Next log sequence

Question

The archive log list gives the following output:
SQL> archive log list
Database log mode Archive Mode
Automatic archival Enabled
Archive destination f:\ora9i\oradata\db01\archive
Oldest online log sequence 26
Next log sequence to archive 28
Current log sequence 28
My question is if the oldest online log sequence is 26, then the next and current log sequence should be 27. But why it is 28?
Regards,
Aqueel.

Answer 1

Generally speaking, if the archiver (ARCn) is up-to-date with archiving, ARCn will begin archiving log 1 as soon as (within a second or so) LGWR switches to log 2. There are no guarantees that this will always happen.
For example, if you have 3 redo logs, and it takes ARCn 15 seconds to write each log off to the archive destination, and somebody submits a large update that fills each log in 3 seconds, then you have a situation like this:
T + 0 Update submitted
T + 3 Log 1 filled switch to log 2
T + 4 ARCn begins archiving log 1
T + 6 Log 2 filled switch to log 3
T + 9 log 3 filled try to switch to log 1
But it is not archived yet so wait - no transactions can happen in the database at all until ARCn is finished.
t + 14 ARCn finishes archiving log 1 LGWR can write and transactions are allowed again
HTH
John