[Qemu-devel] [PULL 03/34] spec: add qcow2 bitmaps extension specification

[Kevin Wolf <kwolf@redhat.com>]

From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

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 <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 docs/specs/qcow2.txt | 221 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 220 insertions(+), 1 deletion(-)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index f236d8c..80cdfd0 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 must be
+                                considered 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 +178,36 @@ 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:
+
+    Byte  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:  Reserved, must be zero.
+
+          8 - 15:  bitmap_directory_size
+                   Size of the bitmap directory in bytes. It is the cumulative
+                   size of all (nb_bitmaps) bitmap headers.
+
+         16 - 23:  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 +402,180 @@ 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 to a virtual disk. This section describes how these bitmaps are stored.
+
+All stored bitmaps are related to the virtual disk stored in the same image, so
+each bitmap size is equal to the virtual disk size.
+
+Each bit of the bitmap is responsible for strictly defined range of the virtual
+disk. For bit number bit_nr the corresponding range (in bytes) will be:
+
+    [bit_nr * bitmap_granularity .. (bit_nr + 1) * bitmap_granularity - 1]
+
+Granularity is a property of the concrete bitmap, see below.
+
+
+=== 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 lengths 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'.
+
+                      2: extra_data_compatible
+                         This flags is meaningful when the extra data is
+                         unknown to the software (currently any extra data is
+                         unknown to Qemu).
+                         If it is set, the bitmap may be used as expected, extra
+                         data must be left as is.
+                         If it is not set, the bitmap must not be used, but
+                         both it and its extra data be left as is.
+
+                    Bits 3 - 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 should be zero. If it is non-zero the
+                    behavior is defined by extra_data_compatible flag.
+
+        variable:   extra_data
+                    Extra data for the bitmap, occupying extra_data_size bytes.
+                    Extra data must never contain references to clusters or in
+                    some other way allocate additional clusters.
+
+        variable:   name
+                    The name of the bitmap (not null terminated), occupying
+                    name_size bytes. 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. All bytes of the padding must be zero.
+
+
+=== Bitmap table ===
+
+Each bitmap is stored using a one-level structure (as opposed to two-level
+structures 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 during reads.
+
+        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_data_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 image file to the corresponding bit of the bitmap can be
+calculated like this:
+
+    bit_offset(byte_nr) =
+        image_offset(byte_nr / granularity / 8) * 8 +
+            (byte_nr / granularity) % 8
+
+If the size of the bitmap data is not a multiple of the cluster size then the
+last cluster of the bitmap data contains some unused tail bits. These bits must
+be zero.
+
+
+=== Dirty tracking bitmaps ===
+
+Bitmaps with 'type' field equal to one are dirty tracking bitmaps.
+
+When the virtual disk is in use dirty tracking bitmap may be 'enabled' or
+'disabled'. While the bitmap is 'enabled', all writes to the virtual disk
+should be reflected in the bitmap. A set bit in the bitmap means that the
+corresponding range of the virtual disk (see above) was written to while the
+bitmap was 'enabled'. An unset bit means that this range was not written to.
+
+The software doesn't have to sync the bitmap in the image file with its
+representation in RAM after each write. Flag 'in_use' should be set while the
+bitmap is not synced.
+
+In the image file the 'enabled' state is reflected by the 'auto' flag. If this
+flag is set, the software must consider the bitmap as 'enabled' and start
+tracking virtual disk changes to this bitmap from the first write to the
+virtual disk. If this flag is not set then the bitmap is disabled.
-- 
1.8.3.1