erofs: update documentation

- Refine highlights for main features;

- Add multi-reference pclusters and fragment description.
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221130095605.4656-1-hsiangkao@linux.alibaba.com

Documentation/filesystems/erofs.rst

@@ -30,12 +30,18 @@ It is implemented to be a better choice for the following scenarios:
    especially for those embedded devices with limited memory and high-density
    hosts with numerous containers.
 
-Here is the main features of EROFS:
+Here are the main features of EROFS:
 
  - Little endian on-disk design;
 
- - 4KiB block size and 32-bit block addresses, therefore 16TiB address space
-   at most for now;
+ - Block-based distribution and file-based distribution over fscache are
+   supported;
+
+ - Support multiple devices to refer to external blobs, which can be used
+   for container images;
+
+ - 4KiB block size and 32-bit block addresses for each device, therefore
+   16TiB address space at most for now;
 
  - Two inode layouts for different requirements:
 
@@ -50,28 +56,29 @@ Here is the main features of EROFS:
    Metadata reserved      8 bytes       18 bytes
    =====================  ============  ======================================
 
- - Metadata and data could be mixed as an option;
-
- - Support extended attributes (xattrs) as an option;
+ - Support extended attributes as an option;
 
- - Support tailpacking data and xattr inline compared to byte-addressed
-   unaligned metadata or smaller block size alternatives;
-
- - Support POSIX.1e ACLs by using xattrs;
+ - Support POSIX.1e ACLs by using extended attributes;
 
  - Support transparent data compression as an option:
    LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
    inplace decompression is also supported to avoid bounce compressed buffers
    and page cache thrashing.
 
+ - Support chunk-based data deduplication and rolling-hash compressed data
+   deduplication;
+
+ - Support tailpacking inline compared to byte-addressed unaligned metadata
+   or smaller block size alternatives;
+
+ - Support merging tail-end data into a special inode as fragments.
+
  - Support direct I/O on uncompressed files to avoid double caching for loop
    devices;
 
  - Support FSDAX on uncompressed images for secure containers and ramdisks in
    order to get rid of unnecessary page cache.
 
- - Support multiple devices for multi blob container images;
-
  - Support file-based on-demand loading with the Fscache infrastructure.
 
 The following git tree provides the file system user-space tools under
@@ -259,7 +266,7 @@ By the way, chunk-based files are all uncompressed for now.
 
 Data compression
 ----------------
-EROFS implements LZ4 fixed-sized output compression which generates fixed-sized
+EROFS implements fixed-sized output compression which generates fixed-sized
 compressed data blocks from variable-sized input in contrast to other existing
 fixed-sized input solutions. Relatively higher compression ratios can be gotten
 by using fixed-sized output compression since nowadays popular data compression
@@ -314,3 +321,6 @@ to understand its delta0 is constantly 1, as illustrated below::
 
 If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
 but it's easy to know the size of such pcluster is 1 lcluster as well.
+
+Since Linux v6.1, each pcluster can be used for multiple variable-sized extents,
+therefore it can be used for compressed data deduplication.