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+.. SPDX-License-Identifier: GPL-2.0
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+
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+The Contents of inode.i\_block
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+------------------------------
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+
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+Depending on the type of file an inode describes, the 60 bytes of
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+storage in ``inode.i_block`` can be used in different ways. In general,
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+regular files and directories will use it for file block indexing
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+information, and special files will use it for special purposes.
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+
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+Symbolic Links
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+~~~~~~~~~~~~~~
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+
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+The target of a symbolic link will be stored in this field if the target
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+string is less than 60 bytes long. Otherwise, either extents or block
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+maps will be used to allocate data blocks to store the link target.
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+
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+Direct/Indirect Block Addressing
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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+
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+In ext2/3, file block numbers were mapped to logical block numbers by
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+means of an (up to) three level 1-1 block map. To find the logical block
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+that stores a particular file block, the code would navigate through
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+this increasingly complicated structure. Notice that there is neither a
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+magic number nor a checksum to provide any level of confidence that the
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+block isn't full of garbage.
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+
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+.. ifconfig:: builder != 'latex'
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+
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+ .. include:: blockmap.rst
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+
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+.. ifconfig:: builder == 'latex'
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+
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+ [Table omitted because LaTeX doesn't support nested tables.]
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+
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+Note that with this block mapping scheme, it is necessary to fill out a
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+lot of mapping data even for a large contiguous file! This inefficiency
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+led to the creation of the extent mapping scheme, discussed below.
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+
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+Notice also that a file using this mapping scheme cannot be placed
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+higher than 2^32 blocks.
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+
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+Extent Tree
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+~~~~~~~~~~~
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+
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+In ext4, the file to logical block map has been replaced with an extent
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+tree. Under the old scheme, allocating a contiguous run of 1,000 blocks
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+requires an indirect block to map all 1,000 entries; with extents, the
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+mapping is reduced to a single ``struct ext4_extent`` with
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+``ee_len = 1000``. If flex\_bg is enabled, it is possible to allocate
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+very large files with a single extent, at a considerable reduction in
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+metadata block use, and some improvement in disk efficiency. The inode
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+must have the extents flag (0x80000) flag set for this feature to be in
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+use.
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+
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+Extents are arranged as a tree. Each node of the tree begins with a
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+``struct ext4_extent_header``. If the node is an interior node
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+(``eh.eh_depth`` > 0), the header is followed by ``eh.eh_entries``
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+instances of ``struct ext4_extent_idx``; each of these index entries
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+points to a block containing more nodes in the extent tree. If the node
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+is a leaf node (``eh.eh_depth == 0``), then the header is followed by
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+``eh.eh_entries`` instances of ``struct ext4_extent``; these instances
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+point to the file's data blocks. The root node of the extent tree is
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+stored in ``inode.i_block``, which allows for the first four extents to
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+be recorded without the use of extra metadata blocks.
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+
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+The extent tree header is recorded in ``struct ext4_extent_header``,
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+which is 12 bytes long:
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+
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+.. list-table::
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+ :widths: 1 1 1 77
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+ :header-rows: 1
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+
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+ * - Offset
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+ - Size
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+ - Name
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+ - Description
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+ * - 0x0
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+ - \_\_le16
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+ - eh\_magic
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+ - Magic number, 0xF30A.
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+ * - 0x2
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+ - \_\_le16
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+ - eh\_entries
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+ - Number of valid entries following the header.
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+ * - 0x4
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+ - \_\_le16
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+ - eh\_max
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+ - Maximum number of entries that could follow the header.
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+ * - 0x6
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+ - \_\_le16
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+ - eh\_depth
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+ - Depth of this extent node in the extent tree. 0 = this extent node
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+ points to data blocks; otherwise, this extent node points to other
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+ extent nodes. The extent tree can be at most 5 levels deep: a logical
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+ block number can be at most ``2^32``, and the smallest ``n`` that
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+ satisfies ``4*(((blocksize - 12)/12)^n) >= 2^32`` is 5.
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+ * - 0x8
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+ - \_\_le32
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+ - eh\_generation
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+ - Generation of the tree. (Used by Lustre, but not standard ext4).
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+
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+Internal nodes of the extent tree, also known as index nodes, are
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+recorded as ``struct ext4_extent_idx``, and are 12 bytes long:
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+
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+.. list-table::
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+ :widths: 1 1 1 77
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+ :header-rows: 1
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+
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+ * - Offset
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+ - Size
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+ - Name
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+ - Description
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+ * - 0x0
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+ - \_\_le32
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+ - ei\_block
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+ - This index node covers file blocks from 'block' onward.
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+ * - 0x4
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+ - \_\_le32
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+ - ei\_leaf\_lo
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+ - Lower 32-bits of the block number of the extent node that is the next
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+ level lower in the tree. The tree node pointed to can be either another
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+ internal node or a leaf node, described below.
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+ * - 0x8
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+ - \_\_le16
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+ - ei\_leaf\_hi
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+ - Upper 16-bits of the previous field.
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+ * - 0xA
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+ - \_\_u16
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+ - ei\_unused
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+ -
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+
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+Leaf nodes of the extent tree are recorded as ``struct ext4_extent``,
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+and are also 12 bytes long:
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+
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+.. list-table::
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+ :widths: 1 1 1 77
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+ :header-rows: 1
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+
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+ * - Offset
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+ - Size
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+ - Name
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+ - Description
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+ * - 0x0
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+ - \_\_le32
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+ - ee\_block
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+ - First file block number that this extent covers.
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+ * - 0x4
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+ - \_\_le16
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+ - ee\_len
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+ - Number of blocks covered by extent. If the value of this field is <=
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+ 32768, the extent is initialized. If the value of the field is > 32768,
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+ the extent is uninitialized and the actual extent length is ``ee_len`