WAITEVENT: "db file sequential read"
This signifies a wait for an I/O read request to complete.
Sequential read reads data into contiguous memory, usually a single-block read, although it is possible to see sequential reads for more than one block.
WAITEVENT: "db file scattered read"
This wait happens when a session is waiting for a multiblock IO to complete.
This typically occurs during full table scans or index fast full scans. Oracle reads up to DB_FILE_MULTIBLOCK_READ_COUNT consecutive blocks at a time and scatters them into buffers in the buffer cache.
WAITEVENT: "log file sync"
When a user session COMMITs (or rolls back), the sessions redo information needs to be flushed to the redo logfile. The user session will post the LGWR to write all redo required from the log buffer to the redo log file. When the LGWR has finished it will post the user session. The user session waits on this wait event while waiting for LGWR to post it back to confirm all redo changes are safely on disk.
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Oracle OCP DBA 9i,
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