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Diffstat (limited to 'Documentation/filesystems/erofs.rst')
| -rw-r--r-- | Documentation/filesystems/erofs.rst | 40 |
1 files changed, 37 insertions, 3 deletions
diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst index 4654ee57c1d5..f200d7874495 100644 --- a/Documentation/filesystems/erofs.rst +++ b/Documentation/filesystems/erofs.rst @@ -58,12 +58,14 @@ Here are the main features of EROFS: - Support extended attributes as an option; + - Support a bloom filter that speeds up negative extended attribute lookups; + - Support POSIX.1e ACLs by using extended attributes; - 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. + LZ4, MicroLZMA and DEFLATE algorithms can be used on a per-file basis; In + addition, inplace decompression is also supported to avoid bounce compressed + buffers and unnecessary page cache thrashing. - Support chunk-based data deduplication and rolling-hash compressed data deduplication; @@ -268,6 +270,38 @@ 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 |