Gentle readers,
I am trying to observe system behavior on our RHEL 5.2 machines. I notice that, it appears to me, based on random iterations of

Code:

dd if=/dev/zero of=/var/tmp/bigfile bs=1024 count=20000

...that if the pdflush is flushing buffers at the time of my write, my write will take about as long as

Code:

dd if=/dev/zero of=/var/tmp/bigfile bs=1024 count=20000 ; sync

This indicates to me that pdflush blocks all write()'s until the flush to disk is complete. The question is: am I correct?

Most of the time, the first dd (without the sync) is much faster than the second. By watching /proc/meminfo, I am able to see approximately when the sync initiated by pdflush takes place. This would be every 30 seconds, as my machine is not filling the disk buffers very quickly. We're talking only a few hundred kilobytes every 30 seconds.

Thanks.
-Mike