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It actually has been around for years: For containers and other sandbox use cases, there will be thousands (and even more) of authenticated (sub)images running on the same host, unlike OS images. Of course, all scenarios can use the same EROFS on-disk format, but bdev-backed mounts just work well for OS images since golden data is dumped into real block devices. However, it's somewhat hard for container runtimes to manage and isolate so many unnecessary virtual block devices safely and efficiently [1]: they just look like a burden to orchestrators and file-backed mounts are preferred indeed. There were already enough attempts such as Incremental FS, the original ComposeFS and PuzzleFS acting in the same way for immutable fses. As for current EROFS users, ComposeFS, containerd and Android APEXs will be directly benefited from it. On the other hand, previous experimental feature "erofs over fscache" was once also intended to provide a similar solution (inspired by Incremental FS discussion [2]), but the following facts show file-backed mounts will be a better approach: - Fscache infrastructure has recently been moved into new Netfslib which is an unexpected dependency to EROFS really, although it originally claims "it could be used for caching other things such as ISO9660 filesystems too." [3] - It takes an unexpectedly long time to upstream Fscache/Cachefiles enhancements. For example, the failover feature took more than one year, and the deamonless feature is still far behind now; - Ongoing HSM "fanotify pre-content hooks" [4] together with this will perfectly supersede "erofs over fscache" in a simpler way since developers (mainly containerd folks) could leverage their existing caching mechanism entirely in userspace instead of strictly following the predefined in-kernel caching tree hierarchy. After "fanotify pre-content hooks" lands upstream to provide the same functionality, "erofs over fscache" will be removed then (as an EROFS internal improvement and EROFS will not have to bother with on-demand fetching and/or caching improvements anymore.) [1] https://github.com/containers/storage/pull/2039 [2] https://lore.kernel.org/r/CAOQ4uxjbVxnubaPjVaGYiSwoGDTdpWbB=w_AeM6YM=zVixsUfQ@mail.gmail.com [3] https://docs.kernel.org/filesystems/caching/fscache.html [4] https://lore.kernel.org/r/cover.1723670362.git.josef@toxicpanda.com Closes: https://github.com/containers/composefs/issues/144 Reviewed-by: Sandeep Dhavale <dhavale@google.com> Reviewed-by: Chao Yu <chao@kernel.org> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com> Link: https://lore.kernel.org/r/20240830032840.3783206-1-hsiangkao@linux.alibaba.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.