From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from fche.csb (vpn-8-169.rdu.redhat.com [10.11.8.169]) by int-mx02.intmail.prod.int.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id p27G2ajf000906 for ; Mon, 7 Mar 2011 11:02:36 -0500 References: <4D6EA3EF.1070401@bartk.us> <4D6EA4E6.9040201@abpni.co.uk> <4D6EC275.6070009@bartk.us> <4D6ECC25.8010502@redhat.com> <20110302174412.06eecd67@bettercgi.com> <4D6EDFE0.2040905@bartk.us> From: Frank Ch. Eigler Date: Mon, 07 Mar 2011 11:02:35 -0500 In-Reply-To: <4D6EDFE0.2040905@bartk.us> (Bart Kus's message of "Wed, 02 Mar 2011 16:25:04 -0800") Message-ID: MIME-Version: 1.0 Subject: Re: [linux-lvm] Tracing IO requests? Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: LVM general discussion and development Bart Kus writes: >>>> issue is why all the reads? It should be 99% writes. >> cp has to read something before it can write it elsewhere. > Ray, my bad, I should have specified, the cp reads from a different > volume/set of drives. [...] One way to try answering such "why" questions is to plop a systemtap probe at an event that should not be happening much, and print a backtrace. In your case you could run this for a little while during the copy: # stap -c 'sleep 2' -e ' probe ioblock.request { if (devname == "sdg2") # adjust to taste if ((rw & 1) == 0) # ! REQ_WRITE if (randint(100) < 2) # 2% of occurrences, if you like { println(devname, rw, size) print_backtrace() } } ' - FChE