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 19772C07545 for ; Wed, 25 Oct 2023 14:33:10 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qvevt-0006c0-1X; Wed, 25 Oct 2023 10:32:32 -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 1qvevb-0006VU-7P for qemu-devel@nongnu.org; Wed, 25 Oct 2023 10:32:14 -0400 Received: from smtp-out1.suse.de ([195.135.220.28]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1qvevX-0003bD-JO for qemu-devel@nongnu.org; Wed, 25 Oct 2023 10:32:10 -0400 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id A8C2F21DDD; Wed, 25 Oct 2023 14:32:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1698244323; h=from:from:reply-to: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=AeIaHaudNwOex2nJBgziT5M5Ge8GpmHX29BdczpdW5M=; b=Vro4Cc1HVgy8PnJ/Zr1aNB9DPWyjmrHHrrI7YxH40yVDfrto0uSMgnkwOomejxhJvgFebb 23P2ieMebFZE8KxmV12zlij5gpOFVdpo80rgc7I7vEBkLuBszt4KNU/2nPE3OebeDbcWw5 sAYrucOd6kMs9v/HwdWx58+Q5XFvhbQ= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1698244323; h=from:from:reply-to: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=AeIaHaudNwOex2nJBgziT5M5Ge8GpmHX29BdczpdW5M=; b=4szJfPSrjuGh9L/IUdusTBk1CE/MhJ5cnIoBn26BPY+nQT7yQq5rD/Vo6v3C2f8LsjnVNU /6OK9DmtTQ9yreCg== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 3A253138E9; Wed, 25 Oct 2023 14:32:03 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id Rk+sAeMmOWUvHwAAMHmgww (envelope-from ); Wed, 25 Oct 2023 14:32:03 +0000 From: Fabiano Rosas To: =?utf-8?Q?Daniel_P=2E_Berrang=C3=A9?= 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 In-Reply-To: References: <20231023203608.26370-1-farosas@suse.de> <20231023203608.26370-29-farosas@suse.de> <878r7svapt.fsf@pond.sub.org> <87msw7ddfp.fsf@suse.de> Date: Wed, 25 Oct 2023 11:32:00 -0300 Message-ID: <87cyx2epsv.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Authentication-Results: smtp-out1.suse.de; none X-Spamd-Result: default: False [-4.97 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; BAYES_HAM(-0.87)[85.68%]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; NEURAL_HAM_LONG(-3.00)[-1.000]; MIME_GOOD(-0.10)[text/plain]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-1.00)[-1.000]; RCPT_COUNT_SEVEN(0.00)[8]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_COUNT_TWO(0.00)[2]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[] Received-SPF: pass client-ip=195.135.220.28; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > On Tue, Oct 24, 2023 at 04:32:10PM -0300, Fabiano Rosas wrote: >> Markus Armbruster writes: >>=20 >> > 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? >>=20 >> 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?