[mirror_ubuntu-hirsute-kernel.git] / Documentation / filesystems / erofs.rst

.. SPDX-License-Identifier: GPL-2.0

======================================
Enhanced Read-Only File System - EROFS
======================================

Overview
========

EROFS file-system stands for Enhanced Read-Only File System. Different
from other read-only file systems, it aims to be designed for flexibility,
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
   immutable and bit-for-bit identical to the official golden image for
   their releases due to security and other considerations and

 - hope to save some extra storage space with guaranteed end-to-end performance
   by using reduced metadata and transparent file compression, especially
   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
   =====================  ============  =====================================

 - Support extended attributes (xattrs) as an option;

 - Support xattr inline and tail-end data inline for all files;

 - Support POSIX.1e ACLs by using xattrs;

 - Support transparent file compression as an option:
   LZ4 algorithm with 4 KB fixed-sized output compression for high performance.

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

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>

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
                                   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
                                   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::

                                |-> aligned with the block size
   ____________________________________________________________
  | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
  |_|__|_|_____|__________|_____|______|__________|_____|______|
  0 +1K

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
	_____________________________________________________________________
	|  ...   | inode |  xattrs  | extents  | data inline | ... | inode ...
	|________|_______|(optional)|(optional)|__(optional)_|_____|__________
		|-> aligned with the inode slot size
		    .                   .
		    .                         .
		.                              .
		.                                    .
	    .                                         .
	    .                                              .
	.____________________________________________________|-> aligned with 4B
	| xattr_ibody_header | shared xattrs | inline xattrs |
	|____________________|_______________|_______________|
	|->    12 bytes    <-|->x * 4 bytes<-|               .
			    .                .                 .
			.                      .                   .
		.                           .                     .
	    ._______________________________.______________________.
	    | id | id | id | id |  ... | id | ent | ... | ent| ... |
	    |____|____|____|____|______|____|_____|_____|____|_____|
					    |-> aligned with 4B
							|-> 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::

        __________________               __________________
       |     i_format     |             |     i_format     |
       |__________________|             |__________________|
       |        ...       |             |        ...       |
       |                  |             |                  |
       |__________________| 32 bytes    |                  |
                                        |                  |
                                        |__________________| 64 bytes

    Xattrs, extents, data inline are followed by the corresponding inode with
    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
	_________________________________________________________________________
	|  ...   | xattr_entry |  xattr data | ... |  xattr_entry | xattr data  ...
	|________|_____________|_____________|_____|______________|_______________

Directories
-----------
All directories are now organized in a compact on-disk format. Note that
each directory block is divided into index and name areas in order to support
random file lookup, and all directory entries are _strictly_ recorded in
alphabetical order in order to support improved prefix binary search
algorithm (could refer to the related source code).

::

		    ___________________________
		    /                           |
		/              ______________|________________
		/              /              | nameoff1       | nameoffN-1
    ____________.______________._______________v________________v__________
    | dirent | dirent | ... | dirent | filename | filename | ... | filename |
    |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
	\                           ^
	\                          |                           * could have
	\                         |                             trailing '\0'
	    \________________________| nameoff0

				Directory block

Note that apart from the offset of the first filename, nameoff0 also indicates
the total number of directory entries in this block since it is no need to
introduce another on-disk field at all.

Compression
-----------
Currently, EROFS supports 4KB fixed-sized output transparent file compression,
as illustrated below::

	    |---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----
	    clusterofs                      clusterofs            clusterofs
	    |                               |                     |   logical data
    _________v_______________________________v_____________________v_______________
    ... |    .        |             |        .    |             |  .          | ...
    ____|____.________|_____________|________.____|_____________|__.__________|____
	|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|
	    size          size          size          size          size
	    .                             .                .                   .
	    .                       .               .                  .
		.                  .              .                .
	_______._____________._____________._____________._____________________
	    ... |             |             |             | ... physical data
	_______|_____________|_____________|_____________|_____________________
		|-> cluster <-|-> cluster <-|-> cluster <-|
		    size          size          size

Currently each on-disk physical cluster can contain 4KB (un)compressed data
at most. For each logical cluster, there is a corresponding on-disk index to
describe its cluster type, physical cluster address, etc.

