Re: Linux Disk Performance/File IO per process

[James Sutherland]

On Mon, 29 Jan 2001, List User wrote:

> Just wanted to 'chime' in here.  Yes this would be noisy and will have
> an affect on system performance however these statistics are what are
> used in conjunction with several others to size systems as well as to
> plan on growth.  If Linux is to be put into an enterprise environment
> these types of statistics will be needed.
> 
> When you start hooking up 100's of 'physical volumes' (be it real
> disks or raided logical drives) this data helps you pin-point
> problems.  I think the idea of having the ability to turn such
> accounting on/off via /proc entry a very nice method of doing things.

Question: how will the extra overhead of checking this configuration
compare with just doing it anyway?

If the code ends up as:

if (stats_enabled)
  counter++;

then you'd be better off keeping stats enabled all the time...

Obviously it'll be a bit more complex, but will the stats code be able to
remove itself completely when disabled, even at runtime??

Might be possible with IBM's dprobes, perhaps...?

> That way you can leave it off for normal run-time but when users
> complain or DBA's et al you can turn it on get some stats for a couple
> hours/days whatever, then turn it back off and plan an upgrade or
> re-create a logical volume or stripping set.

NT allows boot-time (en|dis)abling of stats; they quote a percentage for
the performance hit caused - 4%, or something like that?? Of course, they
don't say whether that's a 486 on a RAID array or a quad Xeon on IDE, so
the accuracy of that figure is a bit questionable...

James.