SQLite Archiver

SQLAR - SQLite Archiver

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This repository contains sources for the “SQLite Archiver” program. This program (named “sqlar”) operates much like “zip”, except that the compressed archive it builds is stored in an SQLite database instead of a ZIP archive.

The motivation for this is to see how much larger an SQLite database file is compared to a ZIP archive containing the same content. The answer depends on the filenames, but 2% seems to be a reasonable guess. In other words, storing files as compressed blobs in an SQLite database file results in a file that is only about 2% larger than storing those same files in a ZIP archive using the same compression.

Compiling

On unix, just type “make”. The SQLite sources are included. The zlib compression library is needed to build.

Usage

To create an archive:

    sqlar ARCHIVE FILES...

All files named in FILES… will be added to the archive. If another file with the same name already exists in the archive, it is replaced. If any of the named FILES is a directory, that directory is scanned recursively.

To see the contents of an archive:

    sqlar -l ARCHIVE

To extract the contents of an archive:

    sqlar -x ARCHIVE [FILES...]

If a FILES argument is provided, then only the named files are extracted. Without a FILES argument, all files are extracted.

All commands can be supplemented with -v for verbose output. For example:

    sqlar -v ARCHIVE FILES..
    sqlar -lv ARCHIVE
    sqlar -xv ARCHIVE

File are normally compressed using zlib prior to being stored as BLOBs in the database. However, if the file is incompressible or if the -n option is used on the command-line, then the file is stored in the database exactly as it appears on disk, without compression.

Storage

The database schema looks like this:

    CREATE TABLE sqlar(
      name TEXT PRIMARY KEY,  -- name of the file
      mode INT,               -- access permissions
      mtime INT,              -- last modification time
      sz INT,                 -- original file size
      data BLOB               -- compressed content
    );

Both directories and empty files have sqlar.sz==0. Directories can be distinguished from empty files because directories have sqlar.data IS NULL. The file is compressed if length(sqlar.blob)<sqlar.sz and is stored as plaintext if length(sqlar.blob)==sqlar.sz.

Symbolic links have sqlar.sz set to -1, and the link target stored as a text value in the sqlar.data field.

SQLAR uses the “zlib format” for compression. ZIP uses the raw deflate format. The difference is that the zlib format contains a two byte compression-type indentification header (0x78 0x9c) and a 4-byte checksum at the end. Thus the “data” for SQLAR is always 6 bytes larger than the equivalent data for ZIP. The SQLAR program uses the zlib format rather than the slightly smaller raw deflate format because that is what the zlib documentation recommends.

SQLAR might someday be extended to support additional compression formats other than deflate. If so, the data field will contain new header values to identify entries compressed using the new formats.

Fuse Filesystem

An SQLite Archive file can be mounted as a Fuse Filesystem using the “sqlarfs” utility, including with this project.

To build the “sqlarfs” utility, run:

    make sqlarfs

Then to mount an SQLite archive as a filesystem, run:

    mkdir ~/fuse
    ./sqlarfs ARCHIVE_NAME -f ~/fuse

Replace ARCHIVE_NAME with the filename of the SQLite archive file to be mounted, of course. The -f option keeps sqlarfs running in the foreground, so that you can unmount the filesystem by simply pressing the interrupt key (usually Ctrl-C).

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