See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
Commit	Line	Data
e66d8631 MCC	1	.. SPDX-License-Identifier: GPL-2.0
	2
	3	======================================
	4	Enhanced Read-Only File System - EROFS
	5	======================================
	6
fdb05364 GX	7	Overview
	8	========
	9
	10	EROFS file-system stands for Enhanced Read-Only File System. Different
	11	from other read-only file systems, it aims to be designed for flexibility,
	12	scalability, but be kept simple and high performance.
	13
	14	It is designed as a better filesystem solution for the following scenarios:
e66d8631	15
fdb05364 GX	16	- read-only storage media or
	17
	18	- part of a fully trusted read-only solution, which means it needs to be
	19	immutable and bit-for-bit identical to the official golden image for
	20	their releases due to security and other considerations and
	21
	22	- hope to save some extra storage space with guaranteed end-to-end performance
	23	by using reduced metadata and transparent file compression, especially
	24	for those embedded devices with limited memory (ex, smartphone);
	25
	26	Here is the main features of EROFS:
e66d8631	27
fdb05364 GX	28	- Little endian on-disk design;
	29
	30	- Currently 4KB block size (nobh) and therefore maximum 16TB address space;
	31
	32	- Metadata & data could be mixed by design;
	33
	34	- 2 inode versions for different requirements:
e66d8631 MCC	35
e66d8631 MCC	36	===================== ============ =====================================
ffafde47	37	compact (v1) extended (v2)
e66d8631 MCC	38	===================== ============ =====================================
	39	Inode metadata size 32 bytes 64 bytes
	40	Max file size 4 GB 16 EB (also limited by max. vol size)
	41	Max uids/gids 65536 4294967296
	42	File change time no yes (64 + 32-bit timestamp)
	43	Max hardlinks 65536 4294967296
	44	Metadata reserved 4 bytes 14 bytes
	45	===================== ============ =====================================
fdb05364 GX	46
	47	- Support extended attributes (xattrs) as an option;
	48
	49	- Support xattr inline and tail-end data inline for all files;
	50
516c115c GX	51	- Support POSIX.1e ACLs by using xattrs;
516c115c GX	52
fdb05364	53	- Support transparent file compression as an option:
ffafde47	54	LZ4 algorithm with 4 KB fixed-sized output compression for high performance.
fdb05364 GX	55
	56	The following git tree provides the file system user-space tools under
	57	development (ex, formatting tool mkfs.erofs):
e66d8631 MCC	58
e66d8631 MCC	59	- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
fdb05364 GX	60
	61	Bugs and patches are welcome, please kindly help us and send to the following
	62	linux-erofs mailing list:
e66d8631 MCC	63
e66d8631 MCC	64	- linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
fdb05364	65
fdb05364 GX	66	Mount options
	67	=============
	68
e66d8631	69	=================== =========================================================
fdb05364 GX	70	(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled
	71	by default if CONFIG_EROFS_FS_XATTR is selected.
	72	(no)acl Setup POSIX Access Control List. Note: acl is enabled
	73	by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
4279f3f9	74	cache_strategy=%s Select a strategy for cached decompression from now on:
e66d8631 MCC	75
	76	========== =============================================
	77	disabled In-place I/O decompression only;
	78	readahead Cache the last incomplete compressed physical
4279f3f9 GX	79	cluster for further reading. It still does
	80	in-place I/O decompression for the rest
	81	compressed physical clusters;
e66d8631	82	readaround Cache the both ends of incomplete compressed
4279f3f9 GX	83	physical clusters for further reading.
	84	It still does in-place I/O decompression
	85	for the rest compressed physical clusters.
e66d8631 MCC	86	========== =============================================
e66d8631 MCC	87	=================== =========================================================
fdb05364 GX	88
	89	On-disk details
	90	===============
	91
	92	Summary
	93	-------
	94	Different from other read-only file systems, an EROFS volume is designed
e66d8631	95	to be as simple as possible::
fdb05364 GX	96
	97	\|-> aligned with the block size
	98	____________________________________________________________
	99	\| \|SB\| \| ... \| Metadata \| ... \| Data \| Metadata \| ... \| Data \|
	100	\|_\|__\|_\|_____\|__________\|_____\|______\|__________\|_____\|______\|
	101	0 +1K
	102
	103	All data areas should be aligned with the block size, but metadata areas
	104	may not. All metadatas can be now observed in two different spaces (views):
e66d8631	105
fdb05364	106	1. Inode metadata space
e66d8631	107
fdb05364	108	Each valid inode should be aligned with an inode slot, which is a fixed
ffafde47	109	value (32 bytes) and designed to be kept in line with compact inode size.
fdb05364 GX	110
	111	Each inode can be directly found with the following formula:
	112	inode offset = meta_blkaddr * block_size + 32 * nid
	113
e66d8631 MCC	114	::
	115
	116	\|-> aligned with 8B
	117	\|-> followed closely
	118	+ meta_blkaddr blocks \|-> another slot
	119	_____________________________________________________________________
	120	\| ... \| inode \| xattrs \| extents \| data inline \| ... \| inode ...
