From patchwork Tue Jan 19 14:15:11 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Stefan Hajnoczi X-Patchwork-Id: 12029899 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=-11.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,MIME_BASE64_TEXT,SPF_HELO_NONE,SPF_PASS 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 E3D00C433E6 for ; Tue, 19 Jan 2021 14:17:35 +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 401322312E for ; Tue, 19 Jan 2021 14:17:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 401322312E Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:56914 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l1rp8-0003bq-CX for qemu-devel@archiver.kernel.org; Tue, 19 Jan 2021 09:17:34 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:37840) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l1rnD-0001xO-2H for qemu-devel@nongnu.org; Tue, 19 Jan 2021 09:15:35 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:49368) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l1rn7-0000lT-FJ for qemu-devel@nongnu.org; Tue, 19 Jan 2021 09:15:34 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611065727; h=from:from: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; bh=wc7GRCDPSOIR7yd1tw+n0BQk5wmXJSW7uUUNEMigbfY=; b=MfOFPDyxXJQ/qH2XcbMEe1FtDUr4Hme+UuamjgO3kT7ynp+kRfkt4+pi/2Myd6ktsW4DMi U2Y5G8toUiPoiLlLIDq+hjZUujvKTPsZ0SC3IoaPGrhPo2aSe4OIcv6TRqtaHYk0jwwg02 uHRI2WRb1ZYRDDHgiybjMRZvZRbJQt0= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-556-nEbdkSZrPNGOhUYbO_zbnA-1; Tue, 19 Jan 2021 09:15:20 -0500 X-MC-Unique: nEbdkSZrPNGOhUYbO_zbnA-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id CF75A1800D41 for ; Tue, 19 Jan 2021 14:15:19 +0000 (UTC) Received: from localhost (ovpn-115-72.ams2.redhat.com [10.36.115.72]) by smtp.corp.redhat.com (Postfix) with ESMTP id B60A760CF0; Tue, 19 Jan 2021 14:15:12 +0000 (UTC) From: Stefan Hajnoczi To: qemu-devel@nongnu.org Subject: [qemu-web PATCH v3] Add virtio-blk and virtio-scsi configuration post Date: Tue, 19 Jan 2021 14:15:11 +0000 Message-Id: <20210119141511.288131-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Received-SPF: pass client-ip=63.128.21.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -12 X-Spam_score: -1.3 X-Spam_bar: - X-Spam_report: (-1.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.195, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MIME_BASE64_TEXT=1.741, RCVD_IN_DNSWL_LOW=-0.7, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Paolo Bonzini , Thomas Huth , =?utf-8?q?Daniel_P_=2E_Berrang=C3=A9?= , Stefan Hajnoczi , slp@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" The second post in the storage series covers virtio-blk and virtio-scsi. It compares the two and offers recommendations that users and tools using QEMU can use as a starting point. Graphs are included comparing the performance of various options. Signed-off-by: Stefan Hajnoczi --- v3: * Mention guest drivers for other operation systems [Thomas] * Reorder virtio-blk/virtio-scsi key points [Thomas] * Mention that virtio-blk is limited to disk emulation and doesn't handle CD-ROM drivers, for example [Thomas] v2: * Mention default storage controllers for non-x86 [danpb] * Remove statement about read-only ISO virtio-blk devices, both Rich Jones and Daniel Berrange commented about this at different times so I decided to drop it completely [danpb] * Clarify PCI slot limits and multi-function discussion for virtio-blk [danpb] * Mention multiple virtio-scsi devices for NUMA tuning and that emulated LUNs can be HDDs or CD-ROMs [danpb] --- ...021-01-19-virtio-blk-scsi-configuration.md | 117 ++++++++++++++++++ screenshots/2020-09-15-scsi-devices.svg | 1 + screenshots/2020-09-15-virtio-blk-vs-scsi.svg | 1 + 3 files changed, 119 insertions(+) create mode 100644 _posts/2021-01-19-virtio-blk-scsi-configuration.md create mode 100644 screenshots/2020-09-15-scsi-devices.svg create mode 100644 screenshots/2020-09-15-virtio-blk-vs-scsi.svg diff --git a/_posts/2021-01-19-virtio-blk-scsi-configuration.md b/_posts/2021-01-19-virtio-blk-scsi-configuration.md new file mode 100644 index 0000000..a9c7a35 --- /dev/null +++ b/_posts/2021-01-19-virtio-blk-scsi-configuration.md @@ -0,0 +1,117 @@ +--- +layout: post +title: "Configuring virtio-blk and virtio-scsi Devices" +date: 2021-01-19 07:00:00 +0000 +author: Stefan Hajnoczi and Sergio Lopez +categories: [storage] +--- +The [previous article](https://www.qemu.org/2020/09/14/qemu-storage-overview/) +in this series introduced QEMU storage concepts. Now we move on to look at the +two most popular emulated storage controllers for virtualization: virtio-blk +and virtio-scsi. + +This post provides recommendations for configuring virtio-blk and virtio-scsi +and how to choose between the two devices. The recommendations provide good +performance in a wide range of use cases and are suitable as default settings +in tools that use QEMU. + +## Virtio storage devices +### Key points +* Prefer virtio storage devices over other emulated storage controllers. +* Use the latest virtio drivers. + +Virtio devices are recommended over other emulated storage controllers as they +are generally the most performant