From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 1EC03C07545 for ; Wed, 25 Oct 2023 14:20:46 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qvekJ-0001xu-0Z; Wed, 25 Oct 2023 10:20:31 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qvekH-0001xR-KC for qemu-devel@nongnu.org; Wed, 25 Oct 2023 10:20:29 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qvekF-0001Ph-Ju for qemu-devel@nongnu.org; Wed, 25 Oct 2023 10:20:29 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698243625; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HTHIquqGKQVLh4UU0qJcdoOUiAR9ffQJjfxMqZ5kij0=; b=AZmjo9vcGNA5mshzLrno50N6lIgFeRqXDc1LOCiW7rjq1aXslyaLtz3WwQ8oSgOZWumH6L VPvl/gzKC9OeX4vevouYIBeJog6QukWI3ZLem9kmhQuLkFth5Kszuqi0Jlcr5G537RejDY 8WPW470aGSnxVptazWXSZRuPGgUbnhA= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-513-PgurnrfuOLCykOSWTX5sjg-1; Wed, 25 Oct 2023 10:20:23 -0400 X-MC-Unique: PgurnrfuOLCykOSWTX5sjg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id DF60A828AC6; Wed, 25 Oct 2023 14:20:22 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.154]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 926F425C0; Wed, 25 Oct 2023 14:20:19 +0000 (UTC) Date: Wed, 25 Oct 2023 15:20:16 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Fabiano Rosas Cc: qemu-devel@nongnu.org, armbru@redhat.com, Juan Quintela , Peter Xu , Leonardo Bras , Claudio Fontana , Eric Blake Subject: Re: [PATCH v2 06/29] migration: Add auto-pause capability Message-ID: References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-7-farosas@suse.de> <87y1fqerev.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87y1fqerev.fsf@suse.de> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.1 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Wed, Oct 25, 2023 at 10:57:12AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé writes: > > > On Mon, Oct 23, 2023 at 05:35:45PM -0300, Fabiano Rosas wrote: > >> Add a capability that allows the management layer to delegate to QEMU > >> the decision of whether to pause a VM and perform a non-live > >> migration. Depending on the type of migration being performed, this > >> could bring performance benefits. > > > > I'm not really see what problem this is solving. > > > > Well, this is the fruit of your discussion with Peter Xu in the previous > version of the patch. > > To recap: he thinks QEMU is doing useless work with file migrations > because they are always asynchronous. He thinks we should always pause > before doing fixed-ram migration. You said that libvirt would rather use > fixed-ram for a more broad set of savevm-style commands, so you'd rather > not always pause. I'm trying to cater to both of your wishes. This new > capability is the middle ground I came up with. > > So fixed-ram would always pause the VM, because that is the primary > use-case, but libvirt would be allowed to say: don't pause this time. If the VM is going to be powered off immediately after saving a snapshot then yes, you might as well pause it, but we can't assume that will be the case. An equally common use case would be for saving periodic snapshots of a running VM. This should be transparent such that the VM remains running the whole time, except a narrow window at completion of RAM/state saving where we flip the disk snapshots, so they are in sync with the RAM snapshot. IOW, save/restore to disk can imply paused, but snapshotting should not imply paused. So I don't see an unambiguous rationale that we should diverge when fixed-ram is set and auto-pause the VM. > > Mgmt apps are perfectly capable of pausing the VM before issuing > > the migrate operation. > > > > Right. But would QEMU be allowed to just assume that if a VM is paused > at the start of migration it can then go ahead and skip all dirty page > mechanisms? Skipping dirty page tracking would imply that the mgmt app cannot resume CPUs without either letting the operation complete, or aborting it. That is probably a reasonable assumption, as I can't come up with a use case for starting out paused and then later resuming, unless there was a scearnio where you needed to synchronous something external with the start of migration. Sychronizing storage though is something that happens at the end of migration instead. > Without pausing, we're basically doing *live* migration into a static > file that will be kept on disk for who knows how long before being > restored on the other side. We could release the src QEMU resources (a > bit) earlier if we paused the VM beforehand. Can we really release resources early ? If the save operation fails right at the end, we want to be able to resume execution of CPUs, which assumes all resources are still available, otherwise we have a failure scenario where we've not successfully saved to disk and also don't still have the running QEMU. > We're basically talking about whether we want the VM to be usable in the > (hopefully) very short time between issuing the migration command and > the migration being finished. We might be splitting hairs here, but we > need some sort of consensus. The time may not be very short for large VMs. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|