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docs: filesystems: convert erofs.txt to ReST

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- Add a SPDX header;
- Add a document title;
- Some whitespace fixes and new line breaks;
- Mark literal blocks as such;
- Add table markups;
- Add lists markups;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/402d1d2f7252b8a683f7a9c6867bc5428da64026.1581955849.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2020-02-17 17:12:01 +01:00 committed by Jonathan Corbet
parent 06dedb45b7
commit e66d8631dd
2 changed files with 103 additions and 73 deletions
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69
Documentation/filesystems/erofs.txt → Documentation/filesystems/erofs.rst
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@ -1,3 +1,9 @@
.. SPDX-License-Identifier: GPL-2.0
======================================
Enhanced Read-Only File System - EROFS
======================================
Overview
========
@ -6,6 +12,7 @@ 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
@ -17,6 +24,7 @@ It is designed as a better filesystem solution for the following scenarios:
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;
@ -24,13 +32,17 @@ Here is the main features of EROFS:
- 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;
@ -43,29 +55,36 @@ Here is the main features of EROFS:
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
===============
@ -73,7 +92,7 @@ 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
____________________________________________________________
@ -83,13 +102,17 @@ to be as simple as possible:
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
@ -117,7 +140,7 @@ may not. All metadatas can be now observed in two different spaces (views):
|-> 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 |
@ -132,16 +155,19 @@ may not. All metadatas can be now observed in two different spaces (views):
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.
@ -149,6 +175,8 @@ may not. All metadatas can be now observed in two different spaces (views):
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
_________________________________________________________________________
@ -163,13 +191,15 @@ 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____|
| dirent | dirent | ... | dirent | filename | filename | ... | filename |
|___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
\ ^
\ | * could have
\ | trailing '\0'
@ -184,14 +214,14 @@ introduce another on-disk field at all.
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
| | | logical data
_________v_______________________________v_____________________v_______________
... | . | | . | | . | ...
____|____.________|_____________|________.____|_____________|__.__________|____
_________v_______________________________v_____________________v_______________
... | . | | . | | . | ...
____|____.________|_____________|________.____|_____________|__.__________|____
|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|
size size size size size
. . . .
@ -208,4 +238,3 @@ 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.

1
Documentation/filesystems/index.rst
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@ -61,6 +61,7 @@ Documentation for filesystem implementations.
dlmfs
ecryptfs
efivarfs
erofs
fuse
overlayfs
virtiofs