From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45072) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aLaOn-0005sT-LM for qemu-devel@nongnu.org; Tue, 19 Jan 2016 12:48:59 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aLaOj-0003QB-BF for qemu-devel@nongnu.org; Tue, 19 Jan 2016 12:48:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:33066) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aLaOj-0003Pz-0z for qemu-devel@nongnu.org; Tue, 19 Jan 2016 12:48:53 -0500 Date: Tue, 19 Jan 2016 18:48:49 +0100 From: Kevin Wolf Message-ID: <20160119174849.GH4579@noname.redhat.com> References: <1452517517-3953-1-git-send-email-vsementsov@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1452517517-3953-1-git-send-email-vsementsov@virtuozzo.com> Subject: Re: [Qemu-devel] [PATCH v7] spec: add qcow2 bitmaps extension specification List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vladimir Sementsov-Ogievskiy Cc: famz@redhat.com, qemu-devel@nongnu.org, mreitz@redhat.com, stefanha@redhat.com, den@openvz.org, jsnow@redhat.com Am 11.01.2016 um 14:05 hat Vladimir Sementsov-Ogievskiy geschrieben: > The new feature for qcow2: storing bitmaps. > > This patch adds new header extension to qcow2 - Bitmaps Extension. It > provides an ability to store virtual disk related bitmaps in a qcow2 > image. For now there is only one type of such bitmaps: Dirty Tracking > Bitmap, which just tracks virtual disk changes from some moment. > > Note: Only bitmaps, relative to the virtual disk, stored in qcow2 file, > should be stored in this qcow2 file. The size of each bitmap > (considering its granularity) is equal to virtual disk size. > > Signed-off-by: Vladimir Sementsov-Ogievskiy > --- > > v7: > > - Rewordings, grammar. > Max, Eric, John, thank you very much. > > - add last paragraph: remaining bits in bitmap data clusters must be > zero. > > - s/Bitmap Directory/bitmap directory/ and other names like this at > the request of Max. > > v6: > > - reword bitmap_directory_size description > - bitmap type: make 0 reserved > - extra_data_size: resize to 4bytes > Also, I've marked this field as "must be zero". We can always change > it, if we decide allowing managing app to specify any extra data, by > defining some magic value as a top of user extra data.. So, for now > non zeor extra_data_size should be considered as an error. > - swap name and extra_data to give good alignment to extra_data. > > > v5: > > - 'Dirty bitmaps' renamed to 'Bitmaps', as we may have several types of > bitmaps. > - rewordings > - move upper bounds to "Notes about Qemu limits" > - s/should/must somewhere. (but not everywhere) > - move name_size field closer to name itself in bitmap header > - add extra data area to bitmap header > - move bitmap data description to separate section > > docs/specs/qcow2.txt | 172 ++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 171 insertions(+), 1 deletion(-) > > diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt > index 121dfc8..997239d 100644 > --- a/docs/specs/qcow2.txt > +++ b/docs/specs/qcow2.txt > @@ -103,7 +103,18 @@ in the description of a field. > write to an image with unknown auto-clear features if it > clears the respective bits from this field first. > > - Bits 0-63: Reserved (set to 0) > + Bit 0: Bitmaps extension bit > + This bit indicates consistency for the bitmaps > + extension data. > + > + It is an error if this bit is set without the > + bitmaps extension present. > + > + If the bitmaps extension is present but this > + bit is unset, the bitmaps extension data is > + inconsistent. It may as well be consistent, but we don't know. Perhaps something like "must be considered inconsistent" or "is potentially inconsistent". > + > + Bits 1-63: Reserved (set to 0) > > 96 - 99: refcount_order > Describes the width of a reference count block entry (width > @@ -123,6 +134,7 @@ be stored. Each extension has a structure like the following: > 0x00000000 - End of the header extension area > 0xE2792ACA - Backing file format name > 0x6803f857 - Feature name table > + 0x23852875 - Bitmaps extension > other - Unknown header extension, can be safely > ignored > > @@ -166,6 +178,34 @@ the header extension data. Each entry look like this: > terminated if it has full length) > > > +== Bitmaps extension == > + > +The bitmaps extension is an optional header extension. It provides the ability > +to store bitmaps related to a virtual disk. For now, there is only one bitmap > +type: the dirty tracking bitmap, which tracks virtual disk changes from some > +point in time. I have one major problem with this patch, and it starts here. The spec talks about dirty tracking bitmaps all the way, but it never really defines what a dirty tracking bitmap even contains. It has a few hints here and there, but they aren't consistent. Here's the first hint: They track "virtual disk changes", which implies they track guest clusters rather than host clusters. > +The data of the extension should be considered consistent only if the > +corresponding auto-clear feature bit is set, see autoclear_features above. > + > +The fields of the bitmaps extension are: > + > + 0 - 3: nb_bitmaps > + The number of bitmaps contained in the image. Must be > + greater than or equal to 1. > + > + Note: Qemu currently only supports up to 65535 bitmaps per > + image. > + > + 4 - 7: bitmap_directory_size > + Size of the bitmap directory in bytes. It is the cumulative > + size of all (nb_bitmaps) bitmap headers. > + > + 8 - 15: bitmap_directory_offset > + Offset into the image file at which the bitmap directory > + starts. Must be aligned to a cluster boundary. > + > + > == Host cluster management == > > qcow2 manages the allocation