241 lines
11 KiB
ReStructuredText
241 lines
11 KiB
ReStructuredText
.. 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.
|