[linux-lvm] High loads when writting to LVM

[Paul Warren]

On Wed, Oct 27, 2004 at 11:14:05AM -0400, Judd Tracy wrote:
> I have a system that I am testing out lvm on to get a feeling about 
> stability and performance.  Stability right now seems good, but I am a 
> little worried about the performance currently I am writing out a large 
> file ~1TB using dd if=/dev/zero of=test bs=1M and am noticing the load 
> seams extreamly high "5.21, 5.26, 5.17".  Most of the work is being done 
> in dd but I also notice that kswapd0 is also working fairly heavily.  Is 
> this to be expected with LVM or do i have things setup wrong.  Write 
> performance seems to be around 100MB/s on an XFS filesystem.

This may be a result of how load average is calculated.  What is the CPU
usage when doing these (see top)?  On a machine with a large number of
processes accessing the disk, it is quite easy for a single, I/O
intensive application (such as your dd) to drive the load up.

The load average is telling you that you have 5 processes blocked
waiting for I/O.  Without the dd, the time taken to service each is
reduced, and the average number blocked at any given time is therefore
lower.

On our combined shell/web/email server, an I/O intensive process will
result in a load of well over 10, yet responsiveness of the system
is not actually that bad, and cpu usage remains low.

Paul