From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:39059) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RQ2sw-00014b-8a for qemu-devel@nongnu.org; Mon, 14 Nov 2011 15:12:07 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RQ2sv-0001qe-45 for qemu-devel@nongnu.org; Mon, 14 Nov 2011 15:12:06 -0500 Received: from mail-yw0-f45.google.com ([209.85.213.45]:64266) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RQ2su-0001qa-VW for qemu-devel@nongnu.org; Mon, 14 Nov 2011 15:12:05 -0500 Received: by ywa17 with SMTP id 17so3206666ywa.4 for ; Mon, 14 Nov 2011 12:12:04 -0800 (PST) Message-ID: <4EC17610.3000401@codemonkey.ws> Date: Mon, 14 Nov 2011 14:12:00 -0600 From: Anthony Liguori MIME-Version: 1.0 References: <1321113420-3252-1-git-send-email-aliguori@us.ibm.com> <1321113420-3252-5-git-send-email-aliguori@us.ibm.com> <4EC1214B.8050105@us.ibm.com> <4EC170C9.5060202@codemonkey.ws> <4EC175D5.1000906@redhat.com> In-Reply-To: <4EC175D5.1000906@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 5/7] block: allow migration to work with image files (v2) List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Lucas Meneghel Rodrigues , Stefan Hajnoczi , quintela@redhat.com, "Michael S. Tsirkin" , qemu-devel@nongnu.org, Avi Kivity On 11/14/2011 02:11 PM, Kevin Wolf wrote: > Am 14.11.2011 20:49, schrieb Anthony Liguori: >> On 11/14/2011 01:46 PM, Juan Quintela wrote: >>> Anthony Liguori wrote: >>>> On 11/14/2011 07:11 AM, Juan Quintela wrote: >>>>> >>>>>> diff --git a/cpus.c b/cpus.c >>>>>> index 82530c4..ae5ec99 100644 >>>>>> --- a/cpus.c >>>>>> +++ b/cpus.c >>>>>> @@ -398,6 +398,7 @@ static void do_vm_stop(RunState state) >>>>>> vm_state_notify(0, state); >>>>>> qemu_aio_flush(); >>>>>> bdrv_flush_all(); >>>>>> + bdrv_invalidate_cache_all(); >>>>>> monitor_protocol_event(QEVENT_STOP, NULL); >>>>>> } >>>>> >>>>> This is too much. Reopening all qcow2 images each time that we stop the >>>>> vm looks excesive, no? >>>> >>>> This general code came in via: >>>> >>>> http://mid.gmane.org/cover.1290613959.git.mst@redhat.com >>>> >>>> That series made migration stable after issuing a stop operation. I >>>> believe the justification was for debugging purposes or something like >>>> that. >>>> >>>> At any rate, invalidating the cache is part of what's required to make >>>> things stable. If you look at something like cache=unsafe, the only >>>> way the metadata will get flushed if via a bdrv_close since bdrv_flush >>>> is a nop. >>>> >>>> So this is needed as long as we care about supporting this use-case. >>> >>> Then we need a "proper" qcow2 invalidate call. Doing in qemu toplevel: >>> >>> (qemu)stop >>> >>> And now all your qcow2 block devices are closed, or perhaps failing to >>> re-open() looks too much to me (TM). >>> >>> Kevin? >> >> Look closely at the patch. It doesn't actually close()/open() anything. >> >> It just invokes the bdrv_close() routine which calls the free functions on the >> l1/l2 caching functions. bdrv_open() doesn't actually open anything (it assumes >> the file is already open. It just reads the header and metadata over again. >> >> For something that's basically a hack, it turned out to work very cleanly :-) > > But why do we need to do it on stop? > > I don't think it makes even sense logically: bdrv_invalidate_cache() > means "throw all your caches away and refetch everything from disk". > What do we gain from doing this on stop? To some degree I could > understand if you did it on cont, so that you can modify an image on the > host while the VM is stopped (though I would still consider it criminal > :-)). Michael basically was trying to avoid having a VM's state change after you stopped the guest. With something like cache=unsafe that periodically flushes based on a timer (I think), you want to make sure that that doesn't happen after stop occurs. Regards, Anthony Liguori > > Kevin > >