER SYSTEM ARCHIVELOG CURRENT. 
What is the difference between ALTER SYSTEM SWITCH LOGFILE and ALTER SYSTEM ARCHIVELOG CURRENT, and when do I use each?

Yes, both ALTER SYSTEM SWITCH LOGFILE and ALTER SYSTEM ARCHIVELOG CURRENT will force a log switch, but they do it in different ways!   
Both the SWITCH LOGFILE and ARCHIVELOG CURRENT write a quiesce checkpoint, a firm place whereby that last redo log is a part of the hot backup, but ARCHIVELOG CURRENT waits for the writing to complete.  This can take several minutes for multi-gigabyte redo logs.  
Conversely, the ALTER SYSTEM SWITCH LOGFILE command is very fast and returns control to the caller in less than a second while ALTER SYSTEM ARCHIVELOG CURRENT pauses.  
As we see below, the ALTER SYSTEM SWITCH LOGFILE is fast because it does not wait for the archiver process (ARCH) to complete writing the online redo log to the archivelog log filesystem:
  1. It issues database checkpoint
  2.  It immediately starts writing to the next redo log
  3.  In the background, the “switch logfile” command tells the ARCH background process to copy the “old” redo log file to the redo log filesystem.  
Here are the important differences between ALTER SYSTEM SWITCH LOGFILE and ALTER SYSTEM ARCHIVELOG CURRENT:
  •  RAC:  If you are running RAC, the ALTER SYSTEM ARCHIVELOG CURRENTwill switch the logs on all RAC nodes (instances), whereas ALTER SYSTEM SWITCH LOGFILE will only switch he logfile on the instance where you issue the switch command.  Hence, ALTER SYSTEM ARCHIVELOG CURRENT is a best practice for RAC systems.
  • ALTER SYSTEM  SWITCH LOGFILE is asynchronous:  This command is fast to return to the invoking program because the writing of the redo log to the OS filesystem is done in the background.  There is a very small risk in cases where the ARCH process cannot complete writing the redo log, such as cases where the OS archivelog file directory is out of space.  It is also risky because the calling script may move on to a subsequent step, assuming that the redo has been written.  Some scripts will place a SLEEP 60 command in their backup script to allow time for the redo to complete writing, but this is not a best practice.  
  • ALTER SYSTEM ARCHIVELOG CURRENT is synchronous:  This is faster to return because this command waits until the online redo log has completed the writing of the redo log file to the filesystem.  This command is safer because it waits for the OS to acknowledge (ACK) that the redo log has been successfully written.  Hence, ALTER SYSTEM ARCHIVELOG CURRENT is the best practice for production backup scripts with RMAN.
  • Argument required:  The ALTER SYSTEM ARCHIVELOG CURRENTallows you to specify the thread to archive while the ALTER SYSTEM SWITCH LOGFILE archives only the current thread.  If you do not pass the thread parameter, Oracle will archive all full online redo logs.   

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