Re: Bad iostat numbers

[Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>]

Carlos H. Reimer wrote:
> While collecting performance data I discovered very bad numbers in the 
> I/O subsystem and I would like to know if I´m thinking correctly.
>  
> Here is a typical iostat -x:
>  
>
> avg-cpu:  %user   %nice %system %iowait   %idle
>
>           50.40    0.00    0.50    1.10   48.00
>
>  
>
> Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s 
> avgrq-sz avgqu-sz   await  svctm  %util
>
> sda          0.00   7.80  0.40  6.40   41.60  113.60    20.80    
> 56.80    22.82 570697.50   10.59 147.06 100.00
>
> sdb          0.20   7.80  0.60  6.40   40.00  113.60    20.00    
> 56.80    21.94 570697.50    9.83 142.86 100.00
>
> md1          0.00   0.00  1.20 13.40   81.60  107.20    40.80    
> 53.60    12.93     0.00    0.00   0.00   0.00
>
> md0          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
>
>  
>
> Are they not saturated?

They look it (if I'm reading your typical numbers correctly) - %util 100 
and svctime in the region of 100 ms!

On the face of it, looks like you need something better than a RAID1 
setup - probably RAID10 (RAID5 is probably no good as you are writing 
more than you are reading it seems). However read on...

If this is a sudden change in system behavior, then it is probably worth 
trying to figure out what is causing it (i.e which queries) - for 
instance it might be that you have some new queries that are doing disk 
based sorts (this would mean you really need more memory rather than 
better disk...)

Cheers

Mark