ratarmount 0.13.0

Random Access Read-Only Tar Mount

Random Access Tar Mount (Ratarmount)

Ratarmount collects all file positions inside a TAR so that it can easily jump to and read from any file without extracting it. It, then, mounts the TAR using fusepy for read access just like archivemount. In contrast to libarchive, on which archivemount is based, random access and true seeking is supported. And in contrast to tarindexer, which also collects file positions for random access, ratarmount offers easy access via FUSE and support for compressed TARs.

Capabilities:

• Highly Parallelized: By default, all cores are used for parallelized algorithms like for the gzip, bzip2, and xz decoders. This can yield huge speedups on most modern processors but requires more main memory. It can be controlled or completely turned off using the -P <cores> option.
• Recursive Mounting: Ratarmount will also mount TARs inside TARs inside TARs, ... recursively into folders of the same name, which is useful for the 1.31TB ImageNet data set.
• Mount Compressed Files: You may also mount files with one of the supported compression schemes. Even if these files do not contain a TAR, you can leverage ratarmount's true seeking capabilities when opening the mounted uncompressed view of such a file.
• Read-Only Bind Mounting: Folders may be mounted read-only to other folders for usecases like merging a backup TAR with newer versions of those files residing in a normal folder.
• Union Mounting: Multiple TARs, compressed files, and bind mounted folders can be mounted under the same mountpoint.
• Write Overlay: A folder can be specified as write overlay. All changes below the mountpoint will be redirected to this folder and deletions are tracked so that all changes can be applied back to the archive.

TAR compressions supported for random access:

• BZip2 as provided by indexed_bzip2 as a backend, which is a refactored and extended version of bzcat from toybox. See also the reverse engineered specification.
• Gzip as provided by pragzip or indexed_gzip by Paul McCarthy. See also RFC1952.
• Xz as provided by python-xz by Rogdham or lzmaffi by Tomer Chachamu. See also The .xz File Format.
• Zstd as provided by indexed_zstd by Marco Martinelli. See also Zstandard Compression Format.

Other supported archive formats:

• Rar as provided by rarfile by Marko Kreen. See also the RAR 5.0 archive format.
• Zip as provided by zipfile, which is distributed with Python itself. See also the ZIP File Format Specification.

Table of Contents

1. Installation
   1. Installation via AppImage
   2. System Dependencies for PIP Installation (Rarely Necessary)
   3. PIP Package Installation
2. Benchmarks
3. The Problem
4. The Solution
5. Usage
   1. Metadata Index Cache
   2. Bind Mounting
   3. Union Mounting
   4. File versions
   5. Compressed non-TAR files
   6. Xz and Zst Files
   7. As a Library

Installation

You can install ratarmount either by simply downloading the AppImage or via pip. The latter might require installing additional dependencies.

Installation via AppImage

The AppImage files are attached under "Assets" on the releases page. They require no installation and can be simply executed like a portable executable. If you want to install it, you can simply copy it into any of the folders listed in your PATH.

wget 'https://github.com/mxmlnkn/ratarmount/releases/download/v0.13.0/ratarmount-0.13.0-manylinux2014_x86_64.AppImage'
chmod u+x 'ratarmount-manylinux2014_x86_64.AppImage'
./ratarmount-manylinux2014_x86_64.AppImage --help  # Simple test run
sudo cp ratarmount-manylinux2014_x86_64.AppImage /usr/local/bin/ratarmount  # Example installation

System Dependencies for PIP Installation (Rarely Necessary)

Python 3.6+, preferably pip 19.0+, FUSE, and sqlite3 are required. These should be preinstalled on most systems.

