From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:42798) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aJgBi-0006JL-Qh for qemu-devel@nongnu.org; Thu, 14 Jan 2016 06:35:36 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aJgBe-0000XD-Ne for qemu-devel@nongnu.org; Thu, 14 Jan 2016 06:35:34 -0500 Received: from relay.parallels.com ([195.214.232.42]:60286) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aJgBe-0000Wa-AW for qemu-devel@nongnu.org; Thu, 14 Jan 2016 06:35:30 -0500 References: <1452517517-3953-1-git-send-email-vsementsov@virtuozzo.com> <56944923.2060603@redhat.com> From: "Denis V. Lunev" Message-ID: <569787F6.5020903@openvz.org> Date: Thu, 14 Jan 2016 14:35:18 +0300 MIME-Version: 1.0 In-Reply-To: <56944923.2060603@redhat.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit 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: John Snow , Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org Cc: kwolf@redhat.com, famz@redhat.com, stefanha@redhat.com, mreitz@redhat.com On 01/12/2016 03:30 AM, John Snow wrote: > > On 01/11/2016 08:05 AM, Vladimir Sementsov-Ogievskiy wrote: >> 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. >> + >> + 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. >> + >> +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. >> + >> +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'. >> + >> + 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. >> + >> + 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. >> > s/multiply/multiple/. > > Looks good otherwise, pending the results of our discussion about what > to do about the extra_data_size / type-specific data fields. > > Thanks! > should we re-submit? Is this version good enough to be accepted? :) Den