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 C52A8C3DA79 for ; Mon, 15 Jan 2024 08:12:45 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rPI4a-00006j-Lg; Mon, 15 Jan 2024 03:11:56 -0500 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 1rPI4X-00006S-JX for qemu-devel@nongnu.org; Mon, 15 Jan 2024 03:11:55 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rPI4V-0008H5-5h for qemu-devel@nongnu.org; Mon, 15 Jan 2024 03:11:53 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1705306309; 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:in-reply-to:in-reply-to: references:references; bh=vWMNauP6hlDkzx3iy0jja6XuX4Tr19XUKwjcnFhstjY=; b=TesIhVBF061wPR5hnGwGbsRkm+vpKSqlefIhGD9+XPKdoiZASpFBilOZhs9l2oK0bed5Xe CLrtNdQ+NvIxxnxd0y5vjvq50DeHr4mrqhKHSQ9jYUmYrtCbYjvVG7x+gvDc86sufxcOxw xYtKp0Zd9OOoecBtTrebli+r+CoI+Cc= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-378-Rqnbh1qWPHagapq4uIHsGQ-1; Mon, 15 Jan 2024 03:11:45 -0500 X-MC-Unique: Rqnbh1qWPHagapq4uIHsGQ-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.rdu2.redhat.com [10.11.54.8]) (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 74C253C025C5; Mon, 15 Jan 2024 08:11:45 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.53]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EDC8AC185A6; Mon, 15 Jan 2024 08:11:42 +0000 (UTC) Date: Mon, 15 Jan 2024 08:11:40 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: Fabiano Rosas , qemu-devel@nongnu.org, armbru@redhat.com, Leonardo Bras , Claudio Fontana Subject: Re: [RFC PATCH v3 00/30] migration: File based migration with multifd and fixed-ram Message-ID: References: <20231127202612.23012-1-farosas@suse.de> <87zfxbn2ag.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/2.2.10 (2023-03-25) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.8 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -48 X-Spam_score: -4.9 X-Spam_bar: ---- X-Spam_report: (-4.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.758, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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 Mon, Jan 15, 2024 at 02:22:47PM +0800, Peter Xu wrote: > On Thu, Jan 11, 2024 at 03:38:31PM -0300, Fabiano Rosas wrote: > > Peter Xu writes: > > > > > On Mon, Nov 27, 2023 at 05:25:42PM -0300, Fabiano Rosas wrote: > > >> Hi, > > >> > > >> In this v3: > > >> > > >> Added support for the "file:/dev/fdset/" syntax to receive multiple > > >> file descriptors. This allows the management layer to open the > > >> migration file beforehand and pass the file descriptors to QEMU. We > > >> need more than one fd to be able to use O_DIRECT concurrently with > > >> unaligned writes. > > >> > > >> Dropped the auto-pause capability. That discussion was kind of > > >> stuck. We can revisit optimizations for non-live scenarios once the > > >> series is more mature/merged. > > >> > > >> Changed the multifd incoming side to use a more generic data structure > > >> instead of MultiFDPages_t. This allows multifd to restore the ram > > >> using larger chunks. > > >> > > >> The rest are minor changes, I have noted them in the patches > > >> themselves. > > > > > > Fabiano, > > > > > > Could you always keep a section around in the cover letter (and also in the > > > upcoming doc file fixed-ram.rst) on the benefits of this feature? > > > > > > Please bare with me - I can start to ask silly questions. > > > > > > > That's fine. Ask away! > > > > > I thought it was about "keeping the snapshot file small". But then when I > > > was thinking the use case, iiuc fixed-ram migration should always suggest > > > the user to stop the VM first before migration starts, then if the VM is > > > stopped the ultimate image shouldn't be large either. > > > > > > Or is it about performance only? Where did I miss? > > > > Performance is the main benefit because fixed-ram enables the use of > > multifd for file migration which would otherwise not be > > parallelizable. To use multifd has been the direction for a while as you > > know, so it makes sense. > > > > A fast file migration is desirable because it could be used for > > snapshots with a stopped vm and also to replace the "exec:cat" hack > > (this last one I found out about recently, Juan mentioned it in this > > thread: https://lore.kernel.org/r/87cyx5ty26.fsf@secure.mitica). > > I digged again the history, and started to remember the "live" migration > case for fixed-ram. IIUC that is what Dan mentioned in below email > regarding to the "virDomainSnapshotXXX" use case: > > https://lore.kernel.org/all/ZD7MRGQ+4QsDBtKR@redhat.com/ > > So IIUC "stopped VM" is not always the use case? > > If you agree with this, we need to document these two use cases clearly in > the doc update: > > - "Migrate a VM to file, then destroy the VM" > > It should be suggested to stop the VM first before triggering such > migration in this use case in the documents. > > - "Take a live snapshot of the VM" > > It'll be ideal if there is a portable interface to synchronously track > dirtying of guest pages, but we don't... > > So fixed-ram seems to be the solution for such a portable solution for > taking live snapshot across-platforms as long as async dirty tracking > is still supported on that OS (aka KVM_GET_DIRTY_LOG). If async > tracking is not supported, snapshot cannot be done live on the OS then, > and one needs to use "snapshot-save". > > For this one, IMHO it would be good to mention (from QEMU perspective) > the existance of background-snapshot even though libvirt didn't support > it for some reason. Currently background-snapshot lacks multi-thread > feature (nor O_DIRECT), though, so it may be less performant than > fixed-ram. However if with all features there I believe that's even > more performant. Please consider mention to a degree of detail on > this. > > > > > The size aspect is just an interesting property, not necessarily a > > reason. > > See above on the 2nd "live" use case of fixed-ram. I think in that case, > size is still a matter, then, because that one cannot stop the VM vcpus. > > > It's about having the file bounded to the RAM size. So a running > > guest would not produce a continuously growing file. This is in contrast > > with previous experiments (libvirt code) in using a proxy to put > > multifd-produced data into a file. > > > > I'll add this^ information in a more organized matter to the docs and > > cover letter. Let me know what else I need to clarify. > > Thanks. > > > > > Some notes about fixed-ram by itself: > > > > This series also enables fixed-ram without multifd, which would only > > take benefit of the size property. That is not part of our end goal > > which is to have multifd + fixed-ram, but I kept it nonetheless because > > it helps to debug/reason about the fixed-ram format without conflating > > matters with multifd. > > Yes, makes sense. > > > > > Fixed-ram without multifd also allows the file migration to take benefit > > of direct io because the data portion of the file (pages) will be > > written with alignment. This version of the series does not yet support > > it, but I have a simple patch for the next version. > > > > I also had a - perhaps naive - idea that we could merge the io code + > > fixed-ram first, to expedite things and later bring in the multifd and > > directio enhancements, but the review process ended up not being that > > modular. > > What's the review process issue you're talking about? > > If you can split the series that'll help merging for sure to me. IIRC > there's complexity on passing the o-direct fds around, and not sure whether > that chunk can be put at the last, similarly to split the multifd bits. > > One thing I just noticed is fixed-ram seems to be always preferred for > "file:" migrations. Then can we already imply fixed-ram for "file" URIs? > > I'm even thinking whether we can make it the default and drop the fixed-ram > capability: fixed-ram won't work besides file, and file won't make sense if > not using offsets / fixed-ram. There's at least one problem, where we have > released 8.2 with "file:", so it means it could break users already using > "file:" there. I'm wondering whether that'll be worthwhile considering if > we can drop the (seems redundant..) capability. What do you think? The 'fd' protocol should support 'fixed-ram' too if passed a seekable FD. The 'file' protocol should be able to create save images compatible with older QEMU too IMHO. 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 :|