alter system set dispatchers

A session has issued a statement ALTER SYSTEM SET DISPATCHERS = string and is waiting for the dispatchers to get started.

Wait Time: The session will wait 1 / 100 of a second and check to see if the new dispatchers have started else the session will wait again

batched allocate scn lock request

A session is waiting on another process to allocate a system change number (SCN). If the foreground timed out waiting on a process to get the SCN, the foreground will get the SCN.

Wait Time: The wait time is 1 second on the assumption that an SCN allocation should normally need much less than that

Parameters: None

BFILE check if exists

The session waits to check if an external large object (LOB) exists.

Wait Time: The total elapsed time for the exists call

BFILE check if open

The session waits for an external large object (LOB) to open.

Wait Time: The total elapsed time for the isopen call

BFILE closure

The session waits for an external large object (LOB) to close.

Wait Time: The total elapsed time for the close call

BFILE get length

The session waits on a call to check the size of an external large object (LOB).

Wait Time: The total elapsed time for the call to check the LOB size

BFILE get name object

The session waits on a call to find or generate the external name of a external large object.

Wait Time: The total elapse time for make external file name to complete

BFILE get path object

The session is waiting on a call to find or generate the external path name of an external large object (LOB).

Wait Time: The total elapsed time for make external path to complete

BFILE internal seek

The session waits for a positioning call within the external large object (LOB) to complete.

Wait Time: The total elapse time for the seek to complete

BFILE open

The session waits for an external large object (LOB) to open.

Wait Time: The total elapsed time for the isopen call

BFILE read

The session waits for a read from a external large object (LOB) to complete.

Wait Time: The total elapse time for the read to complete

buffer busy waits

Wait until a buffer becomes available. This event happens because a buffer is either being read into the buffer cache by another session (and the session is waiting for that read to complete) or the buffer is the buffer cache, but in a incompatible mode (that is, some other session is changing the buffer).

Wait Time: Normal wait time is 1 second. If the session was waiting for a buffer during the last wait, then the next wait will be 3 seconds.

buffer deadlock

Oracle does not really wait on this event; the foreground only yields the CPU. Thus, the chances of catching this event are very low. This is not an application induced deadlock, but an assumed deadlock by the cache layer. The cache layer cannot get a buffer in a certain mode within a certain amount of time.

Wait Time: 0 seconds. The foreground process only yields the CPU and will usually be placed at the end of the CPU run queue.

buffer for checkpoint

The buffer could not be checkpointed, because some process is modifying it. This means that after the wait, the DBWR will scan the whole buffer cache again. This could happen during a database close or after a user does a local checkpoint. During this situation the database cannot be closed.

Wait Time: 1 second

buffer latch

The session waits on the buffer hash chain latch. Primarily used in the dump routines.

Wait Time: 1 second

select *
from v$latch a, v$latchname b
where addr = latch addr
and a.latch# = b.latch#;

buffer read retry

This event occurs only if the instance is mounted in shared mode (Oracle Real Application Cluster). During the read of the buffer, the contents changed. This means that either:

• The version number, dba, or the incarnation and sequence number stored in the block no longer match

• The checksum on the block does not match the checksum in the block

The block will be re-read (this may fail up to 3 times), then corruption is assumed and the corrupt block is dumped in the trace file.

Wait Time: The wait time is the elapsed time of the read

checkpoint completed

A session waits for a checkpoint to complete. This could happen, for example, during a close database or a local checkpoint.

Wait Time: 5 seconds

Parameters: None

checkpoint range buffer not saved

During a range checkpoint operation a buffer was found that was not saved or written. Either:

• The session will wait on this event if the write batch is empty and it is the first time that the session waited on this event in the range checkpoint operation

• The current range checkpoint operation will be aborted and a new one will be started to complete the operation

Wait Time: 10 milliseconds

Parameters: None

control file parallel write

This event occurs while the session is writing physical blocks to all control files. This happens when:

• The session starts a control file transaction (to make sure that the control files are up to date in case the session crashes before committing the control file transaction)

• The session commits a transaction to a control file

• Changing a generic entry in the control file, the new value is being written to all control files

Wait Time: The wait time is the time it takes to finish all writes to all control files

control file sequential read

Reading from the control file. This happens in many cases. For example, while:

• Making a backup of the controlfiles

• Sharing information (between instances) from the controlfile

• Reading other blocks from the controlfiles

• Reading the header block

Wait Time: The wait time is the elapsed time of the read

control file single write

This wait is signaled while the control file's shared information is written to disk. This is an atomic operation protected by an enqueue (CF), so that only one session at a time can write to the entire database.

