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Diffstat (limited to 'Documentation/filesystems/squashfs.txt')
| -rw-r--r-- | Documentation/filesystems/squashfs.txt | 34 |
1 files changed, 28 insertions, 6 deletions
diff --git a/Documentation/filesystems/squashfs.txt b/Documentation/filesystems/squashfs.txt index b324c033035..66699afd66c 100644 --- a/Documentation/filesystems/squashfs.txt +++ b/Documentation/filesystems/squashfs.txt @@ -2,7 +2,7 @@ SQUASHFS 4.0 FILESYSTEM ======================= Squashfs is a compressed read-only filesystem for Linux. -It uses zlib compression to compress files, inodes and directories. +It uses zlib/lzo compression to compress files, inodes and directories. Inodes in the system are very small and all blocks are packed to minimise data overhead. Block sizes greater than 4K are supported up to a maximum of 1Mbytes (default block size 128K). @@ -38,7 +38,8 @@ Hard link support: yes no Real inode numbers: yes no 32-bit uids/gids: yes no File creation time: yes no -Xattr and ACL support: no no +Xattr support: yes no +ACL support: no no Squashfs compresses data, inodes and directories. In addition, inode and directory data are highly compacted, and packed on byte boundaries. Each @@ -58,7 +59,7 @@ obtained from this site also. 3. SQUASHFS FILESYSTEM DESIGN ----------------------------- -A squashfs filesystem consists of seven parts, packed together on a byte +A squashfs filesystem consists of a maximum of eight parts, packed together on a byte alignment: --------------- @@ -80,6 +81,9 @@ alignment: |---------------| | uid/gid | | lookup table | + |---------------| + | xattr | + | table | --------------- Compressed data blocks are written to the filesystem as files are read from @@ -192,6 +196,26 @@ This table is stored compressed into metadata blocks. A second index table is used to locate these. This second index table for speed of access (and because it is small) is read at mount time and cached in memory. +3.7 Xattr table +--------------- + +The xattr table contains extended attributes for each inode. The xattrs +for each inode are stored in a list, each list entry containing a type, +name and value field. The type field encodes the xattr prefix +("user.", "trusted." etc) and it also encodes how the name/value fields +should be interpreted. Currently the type indicates whether the value +is stored inline (in which case the value field contains the xattr value), +or if it is stored out of line (in which case the value field stores a +reference to where the actual value is stored). This allows large values +to be stored out of line improving scanning and lookup performance and it +also allows values to be de-duplicated, the value being stored once, and +all other occurences holding an out of line reference to that value. + +The xattr lists are packed into compressed 8K metadata blocks. +To reduce overhead in inodes, rather than storing the on-disk +location of the xattr list inside each inode, a 32-bit xattr id +is stored. This xattr id is mapped into the location of the xattr +list using a second xattr id lookup table. 4. TODOS AND OUTSTANDING ISSUES ------------------------------- @@ -199,9 +223,7 @@ it is small) is read at mount time and cached in memory. 4.1 Todo list ------------- -Implement Xattr and ACL support. The Squashfs 4.0 filesystem layout has hooks -for these but the code has not been written. Once the code has been written -the existing layout should not require modification. +Implement ACL support. 4.2 Squashfs internal cache --------------------------- |