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 X-Spam-Level: X-Spam-Status: No, score=-6.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 86E38C433E1 for ; Tue, 23 Jun 2020 20:46:39 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 5BC5621548 for ; Tue, 23 Jun 2020 20:46:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5BC5621548 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=bugs.launchpad.net Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:57622 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jnpoU-0000rd-Jh for qemu-devel@archiver.kernel.org; Tue, 23 Jun 2020 16:46:38 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:44096) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jnpne-0008Je-KI for qemu-devel@nongnu.org; Tue, 23 Jun 2020 16:45:46 -0400 Received: from indium.canonical.com ([91.189.90.7]:51958) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1jnpnc-0008Rs-5r for qemu-devel@nongnu.org; Tue, 23 Jun 2020 16:45:46 -0400 Received: from loganberry.canonical.com ([91.189.90.37]) by indium.canonical.com with esmtp (Exim 4.86_2 #2 (Debian)) id 1jnpna-0002Es-65 for ; Tue, 23 Jun 2020 20:45:42 +0000 Received: from loganberry.canonical.com (localhost [127.0.0.1]) by loganberry.canonical.com (Postfix) with ESMTP id 10A0A2E810A for ; Tue, 23 Jun 2020 20:45:42 +0000 (UTC) MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Date: Tue, 23 Jun 2020 20:38:49 -0000 From: Eric Blake <1884831@bugs.launchpad.net> To: qemu-devel@nongnu.org X-Launchpad-Notification-Type: bug X-Launchpad-Bug: product=qemu; status=New; importance=Undecided; assignee=None; X-Launchpad-Bug-Information-Type: Public X-Launchpad-Bug-Private: no X-Launchpad-Bug-Security-Vulnerability: no X-Launchpad-Bug-Commenters: eblake tobias-hunger X-Launchpad-Bug-Reporter: TobiasHunger (tobias-hunger) X-Launchpad-Bug-Modifier: Eric Blake (eblake) References: <159294358125.13789.3860026407311199131.malonedeb@wampee.canonical.com> Message-ID: <18e784cc-82ec-0b03-911b-72961cb722cd@redhat.com> Subject: [Bug 1884831] Re: qemu-nbd fails to discard bigger chunks X-Launchpad-Message-Rationale: Subscriber (QEMU) @qemu-devel-ml X-Launchpad-Message-For: qemu-devel-ml Precedence: bulk X-Generated-By: Launchpad (canonical.com); Revision="1cbd0aa39df153c901321817f9b57cf3f232b507"; Instance="production-secrets-lazr.conf" X-Launchpad-Hash: 720aabcb8b2aa4e3370e1358cbed5ce71ac577bd Received-SPF: none client-ip=91.189.90.7; envelope-from=bounces@canonical.com; helo=indium.canonical.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/23 16:25:36 X-ACL-Warn: Detected OS = Linux 3.11 and newer [fuzzy] X-Spam_score_int: -58 X-Spam_score: -5.9 X-Spam_bar: ----- X-Spam_report: (-5.9 / 5.0 requ) BAYES_00=-1.9, HEADER_FROM_DIFFERENT_DOMAINS=1, RCVD_IN_DNSWL_HI=-5, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Bug 1884831 <1884831@bugs.launchpad.net> Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Message-ID: <20200623203849.RTYVdh7-pokBl6YI2df7UO1sQZLDT54YWepBwwaXoKI@z> On 6/23/20 3:19 PM, TobiasHunger wrote: > Public bug reported: > = > This report is moved from systemd to here: > https://github.com/systemd/systemd/issues/16242 > = > A qemu-nbd device reports that it can discard a lot of bytes: > = > cat /sys/block/nbd0/queue/discard_max_bytes > 2199023255040 That smells fishy. It is 0xffffffff * 512. But in reality, the NBD = protocol is (currently) capped at 32 bits, so it cannot handle any = request 4G or larger. It is not qemu-nbd that populates = /sys/block/nbd0/queue/discard_max_bytes, but the kernel. Are you sure = this is not a bug in the kernel's nbd.ko module, where it may be the = case that it is reporting -1 as a 32-bit value which then gets = mistakenly turned into a faulty advertisement? Can you tweak your = software to behave as if /dev/nbd0 had a discard_max_bytes of 0xfffff000 = instead? In fact, to prove the bug is in the kernel's nbd.ko and not in qemu-nbd, = I created an NBD server using nbdkit: # modprobe nbd # nbdkit memory 5G # nbd-client -b 512 localhost /dev/nbd0 # cat /sys/block/nbd0/queue/discard_max_bytes 2199023255040 Same answer, different nbd server. So it's not qemu's fault. > = > And indeed, discard works with small images: > = > $ qemu-img create -f qcow2 /tmp/image.img 2M > $ sudo qemu-nbd --connect=3D/dev/nbd0 /tmp/image.img > $ sudo blkdiscard /dev/nbd0 > = > but not for bigger ones (still smaller than discard_max_bytes): > = > $ qemu-img create -f qcow2 /tmp/image.img 5G > $ sudo qemu-nbd --connect=3D/dev/nbd0 /tmp/image.img > $ sudo blkdiscard /dev/nbd0 > = > ** Affects: qemu > Importance: Undecided > Status: New > = -- = Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org ** Bug watch added: github.com/systemd/systemd/issues #16242 https://github.com/systemd/systemd/issues/16242 -- = You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1884831 Title: qemu-nbd fails to discard bigger chunks Status in QEMU: New Bug description: This report is moved from systemd to here: https://github.com/systemd/systemd/issues/16242 A qemu-nbd device reports that it can discard a lot of bytes: cat /sys/block/nbd0/queue/discard_max_bytes 2199023255040 And indeed, discard works with small images: $ qemu-img create -f qcow2 /tmp/image.img 2M $ sudo qemu-nbd --connect=3D/dev/nbd0 /tmp/image.img $ sudo blkdiscard /dev/nbd0 but not for bigger ones (still smaller than discard_max_bytes): $ qemu-img create -f qcow2 /tmp/image.img 5G $ sudo qemu-nbd --connect=3D/dev/nbd0 /tmp/image.img $ sudo blkdiscard /dev/nbd0 To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1884831/+subscriptions