Wait Time: The wait time is the elapsed time of the write

conversion file read

This event occurs during the creation of a Version 7 controlfile as part of converting a database to Version 7 from Version 6.

Wait Time: The wait time is the elapsed time of the read

db file parallel read

This happens during recovery. Database blocks that need to be changed as part of recovery are read in parallel from the database.

Wait Time: Wait until all of the I/Os are completed

db file parallel write

This event occurs in the DBWR. It indicates that the DBWR is performing a parallel write to files and blocks. The parameter requests indicates the real number of I/Os that are being performed. When the last I/O has gone to disk, the wait ends.

Wait Time: Wait until all of the I/Os are completed

db file scattered read

Similar to db file sequential read, except that the session is reading multiple data blocks.

Wait Time: The wait time is the actual time it takes to do all of the I/Os

db file sequential read

The session waits while a sequential read from the database is performed. This event is also used for rebuilding the control file, dumping datafile headers, and getting the database file headers.

Wait Time: The wait time is the actual time it takes to do the I/O

db file single write

This event is used to wait for the writing of the file headers.

Wait Time: The wait time is the actual time it takes to do the I/O

DFS db file lock

This event occurs only for the DBWR in the Oracle Real Application Cluster. Each DBWR of every instance holds a global lock on each file in shared mode. The instance that is trying to offline the file will escalate the global lock from shared to exclusive. This signals the other instances to synchronize their SGAs with the controlfile before the file can be taken offline. The name of this lock is DF (see Appendix D, " Oracle Enqueue Names" for more information).

Wait Time: 1 second in loop. The DBWR is waiting in a loop (sleep, check) for the other instances to downgrade to NULL mode. During this time, the DBWR cannot perform other tasks such as writing buffers.

DFS lock handle

The session waits for the lock handle of a global lock request. The lock handle identifies a global lock. With this lock handle, other operations can be performed on this global lock (to identify the global lock in future operations such as conversions or release). The global lock is maintained by the DLM.

Wait Time: The session waits in a loop until it has obtained the lock handle from the DLM. Inside the loop there is a wait of 0.5 seconds.

The session needs to get the lock handle.

direct path read

During Direct Path operations the data is asynchronously read from the database files. At some stage the session needs to make sure that all outstanding asynchronous I/O have been completed to disk. This can also happen if during a direct read no more slots are available to store outstanding load requests (a load request could consist of multiple I/Os).

Wait Time: 10 seconds. The session will be posted by the completing asynchronous I/O. It will never wait the entire 10 seconds. The session waits in a tight loop until all outstanding I/Os have completed.

direct path write

During Direct Path operations, the data is asynchronously written to the database files. At some stage the session needs to make sure that all outstanding asynchronous I/O have been completed to disk. This can also happen if, during a direct write, no more slots are available to store outstanding load requests (a load request could consist of multiple I/Os).

Wait Time: 10 seconds. The session will be posted by the completing asynchronous I/O. It will never wait the entire 10 seconds. The session waits in a tight loop until all outstanding I/Os have completed.

dispatcher shutdown

During shutdown immediate or normal, the shutdown process must wait for all the dispatchers to shutdown. As each dispatcher is signaled, the session that causes the shutdown is waits on this event until the requested dispatcher is no longer alive.

Wait Time: 1 second

dispatcher timer

This basically means that the dispatcher is idle and waiting for some work to arrive.

Wait Time: 60 seconds

duplicate cluster key

It is possible for a race condition to occur when creating a new cluster key. If it is found that another process has put the cluster key into the data/index block, then the session waits and retries. The retry should then find a valid cluster key.

Wait Time: 0.01 seconds

enqueue

The session is waiting for a local enqueue. The wait is dependent on the name of the enqueue (see Appendix D, " Oracle Enqueue Names").

Wait Time: Depends on the enqueue name

file identify

The time it takes to identify a file so that it can be opened later.

file open

The time it takes to open the file.

free buffer waits

This will happen if:

• All buffer gets have been suspended. This could happen when a file was read-only and is now read-write. All the existing buffers need to be invalidated since they are not linked to lock elements (needed when mounted parallel (shared)). So cache buffers are not assigned to data block addresses until the invalidation is finished.

• The session moved some dirty buffers to the dirty queue and now this dirty queue is full. The dirty queue needs to be written first. The session will wait on this event and try again to find a free buffer

• This also happens after inspecting free buffer inspected buffers. If no free buffer is found, Oracle waits for one second, and then tries to get the buffer again (depends on the context). For more information, see free buffer inspected.