On Debian-like systems like Ubuntu, you can install/update all dependencies using:

sudo apt install python3 python3-pip fuse sqlite3 unar

On macOS, you have to install macFUSE with:

brew install macfuse

If you are installing on a system for which there exists no manylinux wheel, then you'll have to install dependencies required to build from source:

sudo apt install python3 python3-pip fuse build-essential software-properties-common zlib1g-dev libzstd-dev liblzma-dev cffi

PIP Package Installation

Then, you can simply install ratarmount from PyPI:

pip install ratarmount

Or, if you want to test the latest version:

python3 -m pip install --user --force-reinstall \
    'git+https://github.com/mxmlnkn/ratarmount.git@develop#egginfo=ratarmountcore&subdirectory=core' \
    'git+https://github.com/mxmlnkn/ratarmount.git@develop#egginfo=ratarmount'

If there are troubles with the compression backend dependencies, you can try the pip --no-deps argument. Ratarmount will work without the compression backends. The hard requirements are fusepy and for Python versions older than 3.7.0 dataclasses.

For xz support, lzmaffi will be used if available. Because lzmaffi does not provide wheels and the build from source depends on cffi, which might be missing, only python-xz is a dependency of ratarmount. If there are problems with xz files, please report any encountered issues. But, as a quick workaround, you can try to simply switch out the xz decoder backend by installing lzmaffi manually and ratarmount will use that instead with higher priority:

sudo apt install liblzma-dev
python3 -m pip install --user cffi  # Necessary because of missing pyprojects.toml
python3 -m pip install --user lzmaffi

Benchmarks

• Not shown in the benchmarks, but ratarmount can mount files with preexisting index sidecar files in under a second making it vastly more efficient compared to archivemount for every subsequent mount. Also, archivemount has no progress indicator making it very unlikely the user will wait hours for the mounting to finish. Fuse-archive, an iteration on archivemount, has the --asyncprogress option to give a progress indicator using the timestamp of a dummy file. Note that fuse-archive daemonizes instantly but the mount point will not be usable for a long time and everything trying to use it will hang until then when not using --asyncprogress!
• Getting file contents of a mounted archive is generally vastly faster than archivemount and fuse-archive and does not increase with the archive size or file count resulting in the largest observed speedups to be around 5 orders of magnitude!
• Memory consumption of ratarmount is mostly less than archivemount and mostly does not grow with the archive size. Not shown in the plots, but the memory usage will be much smaller when not specifying -P 0, i.e., when not parallelizing. The gzip backend grows linearly with the archive size because the data for seeking is thousands of times larger than the simple two 64-bit offsets required for bzip2. The memory usage of the zstd backend only seems humongous because it uses mmap to open. The memory used by mmap is not even counted as used memory when showing the memory usage with free or htop.
• For empty files, mounting with ratarmount and archivemount does not seem be bounded by decompression nor I/O bandwidths but instead by the algorithm for creating the internal file index. This algorithm scales linearly for ratarmount and fuse-archive but seems to scale worse than even quadratically for archives containing more than 1M files when using archivemount. Ratarmount 0.10.0 improves upon earlier versions by batching SQLite insertions.
• Mounting bzip2 and xz archives has actually become faster than archivemount and fuse-archive with ratarmount -P 0 on most modern processors because it actually uses more than one core for decoding those compressions. indexed_bzip2 supports block parallel decoding since version 1.2.0.
• Gzip compressed TAR files are two times slower than archivemount during first time mounting. It is not totally clear to me why that is because streaming the file contents after the archive being mounted is comparably fast, see the next benchmarks below. In order to have superior speeds for both of these, I am experimenting with a parallelized gzip decompressor like the prototype pugz offers for non-binary files only.
• For the other cases, mounting times become roughly the same compared to archivemount for archives with 2M files in an approximately 100GB archive.
• Getting a lot of metadata for archive contents as demonstrated by calling find on the mount point is an order of magnitude slower compared to archivemount. Because the C-based fuse-archive is even slower than ratarmount, the difference is very likely that archivemount uses the low-level FUSE interface while ratarmount and fuse-archive use the high-level FUSE interface.

