Earlier there was a single redo allocation latch available for the entire log buffer. This was a major contention point as till the time one won’t be able to pin the latch, he wont be able to write his changed redo vectors in the log buffer. From 10.1 onwards, the log buffer is broken in to smaller sub-buffers called Private Redo Strands(shown in the x$kcrfstrand). These are also allocated in the shared pool. These all are written by redo allocation latches. There are allocation latches now for each private strand. The data when changed now, doesn’t contend to go into the log buffer as like before but goes into the private redo strands and from there, it goes into the log buffer when the buffer pools become full or when the transaction is committed. We can see the private strands shown in the shared pool from the V$sgastat
SQL> select name,pool from V$sgastat
2 where name like ‘%private%’;
NAME POOL
————————– ————
private strands shared pool
KCB private handles shared pool
At log file switch , when the data is still is in the Private redo strands, lgwr may wait to get the flush done by DBWR. Hence the event, log file switch (private strand flush incomplete) may occur
log file switch (private strand flush incomplete)
New wait 10g
Like a “log file switch Completion”
Wait for lgwr to switch log files when generating redo
Solution:
Increase redo log file size
Reduces frequency of switches
What happens when a log file switch occurs:
Get next log file from control file
Get Redo Copy and Redo Allocation latch
Flush redo
Close File
Update Controlfile
Set new file to Current
Set old file to Active
If in Archivelog mode add file to archive list
Open all members of new logfile group
Write the SCN to the headers
Enable redo log generation
DBWR makes a list of blocks that need to be written out in order to over write the Redo log file a list of blocks that need to be written out in order to over write the Redo log file
