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Overview
Comment:Updated stats and a typo fix in the technical overview document.
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SHA3-256: bcae84663de82ab3ad8971fdef99dd32b1f6d4ecf2e9598cc70aa69d6827eb15
User & Date: drh 2020-02-08 16:43:13
Context
2020-02-08
17:53
Various documentation typo fixes. Remove the link to the (now obsolete) Fossil mailing list archive from the front page. (check-in: b45da258 user: drh tags: trunk)
16:43
Updated stats and a typo fix in the technical overview document. (check-in: bcae8466 user: drh tags: trunk)
2020-02-07
16:53
Issue a verbose warning message if a unversion content sync reverts to pull-only due to lack of permission on the server. (check-in: ef9305a4 user: drh tags: trunk)
Changes
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Changes to www/tech_overview.wiki.

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The bulk of the repository database (typically 75 to 85%) consists
of the artifacts that comprise the
[./fileformat.wiki | enduring, global, shared state] of the project.
The artifacts are stored as BLOBs, compressed using
[http://www.zlib.net/ | zlib compression] and, where applicable,
using [./delta_encoder_algorithm.wiki | delta compression].
The combination of zlib and delta compression results in a considerable
space savings.  For the SQLite project, at the time of this writing,

the total size of all artifacts is over 2.0 GB but thanks to the
combined zlib and delta compression, that content only takes up
32 MB of space in the repository database, for a compression ratio
of about 64:1.  The average size of a content BLOB in the database
is around 500 bytes.



Note that the zlib and delta compression is not an inherent part of the
Fossil file format; it is just an optimization.
The enduring file format for Fossil is the unordered
set of artifacts. The compression techniques are just a detail of
how the current implementation of Fossil happens to store these artifacts
efficiently on disk.
................................................................................
the [/help/deconstruct | fossil deconstruct]
command. Individual artifacts can be extracted using the
[/help/artifact | fossil artifact] command.
When accessing the repository database using raw SQL and the
[/help/sqlite3 | fossil sql] command, the extension function
"<tt>content()</tt>" with a single argument which is the SHA1 or
SHA3-256 hash
of an artifact will return the complete undeleted and uncompressed
content of that artifact.

Going the other way, the [/help/reconstruct | fossil reconstruct]
command will scan a directory hierarchy and add all files found to
a new repository database.  The [/help/import | fossil import] command
works by reading the input git-fast-export stream and using it to construct
corresponding artifacts which are then written into the repository database.







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The bulk of the repository database (typically 75 to 85%) consists
of the artifacts that comprise the
[./fileformat.wiki | enduring, global, shared state] of the project.
The artifacts are stored as BLOBs, compressed using
[http://www.zlib.net/ | zlib compression] and, where applicable,
using [./delta_encoder_algorithm.wiki | delta compression].
The combination of zlib and delta compression results in a considerable
space savings.  For the SQLite project (when this paragraph was last
updated on 2020-02-08)
the total size of all artifacts is over 7.1 GB but thanks to the
combined zlib and delta compression, that content only takes less than
97 MB of space in the repository database, for a compression ratio

of about 74:1.  The median size of all content BLOBs after delta
and zlib compression have been applied is 156 bytes.
The median size of BLOBs without compression is 45,312 bytes.

Note that the zlib and delta compression is not an inherent part of the
Fossil file format; it is just an optimization.
The enduring file format for Fossil is the unordered
set of artifacts. The compression techniques are just a detail of
how the current implementation of Fossil happens to store these artifacts
efficiently on disk.
................................................................................
the [/help/deconstruct | fossil deconstruct]
command. Individual artifacts can be extracted using the
[/help/artifact | fossil artifact] command.
When accessing the repository database using raw SQL and the
[/help/sqlite3 | fossil sql] command, the extension function
"<tt>content()</tt>" with a single argument which is the SHA1 or
SHA3-256 hash
of an artifact will return the complete uncompressed
content of that artifact.

Going the other way, the [/help/reconstruct | fossil reconstruct]
command will scan a directory hierarchy and add all files found to
a new repository database.  The [/help/import | fossil import] command
works by reading the input git-fast-export stream and using it to construct
corresponding artifacts which are then written into the repository database.