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 metadata 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 placed after 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.

Long extended attribute name prefixes¶

There are use cases where extended attributes with different values can have only a few common prefixes (such as overlayfs xattrs). The predefined prefixes work inefficiently in both image size and runtime performance in such cases.

The long xattr name prefixes feature is introduced to address this issue. The overall idea is that, apart from the existing predefined prefixes, the xattr entry could also refer to user-specified long xattr name prefixes, e.g. “trusted.overlay.”.

When referring to a long xattr name prefix, the highest bit (bit 7) of erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole represent the index of the referred long name prefix among all long name prefixes. Therefore, only the trailing part of the name apart from the long xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if the full xattr name matches exactly as its long xattr name prefix.

All long xattr prefixes are stored one by one in the packed inode as long as the packed inode is valid, or in the meta inode otherwise. The xattr_prefix_count (of the on-disk superblock) indicates the total number of long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start offset of long name prefixes in the packed/meta inode. Note that, long extended attribute name prefixes are disabled if xattr_prefix_count is 0.

Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4), where len represents the total size of the data part. The data part is actually represented by ‘struct erofs_xattr_long_prefix’, where base_index represents the index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for “trusted.overlay.” long name prefix, while the infix string keeps the string after stripping the short prefix, e.g. “overlay.” for the example above.

Data compression¶

EROFS implements 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.

Since Linux v6.1, each pcluster can be used for multiple variable-sized extents, therefore it can be used for compressed data deduplication.