squashfs: update documentation to include description of xattr layout

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

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:

	 ---------------

	|---------------|

	|    uid/gid	|

	|  lookup table	|

	|---------------|

	|     xattr     |

	|     table	|

	 ---------------

Compressed data blocks are written to the filesystem as files are read from

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

-------------------------------

4.1 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

---------------------------