Re: tuning for large files in xfs

[fitzboy <fitzboy@iparadigms.com>]

Avi Kivity wrote:
> 
> This will overflow. I think that
> 
>      currentPos = drand48() * s.st_size;
> 
> will give better results
> 
>>     currentPos=currentPos%s.st_size;
> 

why would it overflow? Random() returns a 32 bit number, and if I 
multiple that by 32k (basically the number random() returns is the block 
number I am going to), that should never be over 64 bits? It may be over 
to size of the file though, but that is why I do mod s.st_size... and a 
random number mod something is still a random number. Also, with this 
method it is already currentSize aligned...

> 
> Sorry, I wasn't specific enough: please run iostat -x /dev/whatever 1 
> and look at the 'r/s' (reads per second) field. If that agrees with what 
> your test says, you have a block layer or lower problem, otherwise it's 
> a filesystem problem.
> 

I ran it and found an r/s at 165, which basically corresponds to my 6 ms 
access time... when it should be around 3.5ms... so it seems like the 
seeks themselves are taking along time, NOT that I am doing extra seeks...