[linux-lvm] snapshots+highmem+'md raid5'

[Herbert Poetzl]

On Mon, Jun 16, 2003 at 11:32:49AM +0200, Heinz J . Mauelshagen wrote:
> On Sat, Jun 14, 2003 at 10:07:54PM +0200, Herbert Poetzl wrote:
> > On Sat, Jun 14, 2003 at 02:45:11PM +0200, Svetoslav Slavtchev wrote:
> > > Quoting Herbert Poetzl <herbert at 13thfloor.at>:

 ... some highmem live-lock issue [removed] ...

> > I posted my results a month ago, but I digged them
> > out again, because together with your results, it
> > sheds new light on that issue ...
> > 
> > mine          	raw disc    md(raid5)   lvm on md
> > -----------------------------------------------------
> > hdparm          14694.40    34129.92    18667.52
> > dd 1024k        14988.22    34732.56    18647.98
> > dd 32k          15516.06    33945.48    18862.67
> > 
> > yours          	raw disc    md(raid5)   lvm on md
> > -----------------------------------------------------
> > hdparm          46151.68    60682.24    49459.20 
> > 
> > in both cases the lvm lies about 3MB over the raw
> > disc throughput, regardless of the md values ...
> 
> The reason probably is LVMs read ahead.

hmm, so it's a feature?

seriously, what can be done about it?
it surely doesn't hurt on a 50M per disk throughput 
system, but 34M/19M seems cruel, especially because 
I'm thinking of adding one or two more discs to the 
system, which probably leads to a 50M/19M situation 
with lvm?

TIA,
Herbert

PS: why am I receiving your emails twice, with 
alternating From headers? I actually get them
three times as I'm subscribed to the list too ;)

Heinz.Mauelshagen at t-online.de (Heinz J . Mauelshagen)
"Heinz J . Mauelshagen" <mauelshagen at sistina.com>
(one list copy)