Database Reference
In-Depth Information
Figure 2-2.
Redo protected until the modified (dirty) buffer is written to disk
At time 1, Block A is read from Data File AA into the buffer cache and modified. At time 2 the redo-change vector
information (how the block changed) is written to the log buffer. At time 3 the log-writer process writes the Block
A change-vector information to online redo log 1. At time 4 a log switch occurs, and online redo log 2 becomes the
current online redo log.
Now, suppose that online redo log 2 fills up quickly and another log switch occurs, at which point the log writer
attempts to write to online redo log 1. The log writer isn't allowed to overwrite information in online redo log 1
until the database block writer writes Block A to Data File AA. Until Block A is written to Data File AA, Oracle needs
information in the online redo logs to recover this block in the event of a power failure or shutdown abort. Before
Oracle overwrites information in the online redo logs, it ensures that blocks protected by redo have been written to
disk. If these modified blocks haven't been written to disk, Oracle temporarily suspends processing until this occurs.
There are a few ways to resolve this issue:
•
Add more redo log groups.
FAST_START_MTTR_TARGET
. Doing so causes the database writer process to
write older modified blocks to disk in a shorter time frame.
•
Lower the value of
DB_WRITER_PROCESSES
).
If you notice that the
Checkpoint not complete
message is occurring often (say, several times a day),
I recommend that you add one or more log groups to resolve the issue. Adding an extra redo log gives the database
writer more time to write modified blocks in the database buffer cache to the data files before the associated redo with
a block is overwritten. There is little downside to adding more redo log groups. The main concern is that you could
bump up against the
MAXLOGFILES
value that was used when you created the database. If you need to add more groups
and have exceeded the value of
MAXLOGFILES
, then you must re-create your control file and specify a high value for this
parameter.
•
Tune the database-writer process (modify
