forked from Minki/linux
a1108dcd93
EROFS images should inherit modification time rather than change time, since users and host tooling have no easy way to control change time. To reflect the new timestamp meaning, i_ctime and i_ctime_nsec are renamed to i_mtime and i_mtime_nsec. Link: https://lore.kernel.org/r/20220311041829.3109511-1-dvander@google.com # v1 Signed-off-by: David Anderson <dvander@google.com> [ Gao Xiang: update document as well. ] Reviewed-by: Chao Yu <chao@kernel.org> Link: https://lore.kernel.org/r/20220317114959.106787-1-hsiangkao@linux.alibaba.com # v2 Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
297 lines
14 KiB
ReStructuredText
297 lines
14 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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======================================
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Enhanced Read-Only File System - EROFS
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======================================
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Overview
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========
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EROFS file-system stands for Enhanced Read-Only File System. Different
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from other read-only file systems, it aims to be designed for flexibility,
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scalability, but be kept simple and high performance.
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It is designed as a better filesystem solution for the following scenarios:
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- read-only storage media or
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- part of a fully trusted read-only solution, which means it needs to be
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immutable and bit-for-bit identical to the official golden image for
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their releases due to security and other considerations and
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- hope to minimize extra storage space with guaranteed end-to-end performance
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by using compact layout, transparent file compression and direct access,
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especially for those embedded devices with limited memory and high-density
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hosts with numerous containers;
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Here is the main features of EROFS:
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- Little endian on-disk design;
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- Currently 4KB block size (nobh) and therefore maximum 16TB address space;
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- Metadata & data could be mixed by design;
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- 2 inode versions for different requirements:
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===================== ============ =====================================
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compact (v1) extended (v2)
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===================== ============ =====================================
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Inode metadata size 32 bytes 64 bytes
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Max file size 4 GB 16 EB (also limited by max. vol size)
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Max uids/gids 65536 4294967296
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Per-inode timestamp no yes (64 + 32-bit timestamp)
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Max hardlinks 65536 4294967296
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Metadata reserved 4 bytes 14 bytes
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===================== ============ =====================================
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- Support extended attributes (xattrs) as an option;
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- Support xattr inline and tail-end data inline for all files;
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- Support POSIX.1e ACLs by using xattrs;
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- Support transparent data compression as an option:
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LZ4 algorithm with the fixed-sized output compression for high performance;
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- Multiple device support for multi-layer container images.
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The following git tree provides the file system user-space tools under
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development (ex, formatting tool mkfs.erofs):
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- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
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Bugs and patches are welcome, please kindly help us and send to the following
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linux-erofs mailing list:
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- linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
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Mount options
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=============
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=================== =========================================================
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(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled
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by default if CONFIG_EROFS_FS_XATTR is selected.
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(no)acl Setup POSIX Access Control List. Note: acl is enabled
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by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
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cache_strategy=%s Select a strategy for cached decompression from now on:
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========== =============================================
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disabled In-place I/O decompression only;
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readahead Cache the last incomplete compressed physical
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cluster for further reading. It still does
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in-place I/O decompression for the rest
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compressed physical clusters;
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readaround Cache the both ends of incomplete compressed
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physical clusters for further reading.
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It still does in-place I/O decompression
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for the rest compressed physical clusters.
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========== =============================================
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dax={always,never} Use direct access (no page cache). See
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Documentation/filesystems/dax.rst.
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dax A legacy option which is an alias for ``dax=always``.
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device=%s Specify a path to an extra device to be used together.
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=================== =========================================================
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Sysfs Entries
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=============
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Information about mounted erofs file systems can be found in /sys/fs/erofs.
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Each mounted filesystem will have a directory in /sys/fs/erofs based on its
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device name (i.e., /sys/fs/erofs/sda).
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(see also Documentation/ABI/testing/sysfs-fs-erofs)
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On-disk details
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===============
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Summary
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-------
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Different from other read-only file systems, an EROFS volume is designed
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to be as simple as possible::
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|-> aligned with the block size
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____________________________________________________________
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| |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
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|_|__|_|_____|__________|_____|______|__________|_____|______|
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0 +1K
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All data areas should be aligned with the block size, but metadata areas
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may not. All metadatas can be now observed in two different spaces (views):
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1. Inode metadata space
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Each valid inode should be aligned with an inode slot, which is a fixed
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value (32 bytes) and designed to be kept in line with compact inode size.
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Each inode can be directly found with the following formula:
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inode offset = meta_blkaddr * block_size + 32 * nid
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::
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|-> aligned with 8B
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|-> followed closely
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+ meta_blkaddr blocks |-> another slot
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_____________________________________________________________________
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| ... | inode | xattrs | extents | data inline | ... | inode ...
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|________|_______|(optional)|(optional)|__(optional)_|_____|__________
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|-> aligned with the inode slot size
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. .
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. .
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. .
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. .
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. .
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. .
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.____________________________________________________|-> aligned with 4B
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| xattr_ibody_header | shared xattrs | inline xattrs |
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|____________________|_______________|_______________|
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|-> 12 bytes <-|->x * 4 bytes<-| .
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. . .
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. . .
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. . .
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._______________________________.______________________.
