Common Wait Event Names and What They Mean

Wait Event	Description
buffer busy waits	The session wants to access a data block that is either 1) currently not in memory, but another process has already issued an I/O request to read the block into memory, or 2) in memory but in an incompatible mode (current versus consistent, for example). For example another session is using that block via an insert, update or delete.
control file parallel write	The session 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.
control file sequential read	The session is waiting for blocks to be read from a control file.
db file parallel read	The session has issued multiple I/O requests in parallel to read blocks from data files into memory and is waiting for all requests to complete. This may occur during recovery or during regular activity when a session batches many single block I/O requests together and issues them in parallel.
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.
db file scattered read	The session 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 fast full index scan. Oracle reads up to DB_FILE_MULTIBLOCK_READ_COUNT consecutive blocks at a time and scatters them into buffers in the buffer cache
db file sequential read	The session 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. This call differs from "db file scattered read" in that a sequential read reads data into contiguous memory (whilst a scattered read reads multiple blocks and scatters them into different buffers in the SGA).
direct path read, direct path write	The session has issued asynchronous I/O requests that bypass the buffer cache and is waiting for them to complete. These wait events often involve temporary segments, sorting activity, parallel query or hash joins.
enqueue	The session 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.
free buffer waits	The session needs a free buffer so it can bring a data block into the buffer cache and is waiting for a buffer that is not dirty to become available. This can occur if DBWR is not writing dirty buffers to disk fast enough.
latch free	The session is waiting for a latch held by another session. (This event does not apply to processes that are spinning while waiting for a latch; when a process is spinning, it is not waiting.)
library cache load lock	The session 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.)
library cache pin	The session 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.
log buffer space	The session is waiting for space 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 parallel write	The session 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 sequential read	The session 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 switch completion	The session is waiting for a log file switch to complete, typically so more redo can be generated.
log file sync	The session 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). Probably you are commiting too often
undo segment extension	The session is waiting for an undo segment to be extended or shrunk.
write complete waits	The session is waiting for a requested buffer to be written to disk; the buffer cannot be used while it is being written.