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Clarify negative and overflow sectors in deduplication tables

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Natalia Portillo
2025-08-03 17:58:58 +01:00
parent 45c9b2c17f
commit 12e3995505
2 changed files with 25 additions and 1 deletions
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24
docs/spec/blocks/ddt2.adoc
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@@ -50,6 +50,16 @@ It starts with the following header.
|negative
|The negative displacement of LBA numbers. For media that can have negative LBAs, this establishes the number to substract to the table entry number
|uint64_t
|8 bytes
|blocks
|The number of blocks in the media. This includes all blocks in the media, including the ones represented by the negative displacement as well as overflow displacement.
|uint16_t
|2 bytes
|overflow
|The positive overflow displacement of LBA numbers. For media that can have blocks beyond the end of the user area, this is the number of how many such blocks there are in the image.
|uint64_t
|8 bytes
|start
@@ -243,4 +253,16 @@ Sector is stored at byte offset 0x6A0000 as item number 6 in the data block.
|The sector was unrecorded and each re-read returns random data
|===
When flags are present in a table that has sublevels it applies to all the sectors that shall be present in the subtable, unless the flag specify something else.
When flags are present in a table that has sublevels it applies to all the sectors that shall be present in the subtable, unless the flag specify something else.
===== Negative and Overflow Sectors
In most storage media, the accessible range of blocks or sectors—referred to as the *user area*—represents the logical region intended for data read and write operations.
However, certain media types contain additional blocks situated outside this user area that are accessible through alternate means. These blocks often hold metadata or structural information with significant preservation value. To ensure such data is retained, these sectors must be representable within the deduplication table.
Blocks located before the start of the user area are classified as *negative sectors*. Common examples include the first track pregap or Lead-In areas found on Compact Discs.
Conversely, sectors found beyond the end of the user area are categorized as *overflow sectors*. Examples include replication metadata on floppy disks, typically recorded in track 81, and the Lead-Out area of Compact Discs.
To calculate the precise number of user area sectors represented in the deduplication table, the total number of blocks on the medium is adjusted by subtracting both negative and overflow sectors. This ensures the deduplication map reflects only the standard user-accessible region while retaining awareness of displaced block data.

2
docs/spec/version_history.adoc
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@@ -88,4 +88,6 @@ Deprecate Compact Disc lead-in, first track pregap, lead-out, and floppy disk le
Clarify and correct deduplication table specification.
Rework index version 3 specification.
Clarify negative and overflow blocks in deduplication tables.
|===