133 lines
3.6 KiB
Plaintext
133 lines
3.6 KiB
Plaintext
= Virtual Machine Archive format (VMA) =
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This format contains a header which includes the VM configuration as
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binary blobs, and a list of devices (dev_id, name).
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The actual VM image data is stored inside extents. An extent contains
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up to 64 clusters, and start with a 512 byte header containing
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additional information for those clusters.
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We use a cluster size of 65536, and use 8 bytes for each
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cluster in the header to store the following information:
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* 1 byte dev_id (to identity the drive)
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* 1 byte not used (reserved)
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* 2 bytes zero indicator (mark zero regions (16x4096))
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* 4 bytes cluster number
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We only store non-zero blocks (such block is 4096 bytes).
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Each archive is marked with a uuid. The archive header and all
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extent headers includes that uuid and a MD5 checksum (over header
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data).
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All numbers in VMA archive are stored in Big Endian byte order.
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== VMA Header ==
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Byte 0 - 3: magic
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VMA magic string ("VMA\x00")
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4 - 7: version
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Version number (valid value is 1)
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8 - 23: uuid
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Unique ID, Same uuid is used to mark extents.
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24 - 31: ctime
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Backup time stamp (seconds since epoch)
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32 - 47: md5sum
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Header checksum (from byte 0 to header_size). This field
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is filled with zero to generate the checksum.
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48 - 51: blob_buffer_offset
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Start of blob buffer (multiple of 512)
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52 - 55: blob_buffer_size
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Size of blob buffer (multiple of 512)
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56 - 59: header_size
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Overall size of this header (multiple of 512)
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60 - 2043: reserved
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2044 - 3067: uint32_t config_names[256]
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Offsets into blob_buffer table
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3068 - 4091: uint32_t config_data[256]
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Offsets into blob_buffer table
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4092 - 4095: reserved
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4096 - 12287: VmaDeviceInfoHeader dev_info[256]
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The offset in this table is used as 'dev_id' inside
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the data streams.
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12288 - header_size: Blob buffer
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=== Devive Info Header (VmaDeviceInfoHeader) ===
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This is use to store details about the contained disk images.
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Byte 0 - 3: devive name (offsets into blob_buffer table)
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4 - 7: reserved
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8 - 15: device size in bytes
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16 - 31: reserved
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Note: Devive name 'vmstate' is reserved to store VM RAM state.
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=== Blob buffer ===
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The blob buffer is used to store both configuration file names and
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configuration data.
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This region contain a list of binary data blobs. Each blob starts with
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a 2 byte size field, followed by the actual data.
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== Image Data Streams ==
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The VMA header is followed by the image data stream. Image data is grouped
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with extents, which contains up to 59 clusters from different images.
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=== VMA Extent Header ===
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Byte 0 - 3: magic
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VMA extent magic string ("VMAE")
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4 - 5: reserved
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6 - 7: block_count
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Overall number of contained 4K block
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8 - 23: uuid
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Unique ID, Same uuid as used in the VMA header.
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24 - 39: md5sum
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Header checksum (from byte 0 to header_size). This field
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is filled with zero to generate the checksum.
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40 - 511: blockinfo[59]
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Each 'blockinfo' (8 bytes) give further details about contained clusters:
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Byte 0 - 1: mask
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Bitmap used to indicate non-zero 4K blocks inside the
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cluster.
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2: reserved
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3: dev_id
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Device ID (offset into dev_info table)
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4 - 7: cluster_num
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The extend header if followed by the actual cluster data, where we only
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store non-zero 4K blocks.
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