[linux-block.git] / Documentation / filesystems / erofs.txt

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