Transactions can continue to write redo information to the rest of the
redo log buffer (the portion that isn??™t being written to disk and therefore isn??™t fenced
off by Oracle). In a busy database, transactions may generate enough redo to fill the
remaining unfenced portion of the redo log buffer before the I/O to the disks for the
fenced area of the redo log buffer is complete. If this happens, the transactions will
have to wait for the I/O to complete because there is no more space in the redo log
buffer. This situation can impact performance. The statistic ???redo buffer allocation
Oracle and Memory Resources | 179
retries??? can be used to understand this situation. It is available through V$SYSSTAT
and is an indication of how often a session waited for space in the redo log buffer. An
example of the query you may use to obtain the statistic is:
SELECT name, value FROM V$SYSSTAT
WHERE name = 'redo buffer allocation retries';
Youwou ld monitor these statistics over a period of time to gain insight into the
trend. The values at one point in time reflect the cumulative totals since the instance
was started and aren??™t necessarily meaningful as a single data point.
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