• Reading files from the archive with archivemount are scaling quadratically instead of linearly. This is because archivemount starts reading from the beginning of the archive for each requested I/O block. The block size depends on the program or operating system and should be in the order of 4 kiB. Meaning, the scaling is O( (sizeOfFileToBeCopiedFromArchive / readChunkSize)^2 ). Both, ratarmount and fuse-archive avoid this behavior. Because of this quadratic scaling, the average bandwidth with archivemount seems like it decreases with the file size.
• Reading bz2 and xz are both an order of magnitude faster, as tested on my 12/24-core Ryzen 3900X, thanks to parallelization.
• Memory is bounded in these tests for all programs but ratarmount is a lot more lax with memory because it uses a Python stack and because it needs to hold caches for a constant amount of blocks for parallel decoding of bzip2 and xz files. The zstd backend in ratarmount looks unbounded because it uses mmap, whose memory usage will automatically stop and be freed if the memory limit has been reached.
• The peak for the xz decoder reading speeds happens because some blocks will be cached when loading the index, which is not included in the benchmark for technical reasons. The value for the 1 GiB file size is more realistic.

Further benchmarks can be viewed here.

The Problem

You downloaded a large TAR file from the internet, for example the 1.31TB large ImageNet, and you now want to use it but lack the space, time, or a file system fast enough to extract all the 14.2 million image files.

Existing Partial Solutions

Partial Solutions

Archivemount

Archivemount seems to have large performance issues for too many files and large archive for both mounting and file access in version 0.8.7. A more in-depth comparison benchmark can be found here.

• Mounting the 6.5GB ImageNet Large-Scale Visual Recognition Challenge 2012 validation data set, and then testing the speed with: time cat mounted/ILSVRC2012_val_00049975.JPEG | wc -c takes 250ms for archivemount and 2ms for ratarmount.
• Trying to mount the 150GB ILSVRC object localization data set containing 2 million images was given up upon after 2 hours. Ratarmount takes ~15min to create a ~150MB index and <1ms for opening an already created index (SQLite database) and mounting the TAR. In contrast, archivemount will take the same amount of time even for subsequent mounts.
• Does not support recursive mounting. Although, you could write a script to stack archivemount on top of archivemount for all contained TAR files.

Tarindexer

Tarindex is a command line to tool written in Python which can create index files and then use the index file to extract single files from the tar fast. However, it also has some caveats which ratarmount tries to solve:

• It only works with single files, meaning it would be necessary to loop over the extract-call. But this would require loading the possibly quite large tar index file into memory each time. For example for ImageNet, the resulting index file is hundreds of MB large. Also, extracting directories will be a hassle.
• It's difficult to integrate tarindexer into other production environments. Ratarmount instead uses FUSE to mount the TAR as a folder readable by any other programs requiring access to the contained data.
• Can't handle TARs recursively. In order to extract files inside a TAR which itself is inside a TAR, the packed TAR first needs to be extracted.

TAR Browser

I didn't find out about TAR Browser before I finished the ratarmount script. That's also one of it's cons:

• Hard to find. I don't seem to be the only one who has trouble finding it as it has one star on Github after 7 years compared to 45 stars for tarindexer after roughly the same amount of time.
• Hassle to set up. Needs compilation and I gave up when I was instructed to set up a MySQL database for it to use. Confusingly, the setup instructions are not on its Github but here.
• Doesn't seem to support recursive TAR mounting. I didn't test it because of the MysQL dependency but the code does not seem to have logic for recursive mounting.
• Xz compression also is only block or frame based, i.e., only works faster with files created by pixz or pxz.

Pros:

• supports bz2- and xz-compressed TAR archives

The Solution

Ratarmount creates an index file with file names, ownership, permission flags, and offset information. This sidecar is stored at the TAR file's location or in ~/.ratarmount/. Ratarmount can load that index file in under a second if it exists and then offers FUSE mount integration for easy access to the files inside the archive.

