[CentOS] Very slow disk I/O

Thu Jan 29 14:01:31 UTC 2015
Les Mikesell <lesmikesell at gmail.com>

On Thu, Jan 29, 2015 at 7:07 AM, Jatin Davey <jashokda at cisco.com> wrote:
> [Jatin]
> Yes , entire disk is mounted on a single file system. Will this cause high
> wait time for I/O operations ? Do you think splitting it into multiple
> partitions would help ? Appreciate if you can enlighten me in this regard or
> provide pointers to it.

The extra time goes to move the head back and forth from one file to
the next as different applications write to different files.
Different partitions may make things even worse if they happen to
force those locations to be farther apart.  What you need are more
drives with activity spread over them so you have a better chance of a
head being near where it needs to be.   Adding RAM can help to a
certain extent by allowing more buffering.   Faster disks will help
too, but won't magically eliminate seek time - moving heads is still
going to be your slowest computer operation.

> [Jatin]
> Our workload is a bit high on disk reads / writes and hence i am seeing
> slowness in it when using the top command. We basically run VMs on the RHEL
> 6.5 host. Our target is to house 5-6 application VMs on this host and ship
> this as a product on a rack server.

You can micro-manage this by putting a VM image on its own drive so
you don't lose seek time between the images.  Or make a raid with a
very large number of members and hope the odds work out.

   Les Mikesell
      lesmikesell at gmail.com