	121	\|________\|_______\|(optional)\|(optional)\|__(optional)_\|_____\|__________
	122	\|-> aligned with the inode slot size
	123	. .
	124	. .
	125	. .
	126	. .
	127	. .
	128	. .
	129	.____________________________________________________\|-> aligned with 4B
	130	\| xattr_ibody_header \| shared xattrs \| inline xattrs \|
	131	\|____________________\|_______________\|_______________\|
	132	\|-> 12 bytes <-\|->x * 4 bytes<-\| .
	133	. . .
	134	. . .
	135	. . .
	136	._______________________________.______________________.
	137	\| id \| id \| id \| id \| ... \| id \| ent \| ... \| ent\| ... \|
	138	\|____\|____\|____\|____\|______\|____\|_____\|_____\|____\|_____\|
	139	\|-> aligned with 4B
	140	\|-> aligned with 4B
fdb05364 GX	141
fdb05364 GX	142	Inode could be 32 or 64 bytes, which can be distinguished from a common
e66d8631	143	field which all inode versions have -- i_format::
fdb05364 GX	144
fdb05364 GX	145	__________________ __________________
ffafde47	146	\| i_format \| \| i_format \|
fdb05364 GX	147	\|__________________\| \|__________________\|
	148	\| ... \| \| ... \|
	149	\| \| \| \|
	150	\|__________________\| 32 bytes \| \|
	151	\| \|
	152	\|__________________\| 64 bytes
	153
	154	Xattrs, extents, data inline are followed by the corresponding inode with
ffafde47 GX	155	proper alignment, and they could be optional for different data mappings.
ffafde47 GX	156	_currently_ total 4 valid data mappings are supported:
fdb05364	157
e66d8631	158	== ====================================================================
ffafde47 GX	159	0 flat file data without data inline (no extent);
	160	1 fixed-sized output data compression (with non-compacted indexes);
	161	2 flat file data with tail packing data inline (no extent);
	162	3 fixed-sized output data compression (with compacted indexes, v5.3+).
e66d8631	163	== ====================================================================
fdb05364 GX	164
	165	The size of the optional xattrs is indicated by i_xattr_count in inode
	166	header. Large xattrs or xattrs shared by many different files can be
	167	stored in shared xattrs metadata rather than inlined right after inode.
	168
	169	2. Shared xattrs metadata space
e66d8631	170
fdb05364 GX	171	Shared xattrs space is similar to the above inode space, started with
	172	a specific block indicated by xattr_blkaddr, organized one by one with
	173	proper align.
	174
	175	Each share xattr can also be directly found by the following formula:
	176	xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
	177
e66d8631 MCC	178	::
	179
	180	\|-> aligned by 4 bytes
	181	+ xattr_blkaddr blocks \|-> aligned with 4 bytes
	182	_________________________________________________________________________
	183	\| ... \| xattr_entry \| xattr data \| ... \| xattr_entry \| xattr data ...
	184	\|________\|_____________\|_____________\|_____\|______________\|_______________
fdb05364 GX	185
	186	Directories
	187	-----------
	188	All directories are now organized in a compact on-disk format. Note that
	189	each directory block is divided into index and name areas in order to support
	190	random file lookup, and all directory entries are _strictly_ recorded in
	191	alphabetical order in order to support improved prefix binary search
	192	algorithm (could refer to the related source code).
	193
e66d8631 MCC	194	::
	195
	196	___________________________
	197	/ \|
	198	/ ______________\|________________
	199	/ / \| nameoff1 \| nameoffN-1
	200	____________.______________._______________v________________v__________
	201	\| dirent \| dirent \| ... \| dirent \| filename \| filename \| ... \| filename \|
	202	\|___.0___\|____1___\|_____\|___N-1__\|____0_____\|____1_____\|_____\|___N-1____\|
	203	\ ^
	204	\ \| * could have
	205	\ \| trailing '\0'
	206	\________________________\| nameoff0
fdb05364	207
e66d8631	208	Directory block
fdb05364 GX	209
	210	Note that apart from the offset of the first filename, nameoff0 also indicates
	211	the total number of directory entries in this block since it is no need to
	212	introduce another on-disk field at all.
	213
	214	Compression
	215	-----------
ffafde47	216	Currently, EROFS supports 4KB fixed-sized output transparent file compression,
e66d8631 MCC	217	as illustrated below::
	218
	219	\|---- Variant-Length Extent ----\|-------- VLE --------\|----- VLE -----
	220	clusterofs clusterofs clusterofs
	221	\| \| \| logical data
	222	_________v_______________________________v_____________________v_______________
	223	... \| . \| \| . \| \| . \| ...
	224	____\|____.________\|_____________\|________.____\|_____________\|__.__________\|____
	225	\|-> cluster <-\|-> cluster <-\|-> cluster <-\|-> cluster <-\|-> cluster <-\|
	226	size size size size size
	227	. . . .
	228	. . . .
	229	. . . .
	230	_______._____________._____________._____________._____________________
	231	... \| \| \| \| ... physical data
	232	_______\|_____________\|_____________\|_____________\|_____________________
	233	\|-> cluster <-\|-> cluster <-\|-> cluster <-\|
	234	size size size
fdb05364 GX	235
	236	Currently each on-disk physical cluster can contain 4KB (un)compressed data
	237	at most. For each logical cluster, there is a corresponding on-disk index to
	238	describe its cluster type, physical cluster address, etc.
	239
	240	See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.