Here is a more recent test for version 0.2.0 with the new default SQLite backend:

• TAR size: 124GB
• Contains TARs: yes
• Files in TAR: 1000
• Files in TAR (including recursively in contained TARs): 1.26 million
• Index creation (first mounting): 15m 39s
• Index size: 146MB
• Index loading (subsequent mounting): 0.000s
• Reading a 64kB file: ~4ms
• Running 'find mountPoint -type f | wc -l' (1.26M stat calls): 1m 50s

The reading time for a small file simply verifies the random access by using file seek to be working. The difference between the first read and subsequent reads is not because of ratarmount but because of operating system and file system caches.

Older test with 1.31 TB Imagenet (Fall 2011 release)

The test with the first version of ratarmount (50e8dbb), which used the, as of now removed, pickle backend for serializing the metadata index, for the ImageNet data set:

• TAR size: 1.31TB
• Contains TARs: yes
• Files in TAR: ~26 000
• Files in TAR (including recursively in contained TARs): 14.2 million
• Index creation (first mounting): 4 hours
• Index size: 1GB
• Index loading (subsequent mounting): 80s
• Reading a 40kB file: 100ms (first time) and 4ms (subsequent times)

Index loading is relatively slow with 80s because of the pickle backend, which now has been replaced with SQLite and should take less than a second now.

Usage

Command Line Options

See ratarmount --help.

Full command line options

usage: ratarmount.py [-h] [-c] [-r] [-u] [-P PARALLELIZATION] [-v] [--password PASSWORD]
                     [--verify-mtime] [--index-file INDEX_FILE] [--index-folders INDEX_FOLDERS]
                     [--recursion-depth RECURSION_DEPTH] [-l] [-s]
                     [--transform-recursive-mount-point REGEX_PATTERN REPLACEMENT] [-e ENCODING]
                     [-i] [--gnu-incremental] [--no-gnu-incremental] [-w WRITE_OVERLAY]
                     [--commit-overlay] [-o FUSE] [-f] [-d DEBUG] [-gs GZIP_SEEK_POINT_SPACING]
                     [-p PREFIX] [--password-file PASSWORD_FILE] [--use-backend USE_BACKEND]
                     [--oss-attributions]
                     mount_source [mount_source ...] [mount_point]

With ratarmount, you can:
  - Mount a (compressed) TAR file to a folder for read-only access
  - Mount a compressed file to `<mountpoint>/<filename>`
  - Bind mount a folder to another folder for read-only access
  - Union mount a list of TARs, compressed files, and folders to a mount point
    for read-only access

Optional Arguments:
  --password PASSWORD   Specify a single password which shall be used for RAR and ZIP files.
                        (default: )
  -P PARALLELIZATION, --parallelization PARALLELIZATION
                        If an integer other than 1 is specified, then the threaded parallel bzip2
                        decoder will be used specified amount of block decoder threads. Further
                        threads with lighter work may be started. A value of 0 will use all the
                        available cores (24). (default: 0)
  -h, --help            Show this help message and exit.
  -r, --recursive       Mount archives inside archives recursively. Same as --recursion-depth -1.
                        (default: False)
  -u, --unmount         Unmount the given mount point. Equivalent to calling "fusermount -u".
                        (default: False)
  -v, --version         Print version information and exit.

