RE: High wio on New Hitachi SAN Storage of DB Server

Hi Vivek,

Your note interested me; we have exactly the same configuration and symptoms
but we are still running async_io = TRUE. I concur with your findings on the
extreme performance issues - short term server freezes are occurring. You
mention a bug; is this hearsay or do you have details?

If you've turned of async_io (the default on aix), then you may want to use
DBWR_IO_SLAVES. I've not tried this yet.

Anyway, ours is a HDS9990 with McData Switches attached to a p690 running
AIX 5.2 ML4. The filesystems are jfs2.

I can reproduce the problem just by creating a 10GB datafile, multiple users
doing random i/o can also get the same issue, or indeed when a parallel rman
backup is in progress. However concurrent cp's of multiple files do not
reproduce the issue (hence I believe it is likely an async i/o problem.

Under load Topas shows high wait for i/o and Sar -q shows that swpocc% = 100
and swpocc > 20

My unix admins are currently looking at the async i/o settings as per
metalink note 271444.1, but are heavily loaded and this is not prod (yet) so
the urgency is low.

If you or anyone else has any pointers with this configuration please let me
know.

Kind Regards
Adrian
 
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of VIVEK_SHARMA
Sent: 11 November 2005 18:56
To: Oracle-L
Subject: High wio on New Hitachi SAN Storage of DB Server


Folks,

While doing Application Testing of Hybrid Trans (OLTP mostly though) by
200 users (approx) on a NEWLY configured HITACHI SAN Storage, on DB
Server (of AIX) High wait for IO i.e. wio = 70 % till 1400 Hours is
observed.

NOTE - wio reduced to about 10 % gradually from 1400 Hours to 2000
Hours.

Average sar Output from morning to 1400 Hours:-

13:38:00    %usr    %sys    %wio   %idle
13:43:00       6       5      67      22
13:48:00      10       6      74      10
13:53:00      10       5      66      19
13:58:00       7       5      61      27
14:03:00       5       5      67      22
14:08:00       7       5      74      15
14:13:00       9       6      69      15

CONFIG
======
DB Server = 7 CPUs
OS AIX 5.2
Oracle 8.1.7
Hitachi Storage for DB Server = 40 underlying Disks
Hardware Raid
Init.ora disk_asynch_io = FALSE (When disk_asynch_io is set TRUE there
is extreme performance degradation with AIX 5.2 [seems a bug])


Comments by IBM
===============
The average number of process waiting for IO to complete is 12. This
indicates that these processes are waiting for the IO to complete. This
is the reason why we are seeing an average iowait of 70%.

The seek rate is 95.72% on the hdsdb9960lv  LV's indicates a high degree
of random IO, usually caused by the application or a high degree of disk
fragmentation.


STATSPACK report (will provide any other sections as needed)
================

DB Name         DB Id    Instance     Inst Num Release     OPS Host
------------ ----------- ------------ -------- ----------- ---
------------
ABNPROD         34298189 abnprod             1 8.1.7.4.0   NO  findb

                Snap Id     Snap Time      Sessions
                ------- ------------------ --------
 Begin Snap:          5 15-Oct-05 13:00:55      352
   End Snap:          6 15-Oct-05 14:00:37      352
    Elapsed:                  59.70 (mins)

Cache Sizes
~~~~~~~~~~~
           db_block_buffers:     215000          log_buffer:   18874368
              db_block_size:       8192    shared_pool_size:  754288000

Load Profile
~~~~~~~~~~~~                            Per Second       Per Transaction
                                   ---------------       ---------------
                  Redo size:            298,013.21                837.31
              Logical reads:             55,540.47                156.05
              Block changes:              2,296.74                  6.45
             Physical reads:              3,109.99                  8.74
            Physical writes:                399.33                  1.12
                 User calls:              2,657.16                  7.47
                     Parses:                 64.98                  0.18
                Hard parses:                  5.44                  0.02
                      Sorts:                 75.56                  0.21
                     Logons:                  0.75                  0.00
                   Executes:              1,783.45                  5.01
               Transactions:                355.92

  % Blocks changed per Read:    4.14    Recursive Call %:   15.67
 Rollback per transaction %:   94.71       Rows per Sort:   10.87


Top 5 Wait Events
~~~~~~~~~~~~~~~~~                                             Wait     %
Total
Event                                               Waits  Time (cs)
Wt Time
-------------------------------------------- ------------ ------------
-------
db file sequential read                         6,230,712    1,640,351
43.84
log file sync                                   1,087,475    1,286,467
34.38
db file scattered read                            351,411      416,508
11.13
log file parallel write                           706,201      288,168
7.70
buffer busy waits                                 334,943       69,830
1.87
          -------------------------------------------------------------


Qs How might this issue be approached?
Qs Are there any special O.S. parameters that might be set?

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