[rrd-users] I/O Read overkill

Fabien Wernli wernli at in2p3.fr
Sun Oct 28 11:00:10 CET 2007


On Fri, Oct 26, 2007 at 05:53:33AM -0700, Jason Fesler wrote:
> I wonder if it is the defaults for readahead.  "man blockdev" and search 
> for readahead.   You might read what your defaults were on the old kernel; 
> and temporarilly apply them to your drives under the new kernel, and see 
> what happens with 1.2.10.

yeah I was planning on doing that, however now my production system is
up and running again so... ;)

Here are the values before (RRDs on sda):

# /sbin/blockdev --getra /dev/sda

and after (RRDs on md10 raid1+0):

# /sbin/blockdev --getra /dev/sd[b-e] /dev/md[01] /dev/md10

(siectorsiwe is 512 in both cases)

So I went from 120 to 512. Does that mean 4.266 times more data was being read
off the drive? This doesn't account for the difference though, as I went
from average read of 730KB/s to 36.2MB/s (sorry my previous values were
wrong I mistook Avg for Max), which is a factor of more than 50(!). So
unless the response is non-linear, I guess I have to look elsewhere...
I still suspect a 64bit issue, what do you think?

Now that I use rrdtool 1.2.99999[...] the read value dropped even further:
84KB/s, which is consistent with the announcement by the author (10x more
updates per second).

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