Apparent strange disk behaviour in 6.0

[Brian Candler]

On Sat, Jul 30, 2005 at 03:29:27AM -0700, Julian Elischer wrote:
> >Please use gstat and look at the service times instead of the 
> >busy percentage.
> >
> >
> 
> The snapshot below is typical when doing tar from one drive to another..
> (tar c -C /disk1 f- .|tar x -C /disk2 -f - )
> 
> dT: 1.052  flag_I 1000000us  sizeof 240  i -1
>  L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w    d/s   kBps   ms/d 
> %busy Name
>     0    405    405   1057    0.2      0      0    0.0      0      0    0.0 
> 9.8| ad0
>     0    405    405   1057    0.3      0      0    0.0      0      0    0.0 
> 11.0| ad0s2
>     0    866      3     46    0.4    863   8459    0.7      0      0    0.0 
> 63.8| da0
>    25    866      3     46    0.5    863   8459    0.8      0      0    0.0 
> 66.1| da0s1
>     0    405    405   1057    0.3      0      0    0.0      0      0    0.0 
> 12.1| ad0s2f
>   195    866      3     46    0.5    863   8459    0.8      0      0    0.0 
> 68.1| da0s1d
> 
> even though the process should be disk limitted neither of the disks is 
> anywhere
> near 100%.

Are ad0 and da0 both arrays?

One IDE disk doing 405 reads per second (2.5ms per seek) is pretty good. A
7200rpm drive would have a theoretical average seek time of 1/(7200/60)/2 =
4.2ms, or 7200/60*2 = 240 ops per second. It can be better with read-ahead
caching.

But if really is only 12.1% busy (which the 0.3 ms/r implies), that means it
would be capable of ~3350 operations per second... that's either a seriously
good drive array with tons of cache, or the stats are borked :-)

With a single bog-standard IDE drive tarring up a directory containing some
large .iso images, and piping the output to /dev/null, I get:

 L(q)  ops/s    r/s   kBps   ms/r    w/s   kBps   ms/w   %busy Name
    1    389    388  49318    2.4      1     24    1.4   90.6| ad0s3d

And tarring up /usr/src (again piping to /dev/null) I get:

    1    564    564   5034    1.7      0      0    0.0   95.7| ad0s2e

This is with 5-STABLE as of 2005-05-13 (i.e. a bit after 5.4-RELEASE), and
an AMD 2500+ processor. Interestingly, I get a much higher kBps than your
ad0 - although I'm not actually writing the data out again.

Maybe it would be interesting to pipe the output of your tar to /dev/null
and see how the read performance from ad0 compares with your measured <read
from ad0 plus write to da0> performance? Then try just reading from da0?

Regards,

Brian.