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linux/Documentation/filesystems/erofs.rst
Gao Xiang 6e95d0a018 erofs: update documentation 
- refine the filesystem overview for better description of recent
   new features like FSDAX and Fscache;

 - add the new `fsid' mount option;

 - fix some typos.

Link: https://lore.kernel.org/r/20220527070133.77962-1-hsiangkao@linux.alibaba.com
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
2022-05-29 15:39:04 +08:00

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.. SPDX-License-Identifier: GPL-2.0
======================================
EROFS - Enhanced Read-Only File System
======================================
Overview
========
EROFS filesystem stands for Enhanced Read-Only File System. It aims to form a
generic read-only filesystem solution for various read-only use cases instead
of just focusing on storage space saving without considering any side effects
of runtime performance.
It is designed to meet the needs of flexibility, feature extendability and user
payload friendly, etc. Apart from those, it is still kept as a simple
random-access friendly high-performance filesystem to get rid of unneeded I/O
amplification and memory-resident overhead compared to similar approaches.
It is implemented to be a better choice 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 or other considerations and
- hope to minimize extra storage space with guaranteed end-to-end performance
by using compact layout, transparent file compression and direct access,
especially for those embedded devices with limited memory and high-density
hosts with numerous containers.
Here is the main features of EROFS:
- Little endian on-disk design;
- 4KiB block size and 32-bit block addresses, therefore 16TiB address space
at most for now;
- Two inode layouts for different requirements:
===================== ============ ======================================
compact (v1) extended (v2)
===================== ============ ======================================
Inode metadata size 32 bytes 64 bytes
Max file size 4 GiB 16 EiB (also limited by max. vol size)
Max uids/gids 65536 4294967296
Per-inode timestamp no yes (64 + 32-bit timestamp)
Max hardlinks 65536 4294967296
Metadata reserved 8 bytes 18 bytes
===================== ============ ======================================
- Metadata and data could be mixed as an option;
- Support extended attributes (xattrs) as an option;
- Support tailpacking data and xattr inline compared to byte-addressed
unaligned metadata or smaller block size alternatives;
- Support POSIX.1e ACLs by using xattrs;
- Support transparent data compression as an option:
LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition,
inplace decompression is also supported to avoid bounce compressed buffers
and page cache thrashing.
- Support direct I/O on uncompressed files to avoid double caching for loop
devices;
- Support FSDAX on uncompressed images for secure containers and ramdisks in
order to get rid of unnecessary page cache.
- Support multiple devices for multi blob container images;
- Support file-based on-demand loading with the Fscache infrastructure.
The following git tree provides the file system user-space tools under
development, such as a formatting tool (mkfs.erofs), an on-disk consistency &
compatibility checking tool (fsck.erofs), and a debugging tool (dump.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.
========== =============================================
dax={always,never} Use direct access (no page cache). See
Documentation/filesystems/dax.rst.
dax A legacy option which is an alias for ``dax=always``.
device=%s Specify a path to an extra device to be used together.
fsid=%s Specify a filesystem image ID for Fscache back-end.
=================== =========================================================
Sysfs Entries
=============
Information about mounted erofs file systems can be found in /sys/fs/erofs.
Each mounted filesystem will have a directory in /sys/fs/erofs based on its
device name (i.e., /sys/fs/erofs/sda).
(see also Documentation/ABI/testing/sysfs-fs-erofs)
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 5 data layouts 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+);
4 chunk-based file (v5.15+).
== ====================================================================
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.
Chunk-based files
-----------------
In order to support chunk-based data deduplication, a new inode data layout has
been supported since Linux v5.15: Files are split in equal-sized data chunks
with ``extents`` area of the inode metadata indicating how to get the chunk
data: these can be simply as a 4-byte block address array or in the 8-byte
chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more
details.)
By the way, chunk-based files are all uncompressed for now.
Data compression
----------------
EROFS implements LZ4 fixed-sized output compression which generates fixed-sized
compressed data blocks from variable-sized input in contrast to other existing
fixed-sized input solutions. Relatively higher compression ratios can be gotten
by using fixed-sized output compression since nowadays popular data compression
algorithms are mostly LZ77-based and such fixed-sized output approach can be
benefited from the historical dictionary (aka. sliding window).
In details, original (uncompressed) data is turned into several variable-sized
extents and in the meanwhile, compressed into physical clusters (pclusters).
In order to record each variable-sized extent, logical clusters (lclusters) are
introduced as the basic unit of compress indexes to indicate whether a new
extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now
fixed in block size, as illustrated below::
|<- variable-sized extent ->|<- VLE ->|
clusterofs clusterofs clusterofs
| | |
_________v_________________________________v_______________________v________
... | . | | . | | . ...
____|____._________|______________|________.___ _|______________|__.________
|-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-|
(HEAD) (NONHEAD) (HEAD) (NONHEAD) .
. CBLKCNT . .
. . .
. . .
_______._____________________________.______________._________________
... | | | | ...
_______|______________|______________|______________|_________________
|-> big pcluster <-|-> pcluster <-|
A physical cluster can be seen as a container of physical compressed blocks
which contains compressed data. Previously, only lcluster-sized (4KB) pclusters
were supported. After big pcluster feature is introduced (available since
Linux v5.13), pcluster can be a multiple of lcluster size.
For each HEAD lcluster, clusterofs is recorded to indicate where a new extent
starts and blkaddr is used to seek the compressed data. For each NONHEAD
lcluster, delta0 and delta1 are available instead of blkaddr to indicate the
distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is
also a HEAD lcluster except that its data is uncompressed. See the comments
around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
If big pcluster is enabled, pcluster size in lclusters needs to be recorded as
well. Let the delta0 of the first NONHEAD lcluster store the compressed block
count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy
to understand its delta0 is constantly 1, as illustrated below::
__________________________________________________________
| HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD |
|__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_|
|<----- a big pcluster (with CBLKCNT) ------>|<-- -->|
a lcluster-sized pcluster (without CBLKCNT) ^
If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
but it's easy to know the size of such pcluster is 1 lcluster as well.
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