I get this question all of the time. You are getting warning messages to this effect (this will be found in alert.log on your server):
Thread 1 cannot allocate new log, sequence 1466
Checkpoint not complete
Current log# 3 seq# 1465 mem# 0: /…/…redo03.log

It might say Archival required instead of Checkpoint not complete, but the effect is pretty much the same. This is really something the DBA should be looking out for.
This message will be written to alert.log on the server whenever the database attempts to reuse an online redo log file and finds that it can’t. This happens when DBWn has not yet finished checkpointing the data protected by the redo log or ARCn has not finished copying the redo log file to the archive destination.
At this point, the database effectively halts as far as the end user is concerned. It stops cold. DBWn or ARCn will be given priority to flush the blocks to disk. Upon completion of the checkpoint or archival, everything goes back to normal.
The reason the database suspends user activity is that there is simply no place to record the changes the users are making. Oracle is attempting to reuse an online redo log file, but because either the file would be needed to recover the database in the event of a failure (Checkpoint not complete), or the archiver has not yet finished copying it (Archival required), Oracle must wait (and the end users will wait) until the redo log file can safely be reused.

If you see that your sessions spend a lot of time waiting on a “log file switch,” “log buffer space,” or “log file switch checkpoint or archival incomplete,” you are most likely hitting this. You will notice it during prolonged periods of database modifications if your log files are sized incorrectly or because DBWn and ARCn need to be tuned by the DBA or system administrator. I frequently see this issue with the “starter” database that has not been customized. The “starter” database typically sizes the redo logs far too small for any significant amount of work (including the initial database build of the data dictionary

itself)­. As soon as you start loading up the database, you will notice that the first 1000 rows go fast, and then things start going in spurts: 1000 go fast, then hang, then go fast, then hang, and so on. These are the indications you are hitting this condition.

There are a couple of things you can do to solve this issue:

•\ Make DBWn faster: Have your DBA tune DBWn by enabling ASYNC I/O, using DBWn I/O slaves, or using multiple DBWn processes. Look at the I/O on the system and see if one disk or a set of disks is “hot” and you need to therefore spread the data out. The same general advice applies for ARCn as well. The pros of this are that you get “something for nothing” here—increased performance without really changing any logic/structures/code. There really are no downsides to this approach.

•\ Add more redo log files: This will postpone the Checkpoint not complete in some cases, and, after a while, it will postpone the Checkpoint not complete so long that it perhaps doesn’t happen (because you gave DBWn enough breathing room to checkpoint). The same applies to the Archival required message. The benefit of this approach is the removal of the “pauses” in your system. The downside is it consumes more disk, but the benefit far outweighs any downside here.

•\ Re-create the log files with a larger size: This will extend the amount of time between the time you fill the online redo log and the time you need to reuse it. The same applies to the Archival required message, if the redo log file usage is “bursty.” If you have a period of massive log generation (nightly loads, batch processes) followed by periods of relative calm, then having larger online redo logs can buy enough time for ARCn to catch up during the calm periods. The pros and cons are identical to the preceding approach of adding more files. Additionally, it may postpone a checkpoint from happening until later, since checkpoints happen at each log switch (at least), and the log switches will now be further apart.

•\ Make checkpointing happen more frequently and more continuously: Use a smaller block buffer cache (not entirely desirable) or various parameter settings such as FAST_START_MTTR_TARGET, LOG_CHECKPOINT_INTERVAL, and LOG_CHECKPOINT_TIMEOUT. This will force DBWn to flush dirty blocks more frequently. The benefit to this approach is that recovery time from a failure is reduced. There will always be less work in the online redo logs to be applied. The downside is that blocks may be written to disk more frequently if they are modified often. The buffer cache will not be as effective as it could be, and it can defeat the block cleanout mechanism discussed in the next section.

The approach you take will depend on your circumstances. This is something that must be fixed at the database level, taking the entire instance into consideration.