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Monitoring Current Blocks

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.


In addition to cr blocks, RAC performance is also a concern when processing current mode blocks. Current mode blocks suffer from latency as well as build and wait time concerns similar to cr blocks. The average latency for a current mode block is calculated with the SELECT:

column "AVG RECEIVE TIME (ms)" format 9999999.9
col inst_id for 9999
prompt GCS CURRENT BLOCKS
select b1.inst_id, b2.value "RECEIVED",
b1.value "RECEIVE TIME",
((b1.value / b2.value) * 10) "AVG RECEIVE TIME (ms)"
from gv$sysstat b1, gv$sysstat b2
where b1.name = 'gc current block receive time' and
b2.name = 'gc current blocks received' and b1.inst_id = b2.inst_id;

INST_ID   RECEIVED RECEIVE TIME AVG RECEIVE TIME (ms)
------ ---------- ------------ ---------------------
     1      22694        68999                  30.4
     2      23931        42090                  17.6

The service time for receiving a current block is calculated in a similar fashion to the value for a cr block, except there is a pin time instead of a build time:

SELECT
   a.inst_id "Instance",
   (a.value+b.value+c.value)/d.value "Current Blk Service Time"
FROM
  GV$SYSSTAT A,
  GV$SYSSTAT B,
  GV$SYSSTAT C,
  GV$SYSSTAT D
WHERE
  A.name = 'gc current block pin time' AND
  B.name = 'gc current block flush time' AND
  C.name = 'gc current block send time' AND
  D.name = 'gc current blocks served' AND
  B.inst_id=A.inst_id AND
  C.inst_id=A.inst_id AND
  D.inst_id=A.inst_id
ORDER BY
  a.inst_id;

 Instance Current Blk Service Time
--------- ------------------------
        1               1.18461603
        2               1.63126376

Instance two is requiring more time to service current blocks.  How is the source of the problem determined? The overall service time can be decomposed to determine where the area of concern lies:

SELECT
   A.inst_id "Instance",
   (A.value/D.value) "Current Block Pin",
   (B.value/D.value) "Log Flush Wait",
   (C.value/D.value) "Send Time"
FROM
  GV$SYSSTAT A,
  GV$SYSSTAT B,
  GV$SYSSTAT C,
  GV$SYSSTAT D
WHERE
  A.name = 'gc current block build time' AND
  B.name = 'gc current block flush time' AND
  C.name = 'gc current block send time' AND
  D.name = 'gc current blocks served' AND
  B.inst_id=a.inst_id AND
  C.inst_id=a.inst_id AND
  D.inst_id=a.inst_id
ORDER BY
  A.inst_id;

 Instance Current Block Pin Log Flush Wait  Send Time
--------- ----------------- -------------- ----------
        1         .69366887     .472058762 .018196236
        2        1.07740715     .480549199 .072346418

In this case, most of the time difference comes from the pin time for the current block in instance two. High pin times could indicate problems at the I/O interface level.

A final set of statistics deals with the average global cache convert time and the average global cache get times. The following SELECT can be used to get this information from the RAC database:

select
  a.inst_id "Instance",
  a.value/b.value "Avg Cache Conv. Time",
  c.value/d.value "Avg Cache Get Time",
  e.value "GC Convert Timeouts"
from
   GV$SYSSTAT A,
   GV$SYSSTAT B,
   GV$SYSSTAT C,
   GV$SYSSTAT D,
   GV$SYSSTAT E
where
  a.name='gc convert time' and
  b.name='gc converts' and
  c.name='gc get time' and
  d.name='gc gets' and
  e.name='gc convert timeouts' and
  b.inst_id=a.inst_id and
  c.inst_id=a.inst_id and
  d.inst_id=a.inst_id and
  e.inst_id=a.inst_id
order by
  a.inst_id;

 Instance Avg Cache Conv. Time Avg Cache Get Time GC Convert Timeouts
--------- -------------------- ------------------ -------------------
        1           1.85812072         .981296356                   0
        2           1.65947528         .627444273                   0

For this database, instance one has the highest convert and get times, as expected, since it is converting and getting from instance two, which is the slow instance. None of the times are excessive, >10-20 ms.

Some things to consider about these values are:

* High convert times indicate excessive global concurrency requirements.  In other words, the instances are swapping a lot of blocks over the interconnect.

* Large values or rapid increases in the gets, converts, or average times indicate GCS contention.

* Latencies for resource operations may be high due to excessive system loads.

* The gv$system_event view can be used to review the time_waited statistics for various GCS events if the get or convert times become significant. STATSPACK is good for this.

* Values other than zero for the GC converts timeouts indicate system contention or congestion. Timeouts should not occur and indicate a serious performance problem.

 


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.

http://www.rampant-books.com/book_2004_1_10g_grid.htm


 

 
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