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  Oracle Tips by Burleson

RAC I/O bandwidth issues

In general, Oracle RAC systems with bandwidth saturation may manifest high buffer busy waitsand write complete waits. Sometimes enqueue and db file parallel writes may also be seen.  For example:

                                    Total       Wait    wait     Waits   Event
Wait                                Waits   Timeouts   Time (cs)  (ms)   /txn
---------------------------- ------------ ---------- ----------- ----- ------

buffer busy waits
                 225,559    211,961  24,377,029  1081    4.0
                           25,731     21,756   6,786,722  2638    0.5
Parallel Query Idle Wait - S        9,980      7,929   1,762,606  1766    0.2
SQL*Net message from dblink       435,845          0   1,288,965    30    7.7
db file parallel write              4,252          0   1,287,023  3027    0.1
write complete waits                5,872      5,658     581,066   990    0.1
db file sequential read         1,249,981          0     510,793     4   22.0

If there are write-based disk bandwidth issues, the Oracle database writer (DBWR) process might show slow performance in the form of:

  • Low number of DBWR Timeouts

  • Higher than average "dirty buffers inspected"

  • The ratio of “free buffers inspected” to “free buffers requested” exceeds 5%

Statistic                          Total   per Second    per Trans
----------------------- ---------------- ------------ ------------
consistent changes                43,523         12.1          2.4    
free buffer inspected              6,087          1.7          0.3
free buffer requested            416,010        115.6         23.1     logons cumulative                 15,718          4.4          0.9    
physical writes                   24,757          6.9          1.4
write requests                       634          0.2          0.0

While buffer busy waits are most commonly associated with segment header contention onside the data buffer pool (db_cache_size, etc.), buffer busy waits are also caused by disk I/O issues.

MetaLink note: 155971.1 suggests that buffer-busy waits can signal I/O bandwidth saturation:

The cause for the buffer busy waitsand other related waits might be a saturated disk controller or subsystem impacting the database's ability to read or write blocks. The disk/controller may be saturated because of the many core dumps occurring simultaneously requiring hundreds of megabytes each.

When there is disk controller contention causing buffer busy waits, one approach is to move the offending data files to a device with greater bandwidth.

The above book excerpt is from:

Oracle RAC & Tuning with Solid State Disk

Expert Secrets for High Performance Clustered Grid Computing

ISBN 0-9761573-5-7

Donald K. Burleson & Mike Ault


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