docs: filesystems: convert erofs.txt to ReST

.. SPDX-License-Identifier: GPL-2.0

======================================

Enhanced Read-Only File System - EROFS

======================================

Overview

========

scalability, but be kept simple and high performance.

It is designed as a better filesystem solution for the following scenarios:

 - read-only storage media or

 - part of a fully trusted read-only solution, which means it needs to be

   for those embedded devices with limited memory (ex, smartphone);

Here is the main features of EROFS:

 - Little endian on-disk design;

 - Currently 4KB block size (nobh) and therefore maximum 16TB address space;

 - Metadata & data could be mixed by design;

 - 2 inode versions for different requirements:

   =====================  ============  =====================================

                          compact (v1)  extended (v2)

   Inode metadata size:   32 bytes      64 bytes

   Max file size:         4 GB          16 EB (also limited by max. vol size)

   Max uids/gids:         65536         4294967296

   File change time:      no            yes (64 + 32-bit timestamp)

   Max hardlinks:         65536         4294967296

   Metadata reserved:     4 bytes       14 bytes

   =====================  ============  =====================================

   Inode metadata size    32 bytes      64 bytes

   Max file size          4 GB          16 EB (also limited by max. vol size)

   Max uids/gids          65536         4294967296

   File change time       no            yes (64 + 32-bit timestamp)

   Max hardlinks          65536         4294967296

   Metadata reserved      4 bytes       14 bytes

   =====================  ============  =====================================

 - Support extended attributes (xattrs) as an option;

The following git tree provides the file system user-space tools under

development (ex, formatting tool mkfs.erofs):

>> git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git

- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git

Bugs and patches are welcome, please kindly help us and send to the following

linux-erofs mailing list:

>> linux-erofs mailing list   <linux-erofs@lists.ozlabs.org>

- linux-erofs mailing list   <linux-erofs@lists.ozlabs.org>

Mount options

=============

===================    =========================================================

(no)user_xattr         Setup Extended User Attributes. Note: xattr is enabled

                       by default if CONFIG_EROFS_FS_XATTR is selected.

(no)acl                Setup POSIX Access Control List. Note: acl is enabled

                       by default if CONFIG_EROFS_FS_POSIX_ACL is selected.

cache_strategy=%s      Select a strategy for cached decompression from now on:

                         disabled: In-place I/O decompression only;

                        readahead: Cache the last incomplete compressed physical

		       ==========  =============================================

                         disabled  In-place I/O decompression only;

                        readahead  Cache the last incomplete compressed physical

                                   cluster for further reading. It still does

                                   in-place I/O decompression for the rest

                                   compressed physical clusters;

                       readaround: Cache the both ends of incomplete compressed

                       readaround  Cache the both ends of incomplete compressed

                                   physical clusters for further reading.

                                   It still does in-place I/O decompression

                                   for the rest compressed physical clusters.

		       ==========  =============================================

===================    =========================================================

On-disk details

===============

Summary

-------

Different from other read-only file systems, an EROFS volume is designed

to be as simple as possible:

to be as simple as possible::

                                |-> aligned with the block size

   ____________________________________________________________

All data areas should be aligned with the block size, but metadata areas

may not. All metadatas can be now observed in two different spaces (views):

 1. Inode metadata space

    Each valid inode should be aligned with an inode slot, which is a fixed

    value (32 bytes) and designed to be kept in line with compact inode size.

    Each inode can be directly found with the following formula:

         inode offset = meta_blkaddr * block_size + 32 * nid

    ::

				    |-> aligned with 8B

					    |-> followed closely

	+ meta_blkaddr blocks                                      |-> another slot

							|-> aligned with 4B

    Inode could be 32 or 64 bytes, which can be distinguished from a common

    field which all inode versions have -- i_format:

    field which all inode versions have -- i_format::

        __________________               __________________

       |     i_format     |             |     i_format     |

    proper alignment, and they could be optional for different data mappings.

    _currently_ total 4 valid data mappings are supported:

    ==  ====================================================================

     0  flat file data without data inline (no extent);

     1  fixed-sized output data compression (with non-compacted indexes);

     2  flat file data with tail packing data inline (no extent);

     3  fixed-sized output data compression (with compacted indexes, v5.3+).

    ==  ====================================================================

    The size of the optional xattrs is indicated by i_xattr_count in inode

    header. Large xattrs or xattrs shared by many different files can be

    stored in shared xattrs metadata rather than inlined right after inode.

 2. Shared xattrs metadata space

    Shared xattrs space is similar to the above inode space, started with

    a specific block indicated by xattr_blkaddr, organized one by one with

    proper align.

    Each share xattr can also be directly found by the following formula:

         xattr offset = xattr_blkaddr * block_size + 4 * xattr_id

    ::

			    |-> aligned by  4 bytes

	+ xattr_blkaddr blocks                     |-> aligned with 4 bytes

	_________________________________________________________________________

alphabetical order in order to support improved prefix binary search

algorithm (could refer to the related source code).

::

		    ___________________________

		    /                           |

		/              ______________|________________

Compression

-----------

Currently, EROFS supports 4KB fixed-sized output transparent file compression,

as illustrated below:

as illustrated below::

	    |---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----

	    clusterofs                      clusterofs            clusterofs

describe its cluster type, physical cluster address, etc.

See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.

   dlmfs

   ecryptfs

   efivarfs

   erofs

   fuse

   overlayfs

   virtiofs