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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:
Avg
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
enqueue
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
http://www.rampant-books.com/book_2005_2_rac_ssd_tuning.htm
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