Disk I/O as a bottleneck?

[Nadav Har'El]

On Sat, May 07, 2011, Omer Zak wrote about "Re: Disk I/O as a bottleneck?":
> I suspect that speeding up /usr won't help improve performance that
> much.  The applications, which seem to be sluggish, deal with a lot of
> user data in /home.  Furthermore, this user data varies a lot with time,
> hence it is not that good idea to store it in SSD.

As usual, "it depends on your workload" applies.

In my own personal experience (and naturally, it might differ considerably
from your use case), when I see "sluggish behavior" on a desktop machine,
what is actually happening is that "foreground" activity, such as playing or
working with video files, or such as compilation of a large project, causes
a lot of other pages to be swapped out; And then, when you switch to a
different application, it needs to swap pages in - either program text (code)
directly from the executables, or data pages from the swap partition.
So when you switch to a GUI application, suddenly it takes a second to respond
to a mouse click (it needs to swap in the relevant code and data), and when
you type "ls" in a shell it takes much longer than usual (both the "ls" code
and the directory are not in memory). Not only does fetching all these missing
pages require a lot of seeks, which are slow on hard disks, it's even worse
when that other application (which is using all that user data), continues
to do a lot of seeks, and "competes" with the seeks needed to fetch the missing
pages.

In such a case if your system files - binaries, shared libraries, and swap,
would be on a separate disk, everything might feel more responsive. If that
separate disk had low seek times and hight throughput, it would be especially
quick to recover from swap-outs, so you might see even better interactive
behavior.

Like I said, several of my friends tried this setup (SSD+hard disk) and
liked the improved "feel" of the system (and the faster boot :-)).
I haven't tried it myself, though.

-- 
Nadav Har'El                        |        Sunday, May  8 2011, 4 Iyyar 5771
nyh at math.technion.ac.il             |-----------------------------------------
Phone +972-523-790466, ICQ 13349191 |Christopher Robin Hood steals from the
http://nadav.harel.org.il           |rich and gives to the Pooh.