erofs-utils: update README for the upcoming 1.8

Add descriptions to multi-threaded compression and reproducible builds.

Signed-off-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

README

@@ -54,51 +54,91 @@ mkfs.erofs
 
 Two main kinds of EROFS images can be generated: (un)compressed images.
 
- - For uncompressed images, there will be none of compresssed files in
-   these images.  However, it can decide whether the tail block of a
-   file should be inlined or not properly [1].
+ - For uncompressed images, there will be no compressed files in these
+   images.  However, an EROFS image can contain files which consist of
+   various aligned data blocks and then a tail that is stored inline in
+   order to compact images [1].
 
- - For compressed images, it'll try to use the given algorithms first
+ - For compressed images, it will try to use the given algorithms first
    for each regular file and see if storage space can be saved with
-   compression. If not, fallback to an uncompressed file.
+   compression. If not, it will fall back to an uncompressed file.
 
-How to generate EROFS images (LZ4 for Linux 5.3+, LZMA for Linux 5.16+)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Note that EROFS supports per-file compression configuration, proper
+configuration options need to be enabled to parse compressed files by
+the Linux kernel.
 
-Currently lz4(hc) and lzma are available for compression, e.g.
+How to generate EROFS images
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Compression algorithms could be specified with the command-line option
+`-z` to build a compressed EROFS image from a local directory:
  $ mkfs.erofs -zlz4hc foo.erofs.img foo/
 
-Or leave all files uncompressed as an option:
+Supported algorithms by the Linux kernel:
+ - LZ4 (Linux 5.3+);
+ - LZMA (Linux 5.16+);
+ - DEFLATE (Linux 6.6+);
+ - Zstandard (Linux 6.10+).
+
+Alternatively, generate an uncompressed EROFS from a local directory:
  $ mkfs.erofs foo.erofs.img foo/
 
-In addition, you could specify a higher compression level to get a
-(slightly) better compression ratio than the default level, e.g.
+Additionally, you can specify a higher compression level to get a
+(slightly) smaller image than the default level:
  $ mkfs.erofs -zlz4hc,12 foo.erofs.img foo/
 
-Note that all compressors are still single-threaded for now, thus it
-could take more time on the multiprocessor platform. Multi-threaded
-approach is already in our TODO list.
+Multi-threaded support can be explicitly enabled with the ./configure
+option `--enable-multithreading`; otherwise, single-threaded compression
+will be used for now.  It may take more time on multiprocessor platforms
+if multi-threaded support is not enabled.
+
+Currently, both `-Efragments` (not `-Eall-fragments`) and `-Ededupe`
+don't support multi-threading due to time limitations.
+
+Reproducible builds
+~~~~~~~~~~~~~~~~~~~
+
+Reproducible builds are typically used for verification and security,
+ensuring the same binaries/distributions to be reproduced in a
+deterministic way.
+
+Images generated by the same version of `mkfs.erofs` will be identical
+to previous runs if the same input is specified, and the same options
+are used.
+
+Specifically, variable timestamps and filesystem UUIDs can result in
+unreproducible EROFS images.  `-T` and `-U` can be used to fix them.
 
 How to generate EROFS big pcluster images (Linux 5.13+)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-In order to get much better compression ratios (thus better sequential
-read performance for common storage devices), big pluster feature has
-been introduced since linux-5.13, which is not forward-compatible with
-old kernels.
-
-In details, -C is used to specify the maximum size of each big pcluster
-in bytes, e.g.
+By default, EROFS formatter compresses data into separate one-block
+(e.g. 4KiB) filesystem physical clusters for outstanding random read
+performance.  In other words, each EROFS filesystem block can be
+independently decompressed.  However, other similar filesystems
+typically compress data into "blocks" of 128KiB or more for much smaller
+images.  Users may prefer smaller images for archiving purposes, even if
+random performance is compromised with those configurations, and even
+worse when using 4KiB blocks.
+
+In order to fulfill users' needs, big plusters has been introduced
+since Linux 5.13, in which each physical clusters will be more than one
+blocks.
+
+Specifically, `-C` is used to specify the maximum size of each pcluster
+in bytes:
  $ mkfs.erofs -zlz4hc -C65536 foo.erofs.img foo/
 
-So in that case, pcluster size can be 64KiB at most.
+Thus, in this case, pcluster sizes can be up to 64KiB.
 
-Note that large pcluster size can cause bad random performance, so
-please evaluate carefully in advance. Or make your own per-(sub)file
-compression strategies according to file access patterns if needed.
+Note that large pcluster size can degrade random performance (though it
+may improve sequential read performance for typical storage devices), so
+please evaluate carefully in advance.  Alternatively, you can make
+per-(sub)file compression strategies according to file access patterns
+if needed.
 
-How to generate EROFS images with multiple algorithms (Linux 5.16+)
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+How to generate EROFS images with multiple algorithms
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 It's possible to generate an EROFS image with files in different
 algorithms due to various purposes.  For example, LZMA for archival