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146 lines
3.9 KiB
Plaintext
146 lines
3.9 KiB
Plaintext
=== Deduplication Table (`DDT*`) *_DEPRECATED_*
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The deduplication table is a sequential array of pointers, with each entry corresponding to a sector on the storage media.
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These pointers map sector data to logical content blocks, enabling efficient elimination of duplicate data.
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Every image must include at least one deduplication table of type `UserData`.
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*Deprecation Notice*: This block is deprecated and *MUST NOT* be used in new image files.
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==== Structure Definition
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[source,c]
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#define DDT_MAGIC 0X2A544444
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/**Header for a deduplication table. Table follows it */
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typedef struct DdtHeader
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{
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/**Identifier, <see cref="BlockType.DeDuplicationTable" /> */
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uint32_t identifier;
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/**Type of data pointed by this DDT */
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uint16_t type;
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/**Compression algorithm used to compress the DDT */
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uint16_t compression;
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/**Each entry is ((uint8_t offset in file) << shift) + (sector offset in block) */
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uint8_t shift;
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/**How many entries are in the table */
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uint64_t entries;
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/**Compressed length for the DDT */
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uint64_t cmpLength;
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/**Uncompressed length for the DDT */
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uint64_t length;
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/**CRC64-ECMA of the compressed DDT */
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uint64_t cmpCrc64;
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/**CRC64-ECMA of the uncompressed DDT */
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uint64_t crc64;
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} DdtHeader;
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==== Field Descriptions
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[cols="2,2,2,6",options="header"]
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|===
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|Type
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|Size
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|Name
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|Description
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|uint32_t
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|4 bytes
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|identifier
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|The deduplication table identifier, always `DDT*`
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|uint16_t
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|2 bytes
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|type
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|The data type pointed by this table. See Annex B.
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|uint16_t
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|2 bytes
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|compression
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|The compression algorithm used in the table. See Annex C.
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|uint8_t
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|1 byte
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|shift
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|The shift used to calculate the position of a sector in a data block pointed by this table.
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|uint64_t
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|8 bytes
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|entries
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|How many pointers follow this header.
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|uint32_t
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|4 bytes
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|cmpLength
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|The size in bytes of the compressed table that follows this header.
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|uint32_t
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|4 bytes
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|length
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|The size in bytes of the table block when decompressed.
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|uint64_t
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|8 bytes
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|cmpCrc64
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|The CRC64-ECMA checksum of the compressed table that follows this header.
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|uint64_t
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|8 bytes
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|crc64
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|The CRC64-ECMA checksum of the decompressed table.
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|===
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==== Deduplication Table Entries
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Each entry in the deduplication table references a specific data block and a particular item within that block.
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===== Mapping Logic
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- Entry 0 corresponds to data associated with LBA 0 of the media; subsequent entries map sequentially.
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- The pointer value for an entry is computed using the formula:
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+
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[source]
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pointer = (byte_offset_of_block << shift) + item_index_in_block
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+
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For example, a raw pointer value of `0x8003` in a table with a `shift` of 5 resolves as follows:
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- Byte offset: `0x400` → `1024`
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- Item index: `0x3` → `3`
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- Therefore, the pointer targets item 3 within the data block located at byte offset `1024` in the file.
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==== Special Case – Corrected Sector Tables
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Deduplication tables of type `CdSectorPrefixCorrected` and `CdSectorSuffixCorrected` split the entry value using bitmasking:
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- Pointer component: `entry & 0x00FFFFFF`
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- Flags component: `entry & 0xFF000000`
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===== Flags
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[cols="2,1,6",options="header"]
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|===
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|Flag
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|Value
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|Description
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|None
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|`0x00000000`
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|The suffix or prefix cannot be regenerated as is stored in the pointed data block.
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|NotDumped
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|`0x10000000`
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|The sector has not been dumped. Ignore the pointer.
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|Correct
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|`0x20000000`
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|The suffix (only for MODE 1 sectors) or prefix is correct and can be regenerated. Ignore the pointer.
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|Mode2Form1Ok
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|`0x30000000`
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|The suffix for MODE 2 sectors is correct, can be regenerated, and corresponds to a MODE 2 Form 1 sector.
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|Mode2Form2Ok
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|`0x40000000`
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|The suffix for MODE 2 sectors is correct, can be regenerated, and corresponds to a MODE 2 Form 2 sector with a valid CRC.
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|Mode2Form2NoCrc
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|`0x50000000`
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|The suffix for MODE 2 sectors is correct, can be regenerated, and corresponds to a MODE 2 Form 2 sector with an empty CRC.
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|=== |