Commit 2109901d authored by Gao Xiang's avatar Gao Xiang
Browse files

erofs: update documentation 



- Refine highlights for main features;

- Add multi-reference pclusters and fragment description.

Signed-off-by: default avatarGao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: default avatarYue Hu <huyue2@coolpad.com>
Reviewed-by: default avatarChao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20221130095605.4656-1-hsiangkao@linux.alibaba.com
parent 27f2a2dc

Documentation/filesystems/erofs.rst

+23 −13
Original line number Diff line number Diff line
@@ -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.