Re: disk i/o performance problem

[Douglas Allan Tutty <dtutty@porchlight.ca>]

On Sat, Jun 02, 2007 at 11:58:00AM -0400, Roberto C. Sánchez wrote:
> On Sat, Jun 02, 2007 at 10:54:51AM -0400, Douglas Allan Tutty wrote:
> > On Thu, May 31, 2007 at 11:02:06PM -0400, WHIRLYCOTT wrote:
> > > Etch: (output of hdparm)
> > > 
> > > 	/dev/sda:
> > > 	Timing cached reads:   1588 MB in  2.00 seconds = 794.41 MB/sec
> > > 	Timing buffered disk reads:  176 MB in  3.03 seconds =  58.15 MB/sec
> >  
> > > CentOS: (output of hdparm)
> > > 
> > > 	/dev/sda:
> > > 	Timing cached reads:   4480 MB in  2.00 seconds = 2242.86 MB/sec
> > > 	Timing buffered disk reads:  176 MB in  3.03 seconds =  58.16 MB/sec
> > 
> > The buffered disk reads are the same since that's the transfer rate from
> > the drive's buffer.  The problem is the cache reads which is the time
> > from you OS's cache.  
> > 
> I thought that the cached reads were from the disk drives onboard cache.
> 

I could certainly be wrong, but that would mean that getting data from
the drive cache was at 17 and 38 times faster than getting data from the
OS (?) buffer and that getting such data was the same on both OSs
(unlikely).  

I don't have hdparm installed to be able to check the man page.

If cached reads are from the drive's cache then it would suggest that
the different OSs are able to configure the drive's firmware
differently; 

I don't know.

Doug.