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