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=-7.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 8C4AEC352A4 for ; Mon, 10 Feb 2020 22:53:48 +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 56D1D20715 for ; Mon, 10 Feb 2020 22:53:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="CWG6n5sK" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 56D1D20715 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]:40720 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j1Hw3-0002NG-Ep for qemu-devel@archiver.kernel.org; Mon, 10 Feb 2020 17:53:47 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:59082) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1j1HvS-0001nf-7i for qemu-devel@nongnu.org; Mon, 10 Feb 2020 17:53:11 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1j1HvO-0007Zy-Fz for qemu-devel@nongnu.org; Mon, 10 Feb 2020 17:53:07 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:53555 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1j1HvO-0007Z4-2q for qemu-devel@nongnu.org; Mon, 10 Feb 2020 17:53:06 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1581375185; 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: in-reply-to:in-reply-to:references:references; bh=M0YL0KwAl91h5En4qIho0R1yy996iG8vS6nW1PYexfM=; b=CWG6n5sKSbBSSwfcPCJ379ZNTFQsLRA05YHMRnnhSTKehhVgieoQUGooFW9Bg/jNj97nzJ 3XpxjkMGlmNbTu7qWdtsZxdS3fN/ahp3kMSz4+m1qrO8rvFQTn11TLkGY16SoiGCcRTFAG sqXyv2BNvAvvsHQzYcQMUdviP/webeE= 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-248-LaBlZp-oPQm4go2tU6HvyQ-1; Mon, 10 Feb 2020 17:53:00 -0500 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 422AD800D48; Mon, 10 Feb 2020 22:52:59 +0000 (UTC) Received: from localhost (unknown [10.36.118.78]) by smtp.corp.redhat.com (Postfix) with ESMTP id 34DE21001B07; Mon, 10 Feb 2020 22:52:56 +0000 (UTC) Date: Mon, 10 Feb 2020 22:52:55 +0000 From: "Richard W.M. Jones" To: Eric Blake Subject: Re: Cross-project NBD extension proposal: NBD_INFO_INIT_STATE Message-ID: <20200210225255.GJ3888@redhat.com> References: <20200210221234.GH3888@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-MC-Unique: LaBlZp-oPQm4go2tU6HvyQ-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 205.139.110.120 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: Vladimir Sementsov-Ogievskiy , Alberto Garcia , "qemu-block@nongnu.org" , QEMU , Max Reitz , "nbd@other.debian.org" , "libguestfs@redhat.com" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Mon, Feb 10, 2020 at 04:29:53PM -0600, Eric Blake wrote: > On 2/10/20 4:12 PM, Richard W.M. Jones wrote: > >On Mon, Feb 10, 2020 at 03:37:20PM -0600, Eric Blake wrote: > >>For now, only 2 of those 16 bits are defined: NBD_INIT_SPARSE (the > >>image has at least one hole) and NBD_INIT_ZERO (the image reads > >>completely as zero); the two bits are orthogonal and can be set > >>independently, although it is easy enough to see completely sparse > >>files with both bits set. > > > >I think I'm confused about the exact meaning of NBD_INIT_SPARSE. Do > >you really mean the whole image is sparse; or (as you seem to have > >said above) that there exists a hole somewhere in the image but we're > >not saying where it is and there can be non-sparse parts of the image? >=20 > As implemented: >=20 > NBD_INIT_SPARSE - there is at least one hole somewhere (allocation > would be required to write to that part of the file), but there may > b allocated data elsewhere in the image. Most disk images will fit > this definition (for example, it is very common to have a hole > between the MBR or GPT and the first partition containing a file > system, or for file systems themselves to be sparse within the > larger block device). I think I'm still confused about why this particular flag would be useful for clients (I can completely understand why clients need NBD_INIT_ZERO). But anyway ... could a flag indicating that the whole image is sparse be useful, either as well as NBD_INIT_SPARSE or instead of it? You could use it to avoid an initial disk trim, which is something that mke2fs does: https://github.com/tytso/e2fsprogs/blob/0670fc20df4a4bbbeb0edb30d82628ea3= 0a80598/misc/mke2fs.c#L2768 and which is painfully slow over NBD for very large devices because of the 32 bit limit on request sizes - try doing mke2fs on a 1E nbdkit memory disk some time. > NBD_INIT_ZERO - all bytes read as zero. >=20 > The combination NBD_INIT_SPARSE|NBD_INIT_ZERO is common (generally, > if you use lseek(SEEK_DATA) to prove the entire image reads as > zeroes, you also know the entire image is sparse), but NBD_INIT_ZERO > in isolation is also possible (especially with the qcow2 proposal of > a persistent autoclear bit, where even with a fully preallocated > qcow2 image you still know it reads as zeroes but there are no > holes). But you are also right that for servers that can advertise > both bits efficiently, NBD_INIT_SPARSE in isolation may be more > common than NBD_INIT_SPARSE|NBD_INIT_ZERO (the former for most disk > images, the latter only for a freshly-created image that happens to > create with zero initialization). >=20 > What's more, in my patches, I did NOT patch qemu to set or consume > INIT_SPARSE; so far, it only sets/consumes INIT_ZERO. Of course, if > we can find a reason WHY qemu should track whether a qcow2 image is > fully-allocated, by demonstrating a qemu-img algorithm that becomes > easier for knowing if an image is sparse (even if our justification > is: "when copying an image, I want to know if the _source_ is > sparse, to know whether I have to bend over backwards to preallocate > the destination"), then using that in qemu makes sense for my v2 > patches. But for v1, my only justification was "when copying an > image, I can skip holes in the source if I know the _destination_ > already reads as zeroes", which only needed INIT_ZERO. >=20 > Some of the nbdkit patches demonstrate the some-vs.-all nature of > the two bits; for example, in the split plugin, I initialize > h->init_sparse =3D false; h->init_zero =3D true; then in a loop over > each file change h->init_sparse to true if at least one file was > sparse, and change h->init_zero to false if at least one file had > non-zero contents. Rich. --=20 Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjon= es Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html