285 lines
14 KiB
ReStructuredText
285 lines
14 KiB
ReStructuredText
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.. 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 save some extra storage space with guaranteed end-to-end performance
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by using reduced metadata and transparent file compression, especially
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for those embedded devices with limited memory (ex, smartphone);
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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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File change time 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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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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=================== =========================================================
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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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|__________________| 32 bytes | |
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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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