From nobody Sun Oct 5 19:23:37 2025 Delivered-To: importer@patchew.org Received-SPF: pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) client-ip=208.118.235.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Authentication-Results: mx.zohomail.com; spf=pass (zoho.com: domain of gnu.org designates 208.118.235.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org Return-Path: Received: from lists.gnu.org (208.118.235.17 [208.118.235.17]) by mx.zohomail.com with SMTPS id 1519749223574804.3044312878251; Tue, 27 Feb 2018 08:33:43 -0800 (PST) Received: from localhost ([::1]:38560 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eqiC2-0005pk-O7 for importer@patchew.org; Tue, 27 Feb 2018 11:33:30 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58940) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1eqi8Y-00036h-BU for qemu-devel@nongnu.org; Tue, 27 Feb 2018 11:29:55 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1eqi8W-0005lA-82 for qemu-devel@nongnu.org; Tue, 27 Feb 2018 11:29:54 -0500 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:35352 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1eqi8S-0005iN-Hv; Tue, 27 Feb 2018 11:29:48 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 1A195814DF58; Tue, 27 Feb 2018 16:29:48 +0000 (UTC) Received: from red.redhat.com (ovpn-122-122.rdu2.redhat.com [10.10.122.122]) by smtp.corp.redhat.com (Postfix) with ESMTP id 96D64AFD60; Tue, 27 Feb 2018 16:29:47 +0000 (UTC) From: Eric Blake To: qemu-devel@nongnu.org Date: Tue, 27 Feb 2018 10:29:41 -0600 Message-Id: <20180227162944.17343-2-eblake@redhat.com> In-Reply-To: <20180227155514.16721-1-eblake@redhat.com> References: <20180227155514.16721-1-eblake@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Tue, 27 Feb 2018 16:29:48 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.8]); Tue, 27 Feb 2018 16:29:48 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'eblake@redhat.com' RCPT:'' X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 66.187.233.73 Subject: [Qemu-devel] [PATCH v4 2/5] qcow2: Document some maximum size constraints X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwolf@redhat.com, berto@igalia.com, qemu-block@nongnu.org, Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail: RSF_0 Z_629925259 SPT_0 Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Although off_t permits up to 63 bits (8EB) of file offsets, in practice, we're going to hit other limits first. Document some of those limits in the qcow2 spec, and how choice of cluster size can influence some of the limits. While at it, notice that since we cannot map any virtual cluster to any address higher than 64 PB (56 bits) (due to the current L1/L2 field encoding stopping at bit 55), it makes little sense to require the refcount table to access host offsets beyond that point. Mark the upper bits of the refcount table entries as reserved to match the L1/L2 table, with no ill effects, since it is unlikely that there are any existing images larger than 64PB in the first place, and thus all existing images already have those bits as 0. If 64PB proves to be too small in the future, we could enlarge all three uses of bit 55 into the reserved bits at that time. However, there is one limit that reserved bits don't help with: for compressed clusters, the L2 layout requires an ever-smaller maximum host offset as cluster size gets larger, down to a 512 TB maximum with 2M clusters. Signed-off-by: Eric Blake -- v4: more wording tweaks v3: new patch --- docs/interop/qcow2.txt | 38 +++++++++++++++++++++++++++++++++++--- 1 file changed, 35 insertions(+), 3 deletions(-) diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt index feb711fb6a8..e32d391e66b 100644 --- a/docs/interop/qcow2.txt +++ b/docs/interop/qcow2.txt @@ -40,7 +40,17 @@ The first cluster of a qcow2 image contains the file hea= der: with larger cluster sizes. 24 - 31: size - Virtual disk size in bytes + Virtual disk size in bytes. + + Note: with a 2 MB cluster size, the maximum + virtual size is 2 EB (61 bits) for a sparse file, + but other sizing limitations in refcount and L1/L2 + tables mean that an image cannot have more than 64 + PB of populated clusters (and an image may hit + other sizing limitations as well, such as + underlying protocol limits). With a 512 byte + cluster size, the maximum virtual size drops to + 128 GB (37 bits). 32 - 35: crypt_method 0 for no encryption @@ -318,6 +328,13 @@ for each host cluster. A refcount of 0 means that the = cluster is free, 1 means that it is used, and >=3D 2 means that it is used and any write access must perform a COW (copy on write) operation. +The refcount table has implications on the maximum host file size; a +larger cluster size is required for the refcount table to cover larger +offsets. Furthermore, all qcow2 metadata must currently reside at +offsets below 64 PB (56 bits) (this limit could be enlarged by putting +reserved bits into use, but only if a similar limit on L1/L2 tables is +revisited at the same time). + The refcounts are managed in a two-level table. The first level is called refcount table and has a variable size (which is stored in the header). The refcount table can cover multiple clusters, however it needs to be contigu= ous @@ -341,7 +358,7 @@ Refcount table entry: Bit 0 - 8: Reserved (set to 0) - 9 - 63: Bits 9-63 of the offset into the image file at which t= he + 9 - 55: Bits 9-55 of the offset into the image file at which t= he refcount block starts. Must be aligned to a cluster boundary. @@ -349,6 +366,8 @@ Refcount table entry: been allocated. All refcounts managed by this refcount= block are 0. + 56 - 63: Reserved (set to 0) + Refcount block entry (x =3D refcount_bits - 1): Bit 0 - x: Reference count of the cluster. If refcount_bits impli= es a @@ -365,6 +384,17 @@ The L1 table has a variable size (stored in the header= ) and may use multiple clusters, however it must be contiguous in the image file. L2 tables are exactly one cluster in size. +The L1 and L2 tables have implications on the maximum virtual file +size; a larger cluster size is required for the guest to have access +to more space. Furthermore, a virtual cluster must currently map to a +host offset below 64 PB (56 bits) (this limit could be enlarged by +putting reserved bits into use, but only if a similar limit on +refcount tables is revisited at the same time). Additionally, with +larger cluster sizes, compressed clusters have a smaller limit on host +cluster mappings (a 2M cluster size requires compressed clusters to +reside below 512 TB (49 bits), where enlarging this would require an +incompatible layout change). + Given a offset into the virtual disk, the offset into the image file can be obtained as follows: @@ -427,7 +457,9 @@ Standard Cluster Descriptor: Compressed Clusters Descriptor (x =3D 62 - (cluster_bits - 8)): Bit 0 - x-1: Host cluster offset. This is usually _not_ aligned to a - cluster or sector boundary! + cluster or sector boundary! If cluster_bits is + small enough that this field includes bits beyond + 55, those upper bits must be set to 0. x - 61: Number of additional 512-byte sectors used for the compressed data, beyond the sector containing the offs= et --=20 2.14.3