From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=40969 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1P5I95-0006ir-CM for qemu-devel@nongnu.org; Mon, 11 Oct 2010 09:10:28 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1P5I94-0003Vh-Gh for qemu-devel@nongnu.org; Mon, 11 Oct 2010 09:10:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:17867) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1P5I94-0003Vb-9K for qemu-devel@nongnu.org; Mon, 11 Oct 2010 09:10:26 -0400 Message-ID: <4CB30CB8.3050806@redhat.com> Date: Mon, 11 Oct 2010 15:10:16 +0200 From: Avi Kivity MIME-Version: 1.0 References: <1286552914-27014-1-git-send-email-stefanha@linux.vnet.ibm.com> <1286552914-27014-7-git-send-email-stefanha@linux.vnet.ibm.com> <4CB182F7.8090100@redhat.com> <20101011103743.GB4078@stefan-thinkpad.transitives.com> In-Reply-To: <20101011103743.GB4078@stefan-thinkpad.transitives.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Anthony Liguori , qemu-devel@nongnu.org, Christoph Hellwig On 10/11/2010 12:37 PM, Stefan Hajnoczi wrote: > On Sun, Oct 10, 2010 at 11:10:15AM +0200, Avi Kivity wrote: > > On 10/08/2010 05:48 PM, Stefan Hajnoczi wrote: > > >This patch implements the read/write state machine. Operations are > > >fully asynchronous and multiple operations may be active at any time. > > > > > >Allocating writes lock tables to ensure metadata updates do not > > >interfere with each other. If two allocating writes need to update the > > >same L2 table they will run sequentially. If two allocating writes need > > >to update different L2 tables they will run in parallel. > > > > > > > Shouldn't there be a flush between an allocating write and an L2 > > update? Otherwise a reuse of a cluster can move logical sectors > > from one place to another, causing a data disclosure. > > > > Can be skipped if the new cluster is beyond the physical image size. > > Currently clusters are never reused and new clusters are always beyond > physical image size. The only exception I can think of is when the > image file size is not a multiple of the cluster size and we round down > to the start of cluster. > > In an implementation that supports TRIM or otherwise reuses clusters > this is a cost. > > > >+ > > >+/* > > >+ * Table locking works as follows: > > >+ * > > >+ * Reads and non-allocating writes do not acquire locks because they do not > > >+ * modify tables and only see committed L2 cache entries. > > > > What about a non-allocating write that follows an allocating write? > > > > 1 Guest writes to sector 0 > > 2 Host reads backing image (or supplies zeros), sectors 1-127 > > 3 Host writes sectors 0-127 > > 4 Guest writes sector 1 > > 5 Host writes sector 1 > > > > There needs to be a barrier that prevents the host and the disk from > > reordering operations 3 and 5, or guest operation 4 is lost. As far > > as the guest is concerned no overlapping writes were issued, so it > > isn't required to provide any barriers. > > > > (based on the comment only, haven't read the code) > > There is no barrier between operations 3 and 5. However, operation 5 > only starts after operation 3 has completed because of table locking. > It is my understanding that *independent* requests may be reordered but > two writes to the *same* sector will not be reordered if write A > completes before write B is issued. > > Imagine a test program that uses pwrite() to rewrite a counter many > times on disk. When the program finishes it prints the counter > variable's last value. This scenario is like operations 3 and 5 above. > If we read the counter back from disk it will be the final value, not > some intermediate value. The writes will not be reordered. Yes, all that is needed is a barrier in program order. So, operation 5 is an allocating write as long as 3 hasn't returned? (at which point it becomes a non-allocating write)? -- error compiling committee.c: too many arguments to function