Wait Time: 1 second

free global transaction table entry

The session is waiting for a free slot in the global transaction table (used by the Distributed Database option). It will wait for 1 second and try again.

Wait Time: 1 second

free process state object

Used during the creation of a process. The session will scan the process table and look for a free process slot. If none can be found, PMON is posted to check if all the processes currently in the process table are still alive. If there are dead processes, then PMON will clean them and make the process slot available to new processes. The waiting process will then rescan the process table to find the new slot.

Wait Time: 1 second

Parameters: None

global cache busy

The session waits to convert a buffer from Shared Current to Exclusive Current status.

Wait Time: 1 second

global cache lock cleanup

PMON is waiting for an LCK process to cleanup the lock context after a foreground process died while doing a global cache lock operation.

Wait Time: 1 second

global cache freelist wait

All releasable locks are used and a new one has been requested. To make a resource element available, a resource element is pinged.

Wait Time: The duration of the resource get operation to ping the resource element

Parameters: None

global cache null to s

The session waits for a resource convert from NULL to SHARED mode on the block identified by file# and block#.

Wait Time: 1 second

global cache null to x

The session waits for a resource convert from NULL to EXCLUSIVE mode on the block identified by file# and block#.

Wait Time: 1 second

global cache open s

The session waits for a resource get in SHARED mode on the block identified by file# and block#.

Wait Time: 1 second

global cache open x

The session waits for a resource get in EXCLUSIVE mode on the block identified by file# and block#.

Wait Time: 1 second

global cache s to x

The session waits for a resource convert from SHARED to EXCLUSIVE mode on the block identified by file# and block#.

Wait Time: 1 second

inactive session

This event is used for two purposes:

• Switching sessions

  If a time-out period has been specified, then wait that amount of time for the session to be detached.

• Killing sessions

  From either KILL SESSION or internal request. Having posted a session that it should kill itself, wait for up to 1 minute for the session to terminate.

Wait Time: 1 second

inactive transaction branch

The session waits for a transaction branch that is currently used by another session.

Wait Time: 1 second

index block split

While trying to find an index key in an index block, Oracle noticed that the index block was being split. Oracle will wait for the split to finish and try to find the key again.

Wait Time: The session will yield the CPU, so there is no actual waiting time

instance recovery

The session waits for SMON to finish the instance, transaction recovery, or sort segment cleanup.

Wait Time: The wait time can vary and depends on the amount of recovery needed

select *
from v$rollstat
where usn = undo segment#;

instance state change

The session waits for SMON to enable or disable cache or transaction recovery. This usually happens during ALTER DATABASE OPEN or CLOSE.

Wait Time: Wait time depends on the amount of time the action takes (that is, the amount of recovery needed)

io done

The session waits for an I/O to complete or it waits for a slave process to become available to submit the I/O request. This event occurs on platforms that do not support asynchronous I/O.

Wait Time: 50 milliseconds

kcl bg acks

The session waits for the background LCK process(es) to finish what they are doing. For example:

• Lock recovery

• Initializing the locks (start up)

• Finalizing the locks (shut down)

Wait Time: 10 seconds

latch activity

This event is used as part of the process of determining whether a latch needs to be cleaned.

Wait Time: 0.05 to 0.1 seconds

select *
from v$latchname
where latch# = number;

latch free

The process waits for a latch that is currently busy (held by another process).

Wait Time: The wait time increases exponentially and does not include spinning on the latch (active waiting). The maximum wait time also depends on the number of latches that the process is holding. There is an incremental wait of up to 2 seconds.

select *
from v$latchname
where latch# = number;

library cache load lock

The session tries to find the load lock for the database object so that it can load the object. The load lock is always obtained in Exclusive mode, so that no other process can load the same object. If the load lock is busy the session will wait on this event until the lock becomes available.

Wait Time: 3 seconds (1 second for PMON)

library cache lock

This event controls the concurrency between clients of the library cache. It acquires a lock on the object handle so that either:

• One client can prevent other clients from accessing the same object

• The client can maintain a dependency for a long time (for example, no other client can change the object)

This lock is also obtained to locate an object in the library cache.

Wait Time: 3 seconds (1 second for PMON)

library cache pin

This event manages library cache concurrency. Pinning an object causes the heaps to be loaded into memory. If a client wants to modify or examine the object, the client must acquire a pin after the lock.

