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