These layers are stored on a registry as a compressed tarball archive. When using the overlay backend, the container runtime fetches and extracts each layer to a different directory. At runtime, each layer is used as a lower layer for the final container overlay filesystem mount. A Containerfile Dockerfile made of the following lines results in a series of different layers:. Currently, users can only perform per-layer deduplication. Images can share layers, but it requires discipline. Having more layers can facilitate deduplication, but it has a runtime cost because the overlay filesystem needs to scan more layers for each lookup and build a directory listing.
The current image format used by container engines, a gzip-compressed tarball, doesn't have enough information to allow for these optimizations. Two candidates are currently supported: eStargz and zstd:chunked. These new formats keep the metadata for each file contained in the tarball, including their checksum. The eStargz format transforms a gzip-compressed layer into an equivalent tarball where each file is compressed individually.
The system can retrieve each file without having to fetch and decompress the entire tarball. The metadata for the layer is appended to the tarball file itself as part of the tar stream.
This is a breaking change since the resulting tarball, once decompressed, has a different digest and contains an additional file. Given some limitations in the container and image API s, our container tools can already consume these images but cannot create them yet.
We've created a new solution named zstd:chunked to address the issue with the eStargz format that changes the DiffID and adds the metadata as part of the tarball. In the zstd:chunked format, the same metadata used by eStargz is added within the compression stream. The zstd decompressor ignores the additional metadata so that the digest for the uncompressed file doesn't change.
In addition, zstd is much faster and compresses better than gzip. When a layer stored in one of these two formats is pulled, the container engine takes these steps:. So, deduplication is happening at pull time as known files are not retrieved and with the storage because the same files are deduplicated with reflinks where supported.
The partial pull feature doesn't require an additional store for the object files, as it reads them directly from the final checked-out directory where they are stored. Currently, tarball extraction happens in a separate process that runs in a chroot.
This prevents specially crafted images from taking advantage of symlinks resolution and creating files outside the target directory.
Since the new deduplication feature needs to access files outside the target directory, the container runtime cannot use the existing extraction code. A new extractor is used when using the partial-pull feature. It needs the openat2 syscall that was added to the Linux kernel 5.
Hamiltonians 4. Electronic Structure Methods 5. Post-processing 6. Other Topics Technical stuff 1. Developer Documentation 2. Reference Documentation 3. API Documentation.
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