6.3.1.2 Monitoring Exadata Smart Flash Cache Using Database Statistics and Wait Events

The following table describes various database statistics that are useful for monitoring Exadata Smart Flash Cache. The statistics are available in various dynamic performance views, including V$SYSSTAT, and may be displayed in the Global Activity Statistics or Instance Activity Statistics section of an AWR report.

Statistic	Description
cell flash cache read hits	The number of read requests that were satisfied by the cache
cell flash cache read hits for smart IO	The number of read requests for smart IO that were satisfied by the cache
cell flash cache read hits for temp IO	The number of read requests for temp IO that were satisfied by the cache
cell flash read hits for controlfile reads	The number of read requests for a database control file that were satisfied by the cache
cell overwrites in flash	Total number of write requests that overwrote an existing cacheline in Exadata Smart Flash Cache that had not been written to disk. In effect, this is the amount of disk I/O saved by using Write-Back mode. This statistic is incremented once per mirror write.
cell partial writes in flash	Total number of write requests written to both Exadata Smart Flash Cache and disk. Part of the data was written to flash, and the rest was written to disk. This statistic is incremented once per mirror write.
cell writes to flash cache	Total number of write requests written entirely to Exadata Smart Flash Cache. This statistic is incremented once per mirror write.
cell writes to flash cache for temp IO	The number of write requests for temporary segments that were absorbed by Exadata Smart Flash Cache
physical read IO requests	Number of physical read requests issued by the database.
physical read requests optimized	Total number of read requests satisfied by using Exadata Smart Flash Cache, and read requests avoided by using storage index or columnar cache.
physical read total bytes	Total amount of I/O bytes for reads issued by the database, whether or not the read was offloaded to the storage servers.
physical read total bytes optimized	Total number of bytes read from Exadata Smart Flash Cache, and bytes avoided by using storage index or columnar cache.
physical read total IO requests	Total number of read requests issued by the database for all activity including application, backup, recovery, and other utilities.
physical write IO requests	Number of physical write requests issued by the database.
physical write total bytes	Total number of IO bytes for writes issued by the database for all activity.
physical write total bytes optimized	Total number of bytes written to Exadata Smart Flash Cache. These writes are synchronized to disk in a lazy manner.
physical write total IO requests	Total number of write requests issued by the database for all activity, including application, backup, recovery and other utilities.

The following table describes database wait events that are useful for monitoring Exadata Smart Flash Cache. The wait events are visible in various dynamic performance views, including V$SESSION, V$SYSTEM_EVENT and V$SESSION_EVENT, and may be displayed in the Wait Event sections of the AWR report.

The latency of the cell single block physical read event typically indicates if the read is satisfied from Exadata Smart Flash Cache. The higher the latency, the higher the likelihood that the request was not satisfied from Exadata Smart Flash Cache.

Wait Event	Description
cell list of blocks physical read	This wait event occurs during recovery or during buffer pre-fetching (rather than performing multiple single-block reads). It is used to monitor database blocks that must be changed as part of recovery and are read in parallel for the database. In V$SESSION, records associated with this event include additional parameters: P1 contains the relevant storage server hash number, which corresponds to V$CELL.CELL_HASHVAL. P2 identifies the disk hash number for the grid disk that contains the data, which corresponds to V$ASM_DISK.HASH_VALUE. P3 specifies the number of bytes processed during the I/O read operation. This wait event is equivalent to db file parallel read for a cell.
cell list of blocks read request	This is a placeholder wait event associated with cell list of blocks physical read, which is visible only during the wait period. After the wait event ends, the placeholder is typically converted to cell list of blocks physical read.
cell multiblock physical read	This wait event represents the time taken to perform all the I/Os for a multi-block database read. In V$SESSION, records associated with this event include additional parameters: P1 contains the relevant storage server hash number, which corresponds to V$CELL.CELL_HASHVAL. P2 identifies the disk hash number for the grid disk that contains the data, which corresponds to V$ASM_DISK.HASH_VALUE. P3 specifies the number of bytes processed during the I/O read operation. This wait event is equivalent to db file scattered read for a cell.
cell multiblock read request	This is a placeholder wait event associated with cell multiblock physical read, which is visible only during the wait period. After the wait event ends, the placeholder is typically converted to cell multiblock physical read.
cell single block physical read	This wait event represents the time taken to perform a single block database I/O. In V$SESSION, records associated with this event include additional parameters: P1 contains the relevant storage server hash number, which corresponds to V$CELL.CELL_HASHVAL. P2 identifies the disk hash number for the grid disk that contains the data, which corresponds to V$ASM_DISK.HASH_VALUE. P3 specifies the number of bytes processed during the I/O read operation. This wait event is equivalent to db file sequential read for a cell. Commencing with the May 2022 Oracle Database release updates (versions 19.15.0.0.220419, 21.6.0.0.220419, and later), this wait event no longer includes I/O from Exadata Smart Flash Cache or database I/O using a Remote Direct Memory Access (RDMA) read.
cell single block physical read: flash cache	This wait event represents the time taken to perform a single block database I/O from Exadata Smart Flash Cache. In V$SESSION, records associated with this event include additional parameters: P1 contains the relevant storage server hash number, which corresponds to V$CELL.CELL_HASHVAL. P2 identifies the disk hash number for the grid disk that contains the data (not the cache location), which corresponds to V$ASM_DISK.HASH_VALUE. P3 specifies the number of bytes processed during the I/O read operation. This wait event was introduced in the May 2022 Oracle Database release updates and is present in Oracle Database versions 19.15.0.0.220419, 21.6.0.0.220419, and later. Previously, cell single block physical read included these waits.
cell single block read request	This is a placeholder wait event associated with a single block database I/O that is visible only during the wait period. After the wait event ends, the placeholder is converted to the appropriate wait event, which is typically one of the cell single block physical read events.
cell interconnect retransmit during physical read	This wait event appears during retransmission for an I/O of a single-block or multiblock read. The cell hash number in the P1 column in the V$SESSION_WAIT view is the same cell identified for cell single block physical read and cell multiblock physical read. The P2 column contains the subnet number to the cell, and the P3 column contains the number of bytes processed during the I/O read operation.

The availability of a specific statistic or wait event is subject to the version of Oracle Database being used.