Wait Time: 3 seconds (1 second for PMON)

lock manager wait for remote message

The lock manager waits for a message from a remote lock manager in the same configuration.

Wait Time: The elapsed time of the wait

log buffer space

Waiting for space in the log buffer because the session is writing data into the log buffer faster than LGWR can write it out. Consider making the log buffer bigger if it is small, or moving the log files to faster disks such as striped disks.

Wait Time: Usually 1 second, but 5 seconds if it is waiting for a Switch Logfile to complete

Parameters: None

log file parallel write

Writing redo records to the redo log files from the log buffer.

Wait Time: Time it takes for the I/Os to complete. Even though redo records are written in parallel, the parallel write is not complete until the last I/O is on disk.

log file sequential read

Waiting for the read from this logfile to return. This is used to read redo records from the log file.

Wait Time: Time it takes to complete the physical I/O (read)

log file single write

Waiting for the write to this logfile to complete. This event is used while updating the header of the logfile. It is signaled when adding a log file member and when incrementing sequence numbers.

Wait Time: Time it takes for the physical I/O (write) to complete

log file switch (archiving needed)

Waiting for a log switch because the log that the LGWR will be switching into has not been archived yet. Check the alert file to make sure that archiving has not stopped due to a failed archive write. To speed archiving, consider adding more archive processes or putting the archive files on striped disks.

Wait Time: 1 second

Parameters: None

log file switch (checkpoint incomplete)

Waiting for a log switch because the session cannot wrap into the next log. Wrapping cannot be performed because the checkpoint for that log has not completed.

Wait Time: 1 second

Parameters: None

log file switch (clearing log file)

Waiting for a log switch because the log is being cleared due to a CLEAR LOGFILE command or implicit clear logfile executed by recovery.

Wait Time: 1 second

Parameters: None

log file switch completion

Waiting for a log switch to complete.

Wait Time: 1 second

Parameters: None

log file sync

When a user session commits, the session's redo information needs to be flushed to the redo logfile. The user session will post the LGWR to write the log buffer to the redo log file. When the LGWR has finished writing, it will post the user session.

Wait Time: The wait time includes the writing of the log buffer and the post.

log switch/archive

Used as part of the ALTER SYSTEM ARCHIVE LOG CHANGE scn statement. The session waits for the current log from all open threads to be archived.

Wait Time: Wait for up to 10 seconds

on-going SCN fetch to complete

Another session is fetching the SCN (system change number). This session waits for the other session finish fetching the SCN.

Wait Time: 1 second

Parameters: None

pending global transaction(s)

This event should happen only during testing. The session waits for pending transactions to clear.

Wait Time: 30 seconds

pipe get

The session waits for a message to be received on the pipe or for the pipe timer to expire.

Wait Time: There is a 5 second wake up (check) and the pipe timer set by the user

pipe put

The session waits for the pipe send timer to expire or for space to be made available in the pipe.

Wait Time: There is the 5 second wakeup (check) and the user-supplied timeout value

PL/SQL lock timer

This event is called through the DBMSLOCK.SLEEP procedure or USERLOCK.SLEEP procedure. This event will most likely originate from procedures written by a user.

Wait Time: The wait time is in hundredths of seconds and is dependent on the user context

pmon timer

This is the main wait event for PMON. When PMON is idle, it is waiting on this event.

Wait Time: Up to 3 seconds, if not posted before

process startup

Wait for a shared server, Dispatcher, or other background process to start.

Wait Time: Wait up to 1 second for a background process to start. If timed out, then re-wait until 5 minutes have passed and signal an error. If the process has started, the event will acknowledge this.

PX dequeue wait

The process is waiting for a message during a parallel execute.

Wait Time: The wait time depends on how quickly the message arrives. Wait times can vary, but it will normally be a short period of time.

PX qref latch

Each parallel execution process has a parallel execution qref latch, which needs to be acquired before the queue buffers can be manipulated.

Wait Time: Wait up to 1 second

PX server shutdown

During normal or immediate shutdown the parallel execution slaves are posted to shutdown cleanly. If any parallel execution slaves are still alive after 10 seconds, they are killed.

Wait Time: Wait up to 0.5 seconds

PX signal server

This event occurs only in Exclusive mode. The query coordinator is signalling the Query Slaves that an error has occurred.

Wait Time: 0.5 seconds

queue messages

The session is waiting on an empty OLTP queue (Advanced Queuing) for a message to arrive so that the session can dequeue that message.