of host clusters by maintaining a reference count > @@ -360,3 +400,133 @@ Snapshot table entry: > > variable: Padding to round up the snapshot table entry size to the > next multiple of 8. > + > + > +== Bitmaps == > + > +As mentioned above, the bitmaps extension provides the ability to store bitmaps > +related a virtual disk. This section describes how these bitmaps are stored. s/related/related to/ > +Note: all bitmaps are related to the virtual disk stored in this image. > + > +=== Bitmap directory === > + > +Each bitmap saved in the image is described in a bitmap directory entry. The > +bitmap directory is a contiguous area in the image file, whose starting offset > +and length are given by the header extension fields bitmap_directory_offset and > +bitmap_directory_size. The entries of the bitmap directory have variable > +length, depending on the length of the bitmap name and extra data. These > +entries are also called bitmap headers. > + > +Structure of a bitmap directory entry: > + > + Byte 0 - 7: bitmap_table_offset > + Offset into the image file at which the bitmap table > + (described below) for the bitmap starts. Must be aligned to > + a cluster boundary. > + > + 8 - 11: bitmap_table_size > + Number of entries in the bitmap table of the bitmap. > + > + 12 - 15: flags > + Bit > + 0: in_use > + The bitmap was not saved correctly and may be > + inconsistent. > + > + 1: auto > + The bitmap must reflect all changes of the virtual > + disk by any application that would write to this qcow2 > + file (including writes, snapshot switching, etc.). The > + type of this bitmap must be 'dirty tracking bitmap'. This suggests that we can represent snapshot switching in a dirty tracking bitmap. Which is almost impossible if we track guest clusters, except if loading a snapshot should mean that all bits in the bitmap are set. However, that feels a bit useless and dirty tracking across snapshots doesn't seem to make a whole lot of sense anyway. Maybe what would make more sense is that a bitmap is tied to a specific snapshot or bitmaps are included in a snapshot, so that you can revert to the dirty status as it was when the snapshot was taken. Or, if you really meant, that the tracking words on a host cluster level, what clusters would be included in the bitmap? Would only the virtual disk be included? VM state? Any metadata? It seems we definitely need a new section on dirty tracking bitmaps that describes what the bitmap actually means and how it's supposed to work with snapshots. I guess we could also talk about how it works with other changes to the image like resizing. > + Bits 2 - 31 are reserved and must be 0. > + > + 16: type > + This field describes the sort of the bitmap. > + Values: > + 1: Dirty tracking bitmap > + > + Values 0, 2 - 255 are reserved. > + > + 17: granularity_bits > + Granularity bits. Valid values: 0 - 63. > + > + Note: Qemu currently doesn't support granularity_bits > + greater than 31. > + > + Granularity is calculated as > + granularity = 1 << granularity_bits > + > + A bitmap's granularity is how many bytes of the image > + accounts for one bit of the bitmap. > + > + 18 - 19: name_size > + Size of the bitmap name. Must be non-zero. > + > + Note: Qemu currently doesn't support values greater than > + 1023. > + > + 20 - 23: extra_data_size > + Size of type-specific extra data. > + > + For now, as no extra data is defined, extra_data_size is > + reserved and must be zero. > + > + variable: Type-specific extra data for the bitmap. We talked about this in the other subthread. > + variable: The name of the bitmap (not null terminated). Must be > + unique among all bitmap names within the bitmaps extension. > + > + variable: Padding to round up the bitmap directory entry size to the > + next multiple of 8. > + > +=== Bitmap table === > + > +Bitmaps are stored using a one-level structure (as opposed to two-level > +structure like for refcounts and guest clusters mapping) for the mapping of > +bitmap data to host clusters. This structure is called the bitmap table. > + > +Each bitmap table has a variable size (stored in the bitmap directory Entry) > +and may use multiple clusters, however, it must be contiguous in the image > +file. > + > +Structure of a bitmap table entry: > + > + Bit 0: Reserved and must be zero if bits 9 - 55 are non-zero. > + If bits 9 - 55 are zero: > + 0: Cluster should be read as all zeros. > + 1: Cluster should be read as all ones. > + > + 1 - 8: Reserved and must be zero. > + > + 9 - 55: Bits 9 - 55 of the host cluster offset. Must be aligned to > + a cluster boundary. If the offset is 0, the cluster is > + unallocated; in that case, bit 0 determines how this > + cluster should be treated when read from. > + > + 56 - 63: Reserved and must be zero. > + > +=== Bitmap data === > + > +As noted above, bitmap data is stored in separate clusters, described by the > +bitmap table. Given an offset (in bytes) into the bitmap data, the offset into > +the image file can be obtained as follows: > + > + image_offset = > + bitmap_table[bitmap_data_offset / cluster_size] + > + (bitmap_data_offset % cluster_size) > + > +This offset is not defined if bits 9 - 55 of bitmap table entry are zero (see > +above). > + > +Given an offset byte_nr into the virtual disk and the bitmap's granularity, the > +bit offset into the bitmap can be calculated like this: > + > + bit_offset = > + image_offset(byte_nr / granularity / 8) * 8 + > + (byte_nr / granularity) % 8 > + > +If the size of the bitmap data is not a multiply of cluster size then the last > +cluster of the bitmap data contains some unused tail bits. These bits must be > +zero. In which order are the bits stored in the bitmap? Kevin