iostat puzzle

Thu Oct 6 19:45:20 UTC 2005

Hi Jeremy,

Thanks for your reply.

I use EMC disk array , and i have 1 HBA  with fibre channel. This can provide 100MB. per second io. 

My test sql is simple :  select count(*) from table_a 
The table size is 1gb. 
When i run this by using aio , (which shows reasonable values in iostat , 100MB.read per second) it takes 15 sec. ( This is also expected with this io rate) 
On the other hand , for the above case , it takes 30 sec. If 500mb. iorate is correct , this must take 3-4 secs.(including the overhead of file system) 
Comparing the r/s values , the above output says :  
r/s :  14344.44 
rkB/s : 568785.19 
%util : 1851.85
Issuing 14344.44 read request per second? Isnt 200-250 read per second io enough to saturate a scsi disk ?
(The %util value is also strange) 
And , according to the iostat 
avg-cpu:  %user   %nice    %sys   %idle 
                20.83    0.00      68.98   10.19 
%sys is 68.98.
For the reasonable io , the r/s value is <500 and same amount of %sys is generated.
Most importanly , as i sad above , it takes longer .

This is a datawarehouse system . Most of the queries are full table scans and i always use parallel query .
The datafiles are not raw , i use ext3. ( This is development ) 
Direct-io which bypasses the page cache and puts data directly to the user process , is supported but not enabled on my server dues to an oracle bug. 
I tuned the oracle in order to increase the perfomance of full table scans. 
Oracle on each io operation via pread or kio , requests 1MB. data from the operating system.Oracle parallel query bypasses its own cache , so each parallel query requests data from the operating system.( Whether this data is supplied from the page cache or from the disk array depends on the io type.) I dont use directio , so page cache is used for my data files. I think this explain the sar -B page in values.
Kind Regards,
tolga

________________________________

From: redhat-list-bounces at redhat.com on behalf of Jeremy Lyon
Sent: Thu 10/6/2005 5:59 PM
To: General Red Hat Linux discussion list
Subject: Re: iostat puzzle

Tolga,

What kind of hardware do you have?  Have you done some tests (with dd
perhaps) to see if you are actually getting this type of throughput?
Update 2 for RHEL 3 is pretty old.  The latest rev of the kernel is
-37.ELsmp.  You may want to consider updating.

Jeremy

redhat-list-bounces at redhat.com wrote on 10/05/2005 11:49:27 PM:

> Hi ,
>
> The below is the iostat output redhat linux 3.0 which shows very large
> values for rkB/s field (568785.19 )
>
> Reading 568785.19 kB per second! Impossible.
>
> $uname -a:
> Linux koccrmdev 2.4.21-15.ELsmp #1 SMP Thu Apr 22 00:18:24 EDT 2004
> i686 i686 i386 GNU/Linux
>
> $ cat /etc/redhat-release
> Red Hat Enterprise Linux ES release 3 (Taroon Update 2)
>
> I have oracle running on this server. I can basically regenerate this
> pattern easily ,this happens when i disable async. io at oracle part  .
>
> With aio enabled , oracle does not use pread or readv system calls and
> i see reasonable values at iostat for the system.
>
> But when i disable it , iostat shows very high values.
>
> For clarity , when i get these results , the only user on the system is
>
> me.
>
> avg-cpu:  %user   %nice    %sys   %idle
>           20.83    0.00   68.98   10.19
>
> Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s
> avgrq-sz avgqu-sz   await  svctm  %util
> /dev/sda   127851.85 7748.15 14344.44 879.63 1137570.37 69007.41
> 568785.19 34503.70    79.25  1966.67   12.91   1.22 1851.85
> /dev/sda1    0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00
>    0.00     0.00    0.00   0.00   0.00
> /dev/sda2    0.00   0.00  0.00  0.00    0.00    0.00     0.00     0.00
>    0.00     0.00    0.00   0.00   0.00
> /dev/sda3  127824.07  37.04 14118.52 77.78 1135540.74  918.52 567770.37
>
>   459.26    80.05  1914.81   13.48   1.30 1851.85
> /dev/sda5   27.78  12.96 18.52  9.26  370.37  177.78   185.19    88.89
>   19.73     2.96   10.67   8.00  22.22
> /dev/sda6    0.00 7698.15 207.41 792.59 1659.26 67911.11   829.63
> 33955.56    69.57    48.70    4.89   4.00 400.00
>
> Can this be a redhat bug?
>
> Kind Regards,
> tolga
>
>
>
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