and fully-featured storage controllers in +QEMU. + +Unlike emulations of hardware storage controllers, virtio-blk and virtio-scsi +are specifically designed and optimized for virtualization. The details of how +they work are published for driver and device implementors in the [VIRTIO +specification](https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html). + +Virtio drivers are available for Linux, Windows, and other operating systems. +Installing the latest version is recommended for the latest bug fixes and +performance enhancements. + +If virtio drivers are not available, the AHCI (SATA) device is widely supported +by modern x86 operating systems and can be used as a fallback. On non-x86 +guests the default storage controller can be used as a fallback. + +## Comparing virtio-blk and virtio-scsi +### Key points +* Prefer virtio-blk in performance-critical use cases. +* Prefer virtio-scsi for attaching more than 28 disks or for full SCSI support. +* With virtio-scsi, use scsi-block for SCSI passthrough and otherwise use scsi-hd. + +Two virtio storage controllers are available: virtio-blk and virtio-scsi. + +### virtio-blk +The virtio-blk device presents a block device to the virtual machine. Each +virtio-blk device appears as a disk inside the guest. virtio-blk was available +before virtio-scsi and is the most widely deployed virtio storage controller. + +The virtio-blk device offers high performance thanks to a thin software stack +and is therefore a good choice when performance is a priority. It does not +support non-disk devices such as CD-ROM drives. + +CD-ROMs and in general any application that sends SCSI commands are better +served by the virtio-scsi device, which has full SCSI support. SCSI passthrough +was removed from the Linux virtio-blk driver in v5.6 in favor of using +virtio-scsi. + +Virtual machines that require access to many disks can hit limits based on +availability of PCI slots, which are under contention with other devices +exposed to the guest, such as NICs. For example a typical i440fx machine type +default configuration allows for about 28 disks. It is possible to use +multi-function devices to pack multiple virtio-blk devies into a single PCI +slot at the cost of losing hotplug support, or additional PCI busses can be +defined. Generally though it is simpler to use a single virtio-scsi PCI adapter +instead. + +### virtio-scsi +The virtio-scsi device presents a SCSI Host Bus Adapter to the virtual machine. +SCSI offers a richer command set than virtio-blk and supports more use cases. + +Each device supports up to 16,383 LUNs (disks) per target and up to 255 +targets. This allows a single virtio-scsi device to handle all disks in a +virtual machine, although defining more virtio-scsi devices makes it possible +to tune for NUMA topology as we will see in a later blog post. + +Emulated LUNs can be exposed as hard disk drives or CD-ROMs. Physical SCSI +devices can be passed through into the virtual machine, including CD-ROM +drives, tapes, and other devices besides hard disk drives. + +Clustering software that uses SCSI Persistent Reservations is supported by virtio-scsi, but not by virtio-blk. + +Performance of virtio-scsi may be lower than virtio-blk due to a thicker software stack, but in many use cases, this is not a significant factor. The following graph compares 4KB random read performance at various queue depths: + +![Comparing virtio-blk and virtio-scsi performance](/screenshots/2020-09-15-virtio-blk-vs-scsi.svg) + +### virtio-scsi configuration +The following SCSI devices are available with virtio-scsi: + +|Device|SCSI Passthrough|Performance| +|------|----------------|-----------| +|scsi-hd|No|Highest| +|scsi-block|Yes|Lower| +|scsi-generic|Yes|Lowest| + +The scsi-hd device is suitable for disk image files and host block devices +when SCSI passthrough is not required. + +The scsi-block device offers SCSI passthrough and is preferred over +scsi-generic due to higher performance. + +The following graph compares the sequential I/O performance of these devices +using virtio-scsi with an iothread: + +![Comparing scsi-hd, scsi-block, and scsi-generic performance](/screenshots/2020-09-15-scsi-devices.svg) + +## Conclusion +The virtio-blk and virtio-scsi offer a choice between a single block device and +a full-fledged SCSI Host Bus Adapter. Virtualized guests typically use one or +both of them depending on functional and performance requirements. This post +compared the two and offered recommendations on how to choose between them. + +The next post in this series will discuss the iothreads feature that both +virtio-blk and virtio-scsi support for increased performance. diff --git a/screenshots/2020-09-15-scsi-devices.svg b/screenshots/2020-09-15-scsi-devices.svg new file mode 100644 index 0000000..c6f8311 --- /dev/null +++ b/screenshots/2020-09-15-scsi-devices.svg @@ -0,0 +1 @@ + \ No newline at end of file diff --git a/screenshots/2020-09-15-virtio-blk-vs-scsi.svg b/screenshots/2020-09-15-virtio-blk-vs-scsi.svg new file mode 100644 index 0000000..87b083c --- /dev/null +++ b/screenshots/2020-09-15-virtio-blk-vs-scsi.svg @@ -0,0 +1 @@ + \ No newline at end of file