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Time spent waiting for disk input/output operations to complete.
Input/output (I/O) is one of the most expensive operations in a database system. SQL statements that are I/O intensive can monopolize memory and disk use and cause other database operations to compete for these resources.
Generally, I/O Wait is caused by poorly-tuned SQL queries or applications which generate a significant amount of logical I/O translating into excessive physical disk usage. In this case, SQL/application tuning can reduce the logical I/O- induced load. However, it could also be caused by poorly-configured disks or storage sub-systems.
Time spent by the various sessions waiting to latch a buffer that is not a physical I/O request.
Time spent by the session being blocked by a latch, waiting for it to be released.
Latches do not need to be locked for the duration of a transaction. They are low-overhead, short-term memory synchronization objects. They are used mostly to protect a row when queried for a connection.
Time spent by the various sessions waiting to synchronize commits to marked transactions.