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 BEF2DC07545 for ; Wed, 25 Oct 2023 14:44:55 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qvf6u-000254-CX; Wed, 25 Oct 2023 10:43:52 -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 1qvf6s-00023O-Ca for qemu-devel@nongnu.org; Wed, 25 Oct 2023 10:43:50 -0400 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 1qvf6p-0005hO-Oe for qemu-devel@nongnu.org; Wed, 25 Oct 2023 10:43:49 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698245026; 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=Yk5gUmBIIK8j96js12RDnv5zhi7f+0qkBf0RJtH/hKE=; b=DYmbrLsdn6tqXhx2oAURPmskbIDe63MB/A0Pq6Dfukj/HXajq9/AmujQE/X/dyK90ohF0f ktjfJJXu+pxny2X9W8TnI07LsJyEu9+2ljqSmr6qY1EyFW1zRPZjc1KWGuAfxFiNH1+Gwi nsFp4LLcBENaisWLFXN3hdFiyp5+SJY= 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-530-I66uofQFOuuhZew9QvaWnA-1; Wed, 25 Oct 2023 10:43:37 -0400 X-MC-Unique: I66uofQFOuuhZew9QvaWnA-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (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 5E0483C23645; Wed, 25 Oct 2023 14:43:37 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.154]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1B04A2026D4C; Wed, 25 Oct 2023 14:43:36 +0000 (UTC) Date: Wed, 25 Oct 2023 15:43:34 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Fabiano Rosas Cc: Markus Armbruster , qemu-devel@nongnu.org, Juan Quintela , Peter Xu , Leonardo Bras , Claudio Fontana , Eric Blake Subject: Re: [PATCH v2 28/29] migration: Add direct-io parameter Message-ID: References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-29-farosas@suse.de> <878r7svapt.fsf@pond.sub.org> <87msw7ddfp.fsf@suse.de> <87cyx2epsv.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <87cyx2epsv.fsf@suse.de> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.4 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: -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_H3=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 11:32:00AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé writes: > > > On Tue, Oct 24, 2023 at 04:32:10PM -0300, Fabiano Rosas wrote: > >> Markus Armbruster writes: > >> > >> > Fabiano Rosas writes: > >> > > >> >> Add the direct-io migration parameter that tells the migration code to > >> >> use O_DIRECT when opening the migration stream file whenever possible. > >> >> > >> >> This is currently only used for the secondary channels of fixed-ram > >> >> migration, which can guarantee that writes are page aligned. > >> >> > >> >> However the parameter could be made to affect other types of > >> >> file-based migrations in the future. > >> >> > >> >> Signed-off-by: Fabiano Rosas > >> > > >> > When would you want to enable @direct-io, and when would you want to > >> > leave it disabled? > >> > >> That depends on a performance analysis. You'd generally leave it > >> disabled unless there's some indication that the operating system is > >> having trouble draining the page cache. > > > > That's not the usage model I would suggest. > > > > Hehe I took a shot at answering but I really wanted to say "ask Daniel". > > > The biggest value of the page cache comes when it holds data that > > will be repeatedly accessed. > > > > When you are saving/restoring a guest to file, that data is used > > once only (assuming there's a large gap between save & restore). > > By using the page cache to save a big guest we essentially purge > > the page cache of most of its existing data that is likely to be > > reaccessed, to fill it up with data never to be reaccessed. > > > > I usually describe save/restore operations as trashing the page > > cache. > > > > IMHO, mgmt apps should request O_DIRECT always unless they expect > > the save/restore operation to run in quick succession, or if they > > know that the host has oodles of free RAM such that existing data > > in the page cache won't be trashed, or > > Thanks, I'll try to incorporate this to some kind of doc in the next > version. > > > if the host FS does not support O_DIRECT of course. > > Should we try to probe for this and inform the user? qemu_open_internall will already do a nice error message. If it gets EINVAL when using O_DIRECT, it'll retry without O_DIRECT and if that works, it'll reoprt "filesystem does not support O_DIRECT" Having said that I see a problem with /dev/fdset handling, because we're only validating O_ACCMODE and that excludes O_DIRECT. If the mgmt apps passes an FD with O_DIRECT already set, then it won't work for VMstate saving which is unaligned. If the mgmt app passes an FD without O_DIRECT set, then we are not setting O_DIRECT for the multifd RAM threads. 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 :|