squashfs: update documentation to include description of xattr layout

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

diff --git a/Documentation/filesystems/squashfs.txt b/Documentation/filesystems/squashfs.txt
index b324c03..203f720 100644 (file)
--- a/Documentation/filesystems/squashfs.txt
+++ b/Documentation/filesystems/squashfs.txt
@@ -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
 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
 
 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
 -----------------------------
 
 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:
 
         ---------------
 alignment:
 
         ---------------


@@ -80,6 +81,9 @@ alignment:
        |---------------|
        |    uid/gid    |
        |  lookup table |
        |---------------|
        |    uid/gid    |
        |  lookup table |

+       |---------------|
+       |     xattr     |
+       |     table     |

         ---------------
 
 Compressed data blocks are written to the filesystem as files are read from
         ---------------
 
 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.
 
 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
 -------------------------------
 
 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
 -------------
 
 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
 ---------------------------
 
 4.2 Squashfs internal cache
 ---------------------------