Positional Options:
  mount_source          The path to the TAR archive to be mounted. If multiple archives and/or
                        folders are specified, then they will be mounted as if the arguments
                        coming first were updated with the contents of the archives or folders
                        specified thereafter, i.e., the list of TARs and folders will be union
                        mounted.
  mount_point           The path to a folder to mount the TAR contents into. If no mount path is
                        specified, the TAR will be mounted to a folder of the same name but
                        without a file extension. (default: None)

Index Options:
  --index-file INDEX_FILE
                        Specify a path to the .index.sqlite file. Setting this will disable
                        fallback index folders. If the given path is ":memory:", then the index
                        will not be written out to disk. (default: None)
  --index-folders INDEX_FOLDERS
                        Specify one or multiple paths for storing .index.sqlite files. Paths will
                        be tested for suitability in the given order. An empty path will be
                        interpreted as the location in which the TAR resides. If the argument
                        begins with a bracket "[", then it will be interpreted as a JSON-formatted
                        list. If the argument contains a comma ",", it will be interpreted as a
                        comma-separated list of folders. Else, the whole string will be
                        interpreted as one folder path. Examples: --index-folders ",~/.foo" will
                        try to save besides the TAR and if that does not work, in ~/.foo. --index-
                        folders '["~/.ratarmount", "foo,9000"]' will never try to save besides the
                        TAR. --index-folder ~/.ratarmount will only test ~/.ratarmount as a
                        storage location and nothing else. Instead, it will first try
                        ~/.ratarmount and the folder "foo,9000". (default:
                        ,~/.cache/ratarmount,~/.ratarmount)
  --verify-mtime        By default, only the TAR file size is checked to match the one in the
                        found existing ratarmount index. If this option is specified, then also
                        check the modification timestamp. But beware that the mtime might change
                        during copying or downloading without the contents changing. So, this
                        check might cause false positives. (default: False)
  -c, --recreate-index  If specified, pre-existing .index files will be deleted and newly created.
                        (default: False)

Recursion Options:
  --recursion-depth RECURSION_DEPTH
                        This option takes precedence over --recursive. Mount archives inside the
                        mounted archives recursively up to the given depth. A negative value
                        represents infinite depth. A value of 0 will turn off recursion (same as
                        not specifying --recursive in the first place). A value of 1 will
                        recursively mount all archives in the given archives but not any deeper.
                        Note that this only has an effect when creating an index. If an index
                        already exists, then this option will be effectively ignored. Recreate the
                        index if you want change the recursive mounting policy anyways. (default:
                        None)
  --transform-recursive-mount-point REGEX_PATTERN REPLACEMENT
                        Specify a regex pattern and a replacement string, which will be applied
                        via Python's re module to the full path of the archive to be recursively
                        mounted. E.g., if there are recursive archives: /folder/archive.tar.gz,
                        you can substitute '[.][^/]+$' to '' and it will be mounted to
                        /folder/archive.tar. Or you can replace '^.*/([^/]+).tar.gz$' to '/' to
                        mount all recursive folders under the top-level without extensions.
                        (default: None)
  -l, --lazy            When used with recursively bind-mounted folders, TAR files inside the
                        mounted folder will only be mounted on first access to it. (default:
                        False)
  -s, --strip-recursive-tar-extension
                        If true, then recursively mounted TARs named <file>.tar will be mounted at
                        <file>/. This might lead to folders of the same name being overwritten, so
                        use with care. The index needs to be (re)created to apply this option!
                        (default: False)

Tar Options:
  --gnu-incremental     Will strip octal modification time prefixes from file paths, which appear
                        in GNU incremental backups created with GNU tar with the --incremental or
                        --listed-incremental options. (default: None)
  --no-gnu-incremental  If specified, will never strip octal modification prefixes and will also
                        not do automatic detection. (default: True)
  -e ENCODING, --encoding ENCODING
                        Specify an input encoding used for file names among others in the TAR.
                        This must be used when, e.g., trying to open a latin1 encoded TAR on an
                        UTF-8 system. Possible encodings:
                        https://docs.python.org/3/library/codecs.html#standard-encodings (default:
                        utf-8)
  -i, --ignore-zeros    Ignore zeroed blocks in archive. Normally, two consecutive 512-blocks
                        filled with zeroes mean EOF and ratarmount stops reading after
                        encountering them. This option instructs it to read further and is useful
                        when reading archives created with the -A option. (default: False)