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| id | id | id | id | ... | id | ent | ... | ent| ... |
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|____|____|____|____|______|____|_____|_____|____|_____|
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|-> aligned with 4B
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|-> aligned with 4B
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Inode could be 32 or 64 bytes, which can be distinguished from a common
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field which all inode versions have -- i_format::
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__________________ __________________
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| i_format | | i_format |
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|__________________| |__________________|
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| ... | | ... |
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| | | |
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|__________________| 32 bytes | |
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| |
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|__________________| 64 bytes
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Xattrs, extents, data inline are followed by the corresponding inode with
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proper alignment, and they could be optional for different data mappings.
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_currently_ total 5 data layouts are supported:
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== ====================================================================
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0 flat file data without data inline (no extent);
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1 fixed-sized output data compression (with non-compacted indexes);
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2 flat file data with tail packing data inline (no extent);
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3 fixed-sized output data compression (with compacted indexes, v5.3+);
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4 chunk-based file (v5.15+).
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== ====================================================================
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The size of the optional xattrs is indicated by i_xattr_count in inode
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header. Large xattrs or xattrs shared by many different files can be
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stored in shared xattrs metadata rather than inlined right after inode.
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2. Shared xattrs metadata space
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Shared xattrs space is similar to the above inode space, started with
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a specific block indicated by xattr_blkaddr, organized one by one with
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proper align.
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Each share xattr can also be directly found by the following formula:
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xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
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::
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|-> aligned by 4 bytes
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+ xattr_blkaddr blocks |-> aligned with 4 bytes
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_________________________________________________________________________
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| ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
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|________|_____________|_____________|_____|______________|_______________
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Directories
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-----------
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All directories are now organized in a compact on-disk format. Note that
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each directory block is divided into index and name areas in order to support
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random file lookup, and all directory entries are _strictly_ recorded in
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alphabetical order in order to support improved prefix binary search
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algorithm (could refer to the related source code).
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::
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___________________________
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/ |
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/ ______________|________________
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/ / | nameoff1 | nameoffN-1
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____________.______________._______________v________________v__________
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| dirent | dirent | ... | dirent | filename | filename | ... | filename |
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|___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
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\ ^
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\ | * could have
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\ | trailing '\0'
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\________________________| nameoff0
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Directory block
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Note that apart from the offset of the first filename, nameoff0 also indicates
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the total number of directory entries in this block since it is no need to
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introduce another on-disk field at all.
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Chunk-based file
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----------------
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In order to support chunk-based data deduplication, a new inode data layout has
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been supported since Linux v5.15: Files are split in equal-sized data chunks
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with ``extents`` area of the inode metadata indicating how to get the chunk
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data: these can be simply as a 4-byte block address array or in the 8-byte
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chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more
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details.)
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By the way, chunk-based files are all uncompressed for now.
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Data compression
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----------------
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EROFS implements LZ4 fixed-sized output compression which generates fixed-sized
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compressed data blocks from variable-sized input in contrast to other existing
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fixed-sized input solutions. Relatively higher compression ratios can be gotten
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by using fixed-sized output compression since nowadays popular data compression
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algorithms are mostly LZ77-based and such fixed-sized output approach can be
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benefited from the historical dictionary (aka. sliding window).
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In details, original (uncompressed) data is turned into several variable-sized
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extents and in the meanwhile, compressed into physical clusters (pclusters).
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In order to record each variable-sized extent, logical clusters (lclusters) are
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introduced as the basic unit of compress indexes to indicate whether a new
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extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now
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fixed in block size, as illustrated below::
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|<- variable-sized extent ->|<- VLE ->|
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clusterofs clusterofs clusterofs
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_________v_________________________________v_______________________v________
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... | . | | . | | . ...
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____|____._________|______________|________.___ _|______________|__.________
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|-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-|
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(HEAD) (NONHEAD) (HEAD) (NONHEAD) .
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. CBLKCNT . .
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. . .
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. . .
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_______._____________________________.______________._________________
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... | | | | ...
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_______|______________|______________|______________|_________________
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|-> big pcluster <-|-> pcluster <-|
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A physical cluster can be seen as a container of physical compressed blocks
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which contains compressed data. Previously, only lcluster-sized (4KB) pclusters
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were supported. After big pcluster feature is introduced (available since
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Linux v5.13), pcluster can be a multiple of lcluster size.
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For each HEAD lcluster, clusterofs is recorded to indicate where a new extent
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starts and blkaddr is used to seek the compressed data. For each NONHEAD
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lcluster, delta0 and delta1 are available instead of blkaddr to indicate the
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distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is
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also a HEAD lcluster except that its data is uncompressed. See the comments
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around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
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If big pcluster is enabled, pcluster size in lclusters needs to be recorded as
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well. Let the delta0 of the first NONHEAD lcluster store the compressed block
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count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy
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to understand its delta0 is constantly 1, as illustrated below::
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__________________________________________________________
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| HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD |
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|__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_|
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|<----- a big pcluster (with CBLKCNT) ------>|<-- -->|
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a lcluster-sized pcluster (without CBLKCNT) ^
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If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
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but it's easy to know the size of such pcluster is 1 lcluster as well.
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