Correlation / causation

On Tuesday morning I walked into a right sh*t storm.

I hadn’t been called but on Monday night performance had been absolutely catastrophic – SLAs missed left, right and centre.

Straight into a meeting where the following observations were made:

  • Performance was somewhat suboptimal the previous night
  • Some long-running queries were observed that aren’t normally top of the lists, these were “uncontrolled” access via Excel or Access, not via the reporting Front End or other application processes
  • When these queries stopped, performance returned to normal

So:

  • Why had these queries brought the database to its knees?
  • Was it stats and the stats job on Sunday night?
  • Was it “plan flips”? (I hear a lot about “plan flips” and them being “bad” – do people really expect plan stability without plan stability features?)
  • Was it cardinality feedback? (Admittedly this does seem to have caused us more positives than negatives particularly when working in conjunction with dynamic sampling when I’m not sure it should be).

At this point, I still hadn’t seen any evidence or symptoms of the issue, just hearsay.

So, what did the problem look like?

Below is a snippet from an AWR report from a “normal” period.
(It’s longer than usually advisable – I just want to show the symptoms over the problem period)

Platform                         CPUs Cores Sockets Memory(GB)
-------------------------------- ---- ----- ------- ----------
Linux x86 64-bit                   24    12       2     141.44

              Snap Id      Snap Time      Sessions Curs/Sess
            --------- ------------------- -------- ---------
Begin Snap:      6678 21-May-12 21:30:26       584      22.8
  End Snap:      6682 21-May-12 23:30:33       565      24.6
   Elapsed:              120.13 (mins)
   DB Time:              582.50 (mins)

Foreground Wait Events
                                                             Avg
                                        %Time Total Wait    wait    Waits   % DB
Event                             Waits -outs   Time (s)    (ms)     /txn   time
-------------------------- ------------ ----- ---------- ------- -------- ------
db file sequential read       2,664,063     0      7,050       3      2.5   20.2
log file sync                 1,027,185     0      3,501       3      1.0   10.0
direct path read                159,706     0      1,191       7      0.2    3.4
db file scattered read           61,649     0        284       5      0.1     .8

Background Wait Events
                                                             Avg
                                        %Time Total Wait    wait    Waits   % bg
Event                             Waits -outs   Time (s)    (ms)     /txn   time
-------------------------- ------------ ----- ---------- ------- -------- ------
log file parallel write         839,190     0      1,465       2      0.8   23.6
db file parallel write          551,198     0        674       1      0.5   10.9
Log archive I/O                  21,277     0        311      15      0.0    5.0
log file sequential read          6,714     0        150      22      0.0    2.4
control file parallel writ       14,311     0         48       3      0.0     .8

and this was what had happened:

              Snap Id      Snap Time      Sessions Curs/Sess
            --------- ------------------- -------- ---------
Begin Snap:      7014 28-May-12 21:30:28       560      22.8
  End Snap:      7018 28-May-12 23:30:01       538      23.7
   Elapsed:              119.54 (mins)
   DB Time:            2,067.54 (mins)

Foreground Wait Events
                                                             Avg
                                        %Time Total Wait    wait    Waits   % DB
Event                             Waits -outs   Time (s)    (ms)     /txn   time
-------------------------- ------------ ----- ---------- ------- -------- ------
log file sync                   399,560     0     83,938     210      1.0   67.7
direct path read                123,631     0     15,639     126      0.3   12.6
db file sequential read         363,158     0      9,406      26      0.9    7.6
db file scattered read           93,140     0      4,083      44      0.2    3.3

Background Wait Events
                                                             Avg
                                        %Time Total Wait    wait    Waits   % bg
Event                             Waits -outs   Time (s)    (ms)     /txn   time
-------------------------- ------------ ----- ---------- ------- -------- ------
db file parallel write          394,366     0      8,048      20      1.0   37.6
log file parallel write         101,489     0      6,197      61      0.2   28.9
Log archive I/O                  12,026     0      1,814     151      0.0    8.5
control file parallel writ       11,240     0      1,793     160      0.0    8.4
log file sequential read          9,587     0      1,179     123      0.0    5.5

So, hang on a second.