Write Overlay Options:
  --commit-overlay      Apply deletions and content modifications done in the write overlay to the
                        archive. (default: False)
  -w WRITE_OVERLAY, --write-overlay WRITE_OVERLAY
                        Specify an existing folder to be used as a write overlay. The folder
                        itself will be union-mounted on top such that files in this folder take
                        precedence over all other existing ones. Furthermore, all file creations
                        and modifications will be forwarded to files in this folder. Modifying a
                        file inside a TAR will copy that file to the overlay folder and apply the
                        modification to that writable copy. Deleting files or folders will update
                        the hidden metadata database inside the overlay folder. (default: None)

Advanced Options:
  --oss-attributions    Show licenses of used libraries.
  --password-file PASSWORD_FILE
                        Specify a file with newline separated passwords for RAR and ZIP files. The
                        passwords will be tried out in order of appearance in the file. (default:
                        )
  --use-backend USE_BACKEND
                        Specify a backend to be used with higher priority for files which might be
                        opened with multiple backends. Arguments specified last will have the
                        highest priority. A comma-separated list may be specified. Possible
                        backends: ['indexed_bzip2', 'indexed_gzip', 'indexed_zstd', 'lzmaffi',
                        'pragzip', 'rarfile', 'xz', 'zipfile'] (default: None)
  -d DEBUG, --debug DEBUG
                        Sets the debugging level. Higher means more output. Currently, 3 is the
                        highest. (default: 1)
  -f, --foreground      Keeps the python program in foreground so it can print debug output when
                        the mounted path is accessed. (default: False)
  -gs GZIP_SEEK_POINT_SPACING, --gzip-seek-point-spacing GZIP_SEEK_POINT_SPACING
                        This only is applied when the index is first created or recreated with the
                        -c option. The spacing given in MiB specifies the seek point distance in
                        the uncompressed data. A distance of 16MiB means that archives smaller
                        than 16MiB in uncompressed size will not benefit from faster seek times. A
                        seek point takes roughly 32kiB. So, smaller distances lead to more
                        responsive seeking but may explode the index size! (default: 16)
  -o FUSE, --fuse FUSE  Comma separated FUSE options. See "man mount.fuse" for help. Example:
                        --fuse "allow_other,entry_timeout=2.8,gid=0". (default: )
  -p PREFIX, --prefix PREFIX
                        [deprecated] Use "-o modules=subdir,subdir=<prefix>" instead. This
                        standard way utilizes FUSE itself and will also work for other FUSE
                        applications. So, it is preferable even if a bit more verbose.The
                        specified path to the folder inside the TAR will be mounted to root. This
                        can be useful when the archive as created with absolute paths. E.g., for
                        an archive created with `tar -P cf /var/log/apt/history.log`, -p
                        /var/log/apt/ can be specified so that the mount target directory
                        >directly< contains history.log. (default: )

Metadata Index Cache

In order to reduce the mounting time, the created index for random access to files inside the tar will be saved to one of these locations. These locations are checked in order and the first, which works sufficiently, will be used. This is the default location order:

1. .index.sqlite
2. ~/.ratarmount/<path to tar: '/' -> '_'>.index.sqlite E.g., ~/.ratarmount/_media_cdrom_programm.tar.index.sqlite

This list of fallback folders can be overwritten using the --index-folders option. Furthermore, an explicitly named index file may be specified using the --index-file option. If --index-file is used, then the fallback folders, including the default ones, will be ignored!

Bind Mounting

The mount sources can be TARs and/or folders. Because of that, ratarmount can also be used to bind mount folders read-only to another path similar to bindfs and mount --bind. So, for:

ratarmount folder mountpoint

all files in folder will now be visible in mountpoint.

Union Mounting

If multiple mount sources are specified, the sources on the right side will be added to or update existing files from a mount source left of it. For example:

ratarmount folder1 folder2 mountpoint

will make both, the files from folder1 and folder2, visible in mountpoint. If a file exists in both multiple source, then the file from the rightmost mount source will be used, which in the above example would be folder2.

