Wait Event – Info

Some of the most common wait events, and what they mean:

 

Wait Event Description
enqueue The process is waiting on an enqueue (a lock you can see in v$lock).
This commonly occurs when one user is trying to update a row in a table
that is currently being updated by another user.
library cache pin  The process wants to pin an object in memory in the library cache for
examination, ensuring no other processes can update the object at the same
time. This happens when you are compiling or parsing a PL/SQL object or
a view.
library cache load lock  The process is waiting for the opportunity to load an object or a piece
of an object into the library cache. (Only one process can load an object
or a piece of an object at a time.)
latch free  The process is waiting for a latch held by another process. (This wait
event does not apply to processes that are spinning while waiting for a
latch; when a process is spinning, it is not waiting.)
buffer busy waits  The process wants to access a data block that is currently not in memory,
but another process has already issued an I/O request to read the block
into memory. (The process is waiting for the other process to finish bringing
the block into memory.)
control file sequential read  The process is waiting for blocks to be read from a control file.
control file parallel write The process has issued multiple I/O requests in parallel to write blocks
to all control files, and is waiting for all of the writes to complete.
log buffer space  The process is waiting for space to become available in the log buffer
(Space becomes available only after LGWR has written the current contents
of the log buffer to disk.) This typically happens when applications generate
redo faster than LGWR can write it to disk.
log file sequential read  The process is waiting for blocks to be read from the online redo log
into memory. This primarily occurs at instance startup and when the ARCH
process archives filled online redo logs.
log file parallel write  The process is waiting for blocks to be written to all online redo
log members in one group. LGWR is typically the only process to see this
wait event. It will wait until all blocks have been written to all members.
log file sync  The process is waiting for LGWR to finish flushing the log buffer to
disk. This occurs when a user commits a transaction. (A transaction is
not considered committed until all of the redo to recover the transaction
has been successfully written to disk.)
db file scattered read  The process has issued an I/O request to read a series of contiguous
blocks from a data file into the buffer cache, and is waiting for the operation
to complete. This typically happens during a full table scan or full index
scan.
db file sequential read  The process has issued an I/O request to read one block from a data
file into the buffer cache, and is waiting for the operation to complete.
This typically happens during an index lookup or a fetch from a table by
ROWID when the required data block is not already in memory. Do not be
misled by the confusing name of this wait event!
db file parallel read  The process has issued multiple I/O requests in parallel to read blocks
from data files into memory, and is waiting for all requests to complete.
The documentation says this wait event occurs only during recovery, but
in fact it also occurs during regular activity when a process batches many
single block I/O requests together and issues them in parallel. (In spite
of the name, you will not see this wait event during parallel query or
parallel DML. In those cases wait events with PX in their names occur instead.)
db file parallel write  The process, typically DBWR, has issued multiple I/O requests in parallel
to write dirty blocks from the buffer cache to disk, and is waiting for
all requests to complete.
direct path read, direct path write The process has issued asynchronous I/O requests that bypass the buffer
cache, and is waiting for them to complete. These wait events typically
involve sort segments.
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