We think that a couple of long-running queries caused this devastation – log file syncs out to 70x our chosen “normal” above and no CPU issues? All IO basically choked.

Doesn’t make sense – wouldn’t that take some serious misconfiguration?

And yet, how could this be a storage issue if it all resolved itself when those long-running queries finished? It was this doubt that meant we spent longer than needed checking and double checking this box and this database before contacting the storage team.

Then to cut a long story short and to miss out a whole bunch of toing and froing and whys and wherefores… the storage team put the focus on another of our databases that uses the same disk groups.

That database didn’t seem to be doing very much:

              Snap Id      Snap Time      Sessions Curs/Sess
            --------- ------------------- -------- ---------
Begin Snap:      8271 28-May-12 21:00:33        71        .9
  End Snap:      8273 28-May-12 23:00:17        72        .9
   Elapsed:              119.74 (mins)
   DB Time:              155.20 (mins)

Although the few things it was doing were similarly slow:

                                                           Avg
                                                          wait   % DB
Event                                 Waits     Time(s)   (ms)   time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
control file sequential read        272,003       6,778     25   72.8 System I/O
db file sequential read              36,346       1,409     39   15.1 User I/O
db file parallel read                 1,127         797    707    8.6 User I/O
log file sync                           300         253    843    2.7 Commit
DB CPU                                               72            .8

But, hang on a sec. What’s that:

Time Model Statistics

Statistic Name                                       Time (s) % of DB Time
------------------------------------------ ------------------ ------------
RMAN cpu time (backup/restore)                       42,305.3        454.3

and

Background Wait Events
                                                             Avg
                                        %Time Total Wait    wait    Waits   % bg
Event                             Waits -outs   Time (s)    (ms)     /txn   time
-------------------------- ------------ ----- ---------- ------- -------- ------
RMAN backup & recovery I/O    1,166,261     0     72,242      62 2.65E+04   60.2

Turns out that it was an RMAN backup, running when it shouldn’t have been, on a 3.5T archive database on one of the other nodes of the cluster and which uses the same disk groups.

SQL> select to_char(start_time,'DD-MON-YYYY HH24:MI')
  2  ,      to_char(end_time,'DD-MON-YYYY HH24:MI')
  3  ,      input_bytes_display
  4  ,      output_bytes_display
  5  ,      time_taken_display  
  6  from v$rman_backup_job_details  
  7  where session_stamp = 784494089  
  8  order by start_time desc  
  9  /

TO_CHAR(START_TIM TO_CHAR(END_TIME, INPUT_BYTES_DISPLAY  OUTPUT_BYTES_DISPLAY TIME_TAKEN_DISPLAY
----------------- ----------------- -------------------- -------------------- --------------------
28-MAY-2012 19:01 28-MAY-2012 23:47     2.99T              532.82G            04:46:30

SQL> 

That correlation was the causation, not those queries and this backup finished just before those long-running queries finished and then everything was ok.

But the long-running queries were just a red-herring and another symptom of everything that was running on the live database was choking.

Sometimes it’s difficult to distinguish between cause and symptom.

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3 Responses to Correlation / causation

  1. hourim says:

    Hi Dom,

    I’ve recently investigated a performance problem which ressembles, in the reason that hurted the performance of the system, to this “correlation/causation” article

    http://hourim.wordpress.com/2012/05/25/tuning-a-batch-job-using-awr/

    It is also very remarkable to see, how the average wait time of db file sequential read and direct path read wait events went from a very good 3 ms and 7 ms in normal situation to a catastrophic 26 ms and 126 ms respectively during RMAN backup.

  2. Graham says:

    Great warning to all of us Dom. It’s soooo easy to jump to the wrong conclusion just because you see a couple long running queries.

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