mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-24 01:10:54 +07:00
docs: filesystems: convert erofs.txt to ReST
- 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
Jonathan Corbet
parent
06dedb45b7
commit
e66d8631dd
@ -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
|
||||
____________________________________________________________
|
||||
@ -83,41 +102,45 @@ 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
|
||||
_____________________________________________________________________
|
||||
| ... | 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
|
||||
::
|
||||
|
||||
|-> 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:
|
||||
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.
|
||||
@ -149,11 +175,13 @@ 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
|
||||
_________________________________________________________________________
|
||||
| ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
|
||||
|________|_____________|_____________|_____|______________|_______________
|
||||
::
|
||||
|
||||
|-> aligned by 4 bytes
|
||||
+ xattr_blkaddr blocks |-> aligned with 4 bytes
|
||||
_________________________________________________________________________
|
||||
| ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
|
||||
|________|_____________|_____________|_____|______________|_______________
|
||||
|
||||
Directories
|
||||
-----------
|
||||
@ -163,19 +191,21 @@ 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
|
||||
___________________________
|
||||
/ |
|
||||
/ ______________|________________
|
||||
/ / | 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
|
||||
@ -184,28 +214,27 @@ 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_______________
|
||||
... | . | | . | | . | ...
|
||||
____|____.________|_____________|________.____|_____________|__.__________|____
|
||||
|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|
|
||||
size size size size size
|
||||
. . . .
|
||||
. . . .
|
||||
. . . .
|
||||
_______._____________._____________._____________._____________________
|
||||
... | | | | ... physical data
|
||||
_______|_____________|_____________|_____________|_____________________
|
||||
|-> cluster <-|-> cluster <-|-> cluster <-|
|
||||
size size size
|
||||
|---- 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.
|
||||
|
||||
@ -61,6 +61,7 @@ Documentation for filesystem implementations.
|
||||
dlmfs
|
||||
ecryptfs
|
||||
efivarfs
|
||||
erofs
|
||||
fuse
|
||||
overlayfs
|
||||
virtiofs
|
||||
|
||||
Loading…
Reference in New Issue
Block a user