From nobody Sat Feb 7 10:15:18 2026 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; envelope-from=libvir-list-bounces@redhat.com; helo=mx1.redhat.com; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=libvir-list-bounces@redhat.com; dmarc=pass(p=none dis=none) header.from=redhat.com Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by mx.zohomail.com with SMTPS id 1549339046812991.9748344633718; Mon, 4 Feb 2019 19:57:26 -0800 (PST) Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B655459440; Tue, 5 Feb 2019 03:57:20 +0000 (UTC) Received: from colo-mx.corp.redhat.com (colo-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.20]) by smtp.corp.redhat.com (Postfix) with ESMTPS id A314660BE8; Tue, 5 Feb 2019 03:57:17 +0000 (UTC) Received: from lists01.pubmisc.prod.ext.phx2.redhat.com (lists01.pubmisc.prod.ext.phx2.redhat.com [10.5.19.33]) by colo-mx.corp.redhat.com (Postfix) with ESMTP id 6677518005CB; Tue, 5 Feb 2019 03:57:13 +0000 (UTC) Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) by lists01.pubmisc.prod.ext.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id x153vBbq027309 for ; Mon, 4 Feb 2019 22:57:11 -0500 Received: by smtp.corp.redhat.com (Postfix) id 9F56A68D48; Tue, 5 Feb 2019 03:57:11 +0000 (UTC) Received: from blue.redhat.com (ovpn-116-162.phx2.redhat.com [10.3.116.162]) by smtp.corp.redhat.com (Postfix) with ESMTP id B3D255C57C; Tue, 5 Feb 2019 03:57:06 +0000 (UTC) From: Eric Blake To: qemu-devel@nongnu.org Date: Mon, 4 Feb 2019 21:56:57 -0600 Message-Id: <20190205035704.26014-2-eblake@redhat.com> In-Reply-To: <20190205035704.26014-1-eblake@redhat.com> References: <20190205035704.26014-1-eblake@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-loop: libvir-list@redhat.com Cc: "reviewer:Incompatible changes" , "open list:Network Block Dev..." , Stefano Garzarella Subject: [libvirt] [PULL 1/8] qemu-nbd: Deprecate qemu-nbd --partition X-BeenThere: libvir-list@redhat.com X-Mailman-Version: 2.1.12 Precedence: junk List-Id: Development discussions about the libvirt library & tools List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: quoted-printable Sender: libvir-list-bounces@redhat.com Errors-To: libvir-list-bounces@redhat.com X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Tue, 05 Feb 2019 03:57:23 +0000 (UTC) Content-Type: text/plain; charset="utf-8" The existing qemu-nbd --partition code claims to handle logical partitions up to 8, since its introduction in 2008 (commit 7a5ca86). However, the implementation is bogus (actual MBR logical partitions form a sort of linked list, with one partition per extended table entry, rather than four logical partitions in a single extended table), making the code unlikely to work for anything beyond -P5 on actual guest images. What's more, the code does not support GPT partitions, which are becoming more popular, and maintaining device subsetting in both NBD and the raw device is unnecessary duplication of effort (even if it is not too difficult). Note that obtaining the offsets of a partition (MBR or GPT) can be learned by using 'qemu-nbd -c /dev/nbd0 file.qcow2 && sfdisk --dump /dev/nbd0', but by the time you've done that, you might as well just mount /dev/nbd0p1 that the kernel creates for you instead of bothering with qemu exporting a subset. Or, keeping to just user-space code, use nbdkit's partition filter, which has already known both GPT and primary MBR partitions for a while, and was just recently enhanced to support arbitrary logical MBR parititions. Start the clock on the deprecation cycle, with examples of how to accomplish device subsetting without using -P. Signed-off-by: Eric Blake Message-Id: <20190125234837.2272-1-eblake@redhat.com> Reviewed-by: Richard W.M. Jones Reviewed-by: Stefano Garzarella --- qemu-deprecated.texi | 33 +++++++++++++++++++++++++++++++++ qemu-nbd.texi | 6 ++++-- qemu-nbd.c | 2 ++ 3 files changed, 39 insertions(+), 2 deletions(-) diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi index 9cc20b365c5..8a6174df0c1 100644 --- a/qemu-deprecated.texi +++ b/qemu-deprecated.texi @@ -160,3 +160,36 @@ Example of legacy encoding: The above, converted to the current supported format: @code{json:@{"file.driver":"rbd", "file.pool":"rbd", "file.image":"name"@}} + +@section Related binaries + +@subsection qemu-nbd --partition (since 4.0.0) + +The ``qemu-nbd --partition $digit'' code (also spelled @option{-P}) +can only handle MBR partitions, and has never correctly handled +logical partitions beyond partition 5. If you know the offset and +length of the partition (perhaps by using @code{sfdisk} within the +guest), you can achieve the effect of exporting just that subset of +the disk by use of the @option{--image-opts} option with a raw +blockdev using the @code{offset} and @code{size} parameters layered on +top of any other existing blockdev. For example, if partition 1 is +100MiB long starting at 1MiB, the old command: + +@code{qemu-nbd -t -P 1 -f qcow2 file.qcow2} + +can be rewritten as: + +@code{qemu-nbd -t --image-opts driver=3Draw,offset=3D1M,size=3D100M,file.d= river=3Dqcow2,file.backing.driver=3Dfile,file.backing.filename=3Dfile.qcow2} + +Alternatively, the @code{nbdkit} project provides a more powerful +partition filter on top of its nbd plugin, which can be used to select +an arbitrary MBR or GPT partition on top of any other full-image NBD +export. Using this to rewrite the above example results in: + +@code{qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &} +@code{nbdkit -f --filter=3Dpartition nbd socket=3D/tmp/sock partition=3D1} + +Note that if you are exposing the export via /dev/nbd0, it is easier +to just export the entire image and then mount only /dev/nbd0p1 than +it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a +subset of the image. diff --git a/qemu-nbd.texi b/qemu-nbd.texi index 386bece4680..d0c51828149 100644 --- a/qemu-nbd.texi +++ b/qemu-nbd.texi @@ -56,8 +56,10 @@ auto-detecting. @item -r, --read-only Export the disk as read-only. @item -P, --partition=3D@var{num} -Only expose MBR partition @var{num}. Understands physical partitions -1-4 and logical partitions 5-8. +Deprecated: Only expose MBR partition @var{num}. Understands physical +partitions 1-4 and logical partition 5. New code should instead use +@option{--image-opts} with the raw driver wrapping a subset of the +original image. @item -B, --bitmap=3D@var{name} If @var{filename} has a qcow2 persistent bitmap @var{name}, expose that bitmap via the ``qemu:dirty-bitmap:@var{name}'' context diff --git a/qemu-nbd.c b/qemu-nbd.c index 1f7b2a03f5d..00c07fd27ea 100644 --- a/qemu-nbd.c +++ b/qemu-nbd.c @@ -787,6 +787,8 @@ int main(int argc, char **argv) flags &=3D ~BDRV_O_RDWR; break; case 'P': + warn_report("The '-P' option is deprecated; use --image-opts w= ith " + "a raw device wrapper for subset exports instead"); if (qemu_strtoi(optarg, NULL, 0, &partition) < 0 || partition < 1 || partition > 8) { error_report("Invalid partition '%s'", optarg); --=20 2.20.1 -- libvir-list mailing list libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list