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137 lines
4.4 KiB
Markdown
137 lines
4.4 KiB
Markdown
# tar-split
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[](https://travis-ci.org/vbatts/tar-split)
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Pristinely disassembling a tar archive, and stashing needed raw bytes and offsets to reassemble a validating original archive.
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## Docs
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Code API for libraries provided by `tar-split`:
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* https://godoc.org/github.com/vbatts/tar-split/tar/asm
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* https://godoc.org/github.com/vbatts/tar-split/tar/storage
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* https://godoc.org/github.com/vbatts/tar-split/archive/tar
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## Install
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The command line utilitiy is installable via:
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```bash
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go get github.com/vbatts/tar-split/cmd/tar-split
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```
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## Usage
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For cli usage, see its [README.md](cmd/tar-split/README.md).
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For the library see the [docs](#docs)
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## Demo
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### Basic disassembly and assembly
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This demonstrates the `tar-split` command and how to assemble a tar archive from the `tar-data.json.gz`
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[youtube video of basic command demo](https://youtu.be/vh5wyjIOBtc)
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### Docker layer preservation
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This demonstrates the tar-split integration for docker-1.8. Providing consistent tar archives for the image layer content.
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[youtube vide of docker layer checksums](https://youtu.be/tV_Dia8E8xw)
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## Caveat
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Eventually this should detect TARs that this is not possible with.
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For example stored sparse files that have "holes" in them, will be read as a
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contiguous file, though the archive contents may be recorded in sparse format.
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Therefore when adding the file payload to a reassembled tar, to achieve
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identical output, the file payload would need be precisely re-sparsified. This
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is not something I seek to fix imediately, but would rather have an alert that
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precise reassembly is not possible.
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(see more http://www.gnu.org/software/tar/manual/html_node/Sparse-Formats.html)
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Other caveat, while tar archives support having multiple file entries for the
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same path, we will not support this feature. If there are more than one entries
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with the same path, expect an err (like `ErrDuplicatePath`) or a resulting tar
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stream that does not validate your original checksum/signature.
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## Contract
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Do not break the API of stdlib `archive/tar` in our fork (ideally find an upstream mergeable solution).
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## Std Version
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The version of golang stdlib `archive/tar` is from go1.4.1, and their master branch around [a9dddb53f](https://github.com/golang/go/tree/a9dddb53f).
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It is minimally extended to expose the raw bytes of the TAR, rather than just the marshalled headers and file stream.
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## Design
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See the [design](concept/DESIGN.md).
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## Stored Metadata
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Since the raw bytes of the headers and padding are stored, you may be wondering
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what the size implications are. The headers are at least 512 bytes per
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file (sometimes more), at least 1024 null bytes on the end, and then various
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padding. This makes for a constant linear growth in the stored metadata, with a
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naive storage implementation.
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First we'll get an archive to work with. For repeatability, we'll make an
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archive from what you've just cloned:
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```bash
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git archive --format=tar -o tar-split.tar HEAD .
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```
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```bash
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$ go get github.com/vbatts/tar-split/cmd/tar-split
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$ tar-split checksize ./tar-split.tar
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inspecting "tar-split.tar" (size 210k)
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-- number of files: 50
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-- size of metadata uncompressed: 53k
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-- size of gzip compressed metadata: 3k
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```
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So assuming you've managed the extraction of the archive yourself, for reuse of
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the file payloads from a relative path, then the only additional storage
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implications are as little as 3kb.
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But let's look at a larger archive, with many files.
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```bash
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$ ls -sh ./d.tar
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1.4G ./d.tar
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$ tar-split checksize ~/d.tar
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inspecting "/home/vbatts/d.tar" (size 1420749k)
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-- number of files: 38718
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-- size of metadata uncompressed: 43261k
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-- size of gzip compressed metadata: 2251k
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```
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Here, an archive with 38,718 files has a compressed footprint of about 2mb.
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Rolling the null bytes on the end of the archive, we will assume a
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bytes-per-file rate for the storage implications.
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| uncompressed | compressed |
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| :----------: | :--------: |
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| ~ 1kb per/file | 0.06kb per/file |
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## What's Next?
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* More implementations of storage Packer and Unpacker
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* More implementations of FileGetter and FilePutter
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* would be interesting to have an assembler stream that implements `io.Seeker`
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## License
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See [LICENSE](LICENSE)
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