Wait Time: The amount of time that the session wants to wait is determined by the parameter wait time

rdbms ipc message

The background processes (LGWR, DBWR, LMS0) use this event to indicate that they are idle and are waiting for the foreground processes to send them an IPC message to do some work.

Wait Time: Up to 3 seconds. The parameter timeout shows the true sleep time.

rdbms ipc message block

This event indicates that all message blocks are in use and that the session had to wait for a message block to become available.

Wait Time: Wait up to 60 seconds

Parameters: None

rdbms ipc reply

This event is used to wait for a reply from one of the background processes.

Wait Time: The wait time is specified by the user and is indicated by the parameter timeout.

redo wait

Defined but not used by the code.

row cache lock

The session is trying to get a data dictionary lock.

Wait Time: Wait up to 60 seconds.

scginq AST call

Called by the session to find the highest lock mode that is held on a resource.

Wait Time: Wait up to 0.2 seconds, but the wait will continue until the NULL mode Acquisition AST has fired.

Parameters: None

single-task message

When running single task, this event indicates that the session waits for the client side of the executable.

Wait Time: Total elapsed time that this session spent in the user application

Parameters: None

smon timer

This is the main idle event for SMON. SMON will be waiting on this event most of the time until it times out or is posted by another process.

Wait Time: 5 minutes (300 seconds)

SQL*Net break/reset to client

The server sends a break or reset message to the client. The session running on the server waits for a reply from the client.

Wait Time: The actual time it takes for the break or reset message to return from the client

SQL*Net break/reset to dblink

Same as SQL*Net break/reset to client, but in this case, the break/reset message is sent to another server process over a database link.

Wait Time: The actual time it takes for the break or reset message to return from the other server process

SQL*Net message from client

The server process (foreground process) waits for a message from the client process to arrive.

Wait Time: The time it took for a message to arrive from the client since the last message was sent to the client

SQL*Net message from dblink

The session waits while the server process (foreground process) receives messages over a database link from another server process.

Wait Time: The time it took for a message to arrive from another server (foreground process) since a message was sent to the other foreground process.

SQL*Net message to client

The server (foreground process) is sending a message to the client.

Wait Time: The actual time the send takes

SQL*Net message to dblink

The server process (foreground process) is sending a message over a database link to another server process.

Wait Time: The actual time the send takes

SQL*Net more data from client

The server is performing another send to the client. The previous operation was also a send to the client.

Wait Time: The time waited depends on the time it took to receive the data (including the waiting time)

SQL*Net more data from dblink

The foreground process is expecting more data from a data base link.

Wait Time: The total time it takes to read the data from the database link (including the waiting time for the data to arrive)

SQL*Net more data to client

The server process is sending more data/messages to the client. The previous operation to the client was also a send.

Wait Time: The actual time it took for the send to complete

SQL*Net more data to dblink

The event indicates that the server is sending data over a database link again. The previous operation over this database link was also a send.

Wait Time: The actual time it takes to send the data to the other server

switch logfile command

The session waits on the user command SWITCH LOGFILE to complete.

Wait Time: 5 seconds

Parameters: None

timer in sksawat

The session waits for the Archiver (ARCH) asynchronous I/O to complete.

Wait Time: 0.01 seconds

Parameters: None

transaction

Wait for a blocking transaction to be rolled back. Continue waiting until the transaction has been rolled back.

Wait Time: 1 second

unbound tx

The session waits to see if there are any transactions that have been started but do not have a Rollback Segment associated with them.

Wait Time: 1 second

Parameters: None

undo segment extension

The undo segment is being extended or shrunk. The session must wait until the operation on the undo segment has finished.

Wait Time: 0.01 seconds

undo segment recovery

PMON is rolling back a dead transaction. The wait continues until rollback finishes.

Wait Time: 3 seconds

undo segment tx slot

Wait for a transaction slot to become available within the selected rollback segment. Continue waiting until the slot is available.

Wait Time: 1 second

virtual circuit status

The session waits for a virtual circuit to return a message type indicated by status.

Wait Time: 30 seconds

WMON goes to sleep

WMON is the UNIX-specific Wait Monitor, that can be used to reduce the number of system calls related to setting timers for posting or waiting in Oracle. You need to set an initialization parameter that enables the WMON process.

Wait Time: Depends on the next timeout

Parameters: None

write complete waits

The session waits for a buffer to be written. The write is caused by normal aging or by a cross-instance call.

Wait Time: 1 second

writes stopped by instance recovery or database suspension

The session is blocked until the instance that started Instance Recovery is finished.

Wait Time: 5 seconds