If you want to update / overwrite a folder with the contents of a given TAR, you can specify the folder both as a mount source and as the mount point:

ratarmount folder file.tar folder

The FUSE option -o nonempty will be automatically added if such a usage is detected. If you instead want to update a TAR with a folder, you only have to swap the two mount sources:

ratarmount file.tar folder folder

File versions

If a file exists multiple times in a TAR or in multiple mount sources, then the hidden versions can be accessed through special .versions folders. For example, consider:

ratarmount folder updated.tar mountpoint

and the file foo exists both in the folder and as two different versions in updated.tar. Then, you can list all three versions using:

ls -la mountpoint/foo.versions/
    dr-xr-xr-x 2 user group     0 Apr 25 21:41 .
    dr-x------ 2 user group 10240 Apr 26 15:59 ..
    -r-x------ 2 user group   123 Apr 25 21:41 1
    -r-x------ 2 user group   256 Apr 25 21:53 2
    -r-x------ 2 user group  1024 Apr 25 22:13 3

In this example, the oldest version has only 123 bytes while the newest and by default shown version has 1024 bytes. So, in order to look at the oldest version, you can simply do:

cat mountpoint/foo.versions/1

Note that these version numbers are the same as when used with tar's --occurrence=N option.

Prefix Removal

Use ratarmount -o modules=subdir,subdir=<prefix> to remove path prefixes using the FUSE subdir module. Because it is a standard FUSE feature, the -o ... argument should also work for other FUSE applications.

When mounting an archive created with absolute paths, e.g., tar -P cf /var/log/apt/history.log, you would see the whole var/log/apt hierarchy under the mount point. To avoid that, specified prefixes can be stripped from paths so that the mount target directory directly contains history.log. Use ratarmount -o modules=subdir,subdir=/var/log/apt/ to do so. The specified path to the folder inside the TAR will be mounted to root, i.e., the mount point.

Compressed non-TAR files

If you want a compressed file not containing a TAR, e.g., foo.bz2, then you can also use ratarmount for that. The uncompressed view will then be mounted to <mountpoint>/foo and you will be able to leverage ratarmount's seeking capabilities when opening that file.

Xz and Zst Files

In contrast to bzip2 and gzip compressed files, true seeking on xz and zst files is only possible at block or frame boundaries. This wouldn't be noteworthy, if both standard compressors for xz and zstd were not by default creating unsuited files. Even though both file formats do support multiple frames and xz even contains a frame table at the end for easy seeking, both compressors write only a single frame and/or block out, making this feature unusable. In order to generate truly seekable compressed files, you'll have to use pixz for xz files. For zstd compressed, you can try with t2sz. The standard zstd tool does not support setting smaller block sizes yet although an issue does exist. Alternatively, you can simply split the original file into parts, compress those parts, and then concatenate those parts together to get a suitable multiframe zst file. Here is a bash function, which can be used for that:

Bash script: createMultiFrameZstd

createMultiFrameZstd()
(
    # Detect being piped into
    if [ -t 0 ]; then
        file=$1
        frameSize=$2
        if [[ ! -f "$file" ]]; then echo "Could not find file '$file'." 1>&2; return 1; fi
        fileSize=$( stat -c %s -- "$file" )
    else
        if [ -t 1 ]; then echo 'You should pipe the output to somewhere!' 1>&2; return 1; fi
        echo 'Will compress from stdin...' 1>&2
        frameSize=$1
    fi
    if [[ ! $frameSize =~ ^[0-9]+$ ]]; then
        echo "Frame size '$frameSize' is not a valid number." 1>&2
        return 1
    fi

