parallels: add format spec

This specifies Parallels image format as implemented in Parallels Cloud
Server 6.10

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Message-id: 1448626806-17591-1-git-send-email-den@openvz.org
CC: Eric Blake <eblake@redhat.com>
CC: John Snow <jsnow@redhat.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 228 insertions, 0 deletions

diff --git a/docs/specs/parallels.txt b/docs/specs/parallels.txt
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+= License =
+
+Copyright (c) 2015 Denis Lunev
+Copyright (c) 2015 Vladimir Sementsov-Ogievskiy
+
+This work is licensed under the terms of the GNU GPL, version 2 or later.
+See the COPYING file in the top-level directory.
+
+= Parallels Expandable Image File Format =
+
+A Parallels expandable image file consists of three consecutive parts:
+    * header
+    * BAT
+    * data area
+
+All numbers in a Parallels expandable image are stored in little-endian byte
+order.
+
+
+== Definitions ==
+
+    Sector    A 512-byte data chunk.
+
+    Cluster   A data chunk of the size specified in the image header.
+              Currently, the default size is 1MiB (2048 sectors). In previous
+              versions, cluster sizes of 63 sectors, 256 and 252 kilobytes were
+              used.
+
+    BAT       Block Allocation Table, an entity that contains information for
+              guest-to-host I/O data address translation.
+
+
+== Header ==
+
+The header is placed at the start of an image and contains the following
+fields:
+
+Bytes:
+   0 - 15:    magic
+              Must contain "WithoutFreeSpace" or "WithouFreSpacExt".
+
+  16 - 19:    version
+              Must be 2.
+
+  20 - 23:    heads
+              Disk geometry parameter for guest.
+
+  24 - 27:    cylinders
+              Disk geometry parameter for guest.
+
+  28 - 31:    tracks
+              Cluster size, in sectors.
+
+  32 - 35:    nb_bat_entries
+              Disk size, in clusters (BAT size).
+
+  36 - 43:    nb_sectors
+              Disk size, in sectors.
+
+              For "WithoutFreeSpace" images:
+              Only the lowest 4 bytes are used. The highest 4 bytes must be
+              cleared in this case.
+
+              For "WithouFreSpacExt" images, there are no such
+              restrictions.
+
+  44 - 47:    in_use
+              Set to 0x746F6E59 when the image is opened by software in R/W
+              mode; set to 0x312e3276 when the image is closed.
+
+              A zero in this field means that the image was opened by an old
+              version of the software that doesn't support Format Extension
+              (see below).
+
+              Other values are not allowed.
+
+  48 - 51:    data_off
+              An offset, in sectors, from the start of the file to the start of
+              the data area.
+
+              For "WithoutFreeSpace" images:
+              - If data_off is zero, the offset is calculated as the end of BAT
+                table plus some padding to ensure sector size alignment.
+              - If data_off is non-zero, the offset should be aligned to sector
+                size. However it is recommended to align it to cluster size for
+                newly created images.
+
+              For "WithouFreSpacExt" images:
+              data_off must be non-zero and aligned to cluster size.
+
+  52 - 55:    flags
+              Miscellaneous flags.
+
+              Bit 0: Empty Image bit. If set, the image should be
+                     considered clear.
+
+              Bits 2-31: Unused.
+
+  56 - 63:    ext_off
+              Format Extension offset, an offset, in sectors, from the start of
+              the file to the start of the Format Extension Cluster.
+
+              ext_off must meet the same requirements as cluster offsets
+              defined by BAT entries (see below).
+
+
+== BAT ==
+
+BAT is placed immediately after the image header. In the file, BAT is a
+contiguous array of 32-bit unsigned little-endian integers with
+(bat_entries * 4) bytes size.
+
+Each BAT entry contains an offset from the start of the file to the
+corresponding cluster. The offset set in clusters for "WithouFreSpacExt" images
+and in sectors for "WithoutFreeSpace" images.
+
+If a BAT entry is zero, the corresponding cluster is not allocated and should
+be considered as filled with zeroes.
+
+Cluster offsets specified by BAT entries must meet the following requirements:
+    - the value must not be lower than data offset (provided by header.data_off
+      or calculated as specified above),
+    - the value must be lower than the desired file size,
+    - the value must be unique among all BAT entries,
+    - the result of (cluster offset - data offset) must be aligned to cluster
+      size.
+
+
+== Data Area ==
+
+The data area is an area from the data offset (provided by header.data_off or
+calculated as specified above) to the end of the file. It represents a
+contiguous array of clusters. Most of them are allocated by the BAT, some may
+be allocated by the ext_off field in the header while other may be allocated by
+extensions. All clusters allocated by ext_off and extensions should meet the
+same requirements as clusters specified by BAT entries.
+
+
+== Format Extension ==
+
+The Format Extension is an area 1 cluster in size that provides additional
+format features. This cluster is addressed by the ext_off field in the header.
+The format of the Format Extension area is the following:
+
+   0 -  7:    magic
+              Must be 0xAB234CEF23DCEA87
+
+   8 - 23:    m_CheckSum
+              The MD5 checksum of the entire Header Extension cluster except
+              the first 24 bytes.
+
+    The above are followed by feature sections or "extensions". The last
+    extension must be "End of features" (see below).
+
+Each feature section has the following format:
+
+   0 -  7:    magic
+              The identifier of the feature:
+              0x0000000000000000 - End of features
+              0x20385FAE252CB34A - Dirty bitmap
+
+   8 - 15:    flags
+              External flags for extension:
+
+              Bit 0: NECESSARY
+                     If the software cannot load the extension (due to an
+                     unknown magic number or error), the file should not be
+                     changed. If this flag is unset and there is an error on
+                     loading the extension, said extension should be dropped.
+
+              Bit 1: TRANSIT
+                     If there is an unknown extension with this flag set,
+                     said extension should be left as is.
+
+              If neither NECESSARY nor TRANSIT are set, the extension should be
+              dropped.
+
+  16 - 19:    data_size
+              The size of the following feature data, in bytes.
+
+  20 - 23:    unused32
+              Align header to 8 bytes boundary.
+
+  variable:   data (data_size bytes)
+
+    The above is followed by padding to the next 8 bytes boundary, then the
+    next extension starts.
+
+    The last extension must be "End of features" with all the fields set to 0.
+
+
+=== Dirty bitmaps feature ===
+
+This feature provides a way of storing dirty bitmaps in the image. The fields
+of its data area are:
+
+   0 -  7:    size
+              The bitmap size, should be equal to disk size in sectors.
+
+   8 - 23:    id
+              An identifier for backup consistency checking.
+
+  24 - 27:    granularity
+              Bitmap granularity, in sectors. I.e., the number of sectors
+              corresponding to one bit of the bitmap. Granularity must be
+              a power of 2.
+
+  28 - 31:    l1_size
+              The number of entries in the L1 table of the bitmap.
+
+  variable:   l1 (64 * l1_size bytes)
+              L1 offset table (in bytes)
+
+A dirty bitmap is stored using a one-level structure for the mapping to host
+clusters - an L1 table.
+
+Given an offset in bytes into the bitmap data, the offset in bytes into the
+image file can be obtained as follows:
+
+    offset = l1_table[offset / cluster_size] + (offset % cluster_size)
+
+If an L1 table entry is 0, the corresponding cluster of the bitmap is assumed
+to be zero.
+
+If an L1 table entry is 1, the corresponding cluster of the bitmap is assumed
+to have all bits set.
+
+If an L1 table entry is not 0 or 1, it allocates a cluster from the data area.