Enhanced Read-Only File System - EROFS

Overview

EROFS file-system stands for Enhanced Read-Only File System. Differentfrom 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 beimmutable and bit-for-bit identical to the official golden image fortheir releases due to security and other considerations and
  • hope to save some extra storage space with guaranteed end-to-end performanceby using reduced metadata and transparent file compression, especiallyfor 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:

       
    Inode metadata size32 bytes64 bytes
    Max file size4 GB16 EB (also limited by max. vol size)
    Max uids/gids655364294967296
    File change timenoyes (64 + 32-bit timestamp)
    Max hardlinks655364294967296
    Metadata reserved4 bytes14 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 underdevelopment (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 followinglinux-erofs mailing list:

Mount options

(no)user_xattrSetup Extended User Attributes. Note: xattr is enabledby default if CONFIG_EROFS_FS_XATTR is selected.
(no)aclSetup POSIX Access Control List. Note: acl is enabledby default if CONFIG_EROFS_FS_POSIX_ACL is selected.
cache_strategy=%s

Select a strategy for cached decompression from now on:

disabledIn-place I/O decompression only;
readaheadCache the last incomplete compressed physicalcluster for further reading. It still doesin-place I/O decompression for the restcompressed physical clusters;
readaroundCache the both ends of incomplete compressedphysical clusters for further reading.It still does in-place I/O decompressionfor the rest compressed physical clusters.

On-disk details

Summary

Different from other read-only file systems, an EROFS volume is designedto 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 areasmay 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 fixedvalue (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 commonfield 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 withproper alignment, and they could be optional for different data mappings._currently_ total 4 valid data mappings are supported:

    0flat file data without data inline (no extent);
    1fixed-sized output data compression (with non-compacted indexes);
    2flat file data with tail packing data inline (no extent);
    3fixed-sized output data compression (with compacted indexes, v5.3+).

    The size of the optional xattrs is indicated by i_xattr_count in inodeheader. Large xattrs or xattrs shared by many different files can bestored 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 witha specific block indicated by xattr_blkaddr, organized one by one withproper 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 thateach directory block is divided into index and name areas in order to supportrandom file lookup, and all directory entries are _strictly_ recorded inalphabetical order in order to support improved prefix binary searchalgorithm (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 indicatesthe total number of directory entries in this block since it is no need tointroduce 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 dataat most. For each logical cluster, there is a corresponding on-disk index todescribe its cluster type, physical cluster address, etc.

See “struct z_erofs_vle_decompressed_index” in erofs_fs.h for more details.