    # Create a temporary file. I avoid simply piping to zstd
    # because it wouldn't store the uncompressed size.
    if [[ -d /dev/shm ]]; then frameFile=$( mktemp --tmpdir=/dev/shm ); fi
    if [[ -z $frameFile ]]; then frameFile=$( mktemp ); fi
    if [[ -z $frameFile ]]; then
        echo "Could not create a temporary file for the frames." 1>&2
        return 1
    fi

    if [ -t 0 ]; then
        true > "$file.zst"
        for (( offset = 0; offset < fileSize; offset += frameSize )); do
            dd if="$file" of="$frameFile" bs=$(( 1024*1024 )) \
               iflag=skip_bytes,count_bytes skip="$offset" count="$frameSize" 2>/dev/null
            zstd -c -q -- "$frameFile" >> "$file.zst"
        done
    else
        while true; do
            dd of="$frameFile" bs=$(( 1024*1024 )) \
               iflag=count_bytes count="$frameSize" 2>/dev/null
            # pipe is finished when reading it yields no further data
            if [[ ! -s "$frameFile" ]]; then break; fi
            zstd -c -q -- "$frameFile"
        done
    fi

    'rm' -f -- "$frameFile"
)

In order to compress a file named foo into a multiframe zst file called foo.zst, which contains frames sized 4MiB of uncompressed ata, you would call it like this:

createMultiFrameZstd foo  $(( 4*1024*1024 ))

It also works when being piped to. This can be useful for recompressing files to avoid having to decompress them first to disk.

lbzip2 -cd well-compressed-file.bz2 | createMultiFrameZstd $(( 4*1024*1024 )) > recompressed.zst

Writable Mounting

The --write-overlay <folder> option can be used to create a writable mount point. The original archive will not be modified.

• File creations will create these files in the specified overlay folder.
• File deletions and renames will be registered in a database that also resides in the overlay folder.
• File modifications will copy the file from the archive into the overlay folder before applying the modification.

This overlay folder can be stored alongside the archive or it can be deleted after unmounting the archive. This is useful when building the executable from a source tarball without extracting. After installation, the intermediary build files residing in the overlay folder can be safely removed.

If it is desired to apply the modifications to the original archive, then the --commit-overlay can be prepended to the original ratarmount call.

Here is an example for applying modifications to a writable mount and then committing those modifications back to the archive:

1. Mount it with a write overlay and add new files. The original archive is not modified.

   ratarmount --write-overlay example-overlay example.tar example-mount-point
   echo "Hello World" > example-mount-point/new-file.txt
   

2. Unmount. Changes persist solely in the overlay folder.

   fusermount -u example-mount-point
   

3. Commit changes to the original archive.

   ratarmount --commit-overlay --write-overlay example-overlay example.tar example-mount-point
   

   Output:

   To commit the overlay folder to the archive, these commands have to be executed:
   
       tar --delete --null --verbatim-files-from --files-from='/tmp/tmp_ajfo8wf/deletions.lst' \
           --file 'example.tar' 2>&1 |
          sed '/^tar: Exiting with failure/d; /^tar.*Not found in archive/d'
       tar --append -C 'zlib-wiki-overlay' --null --verbatim-files-from --files-from='/tmp/tmp_ajfo8wf/append.lst' --file 'example.tar'
   
   Committing is an experimental feature!
   Please confirm by entering "commit". Any other input will cancel.
   > 
   Committed successfully. You can now remove the overlay folder at example-overlay.
   

4. Verify the modifications to the original archive.

   tar -tvlf example.tar
   

   Output:

   -rw-rw-r-- user/user 652817 2022-08-08 10:44 example.txt
   -rw-rw-r-- user/user     12 2023-02-16 09:49 new-file.txt
   

5. Remove the obsole write overlay folder.

   rm -r example-overlay
   

As a Library

Ratarmount can also be used as a library. Using ratarmountcore, files inside archives can be accessed directly from Python code without requiring FUSE. For a more detailed description, see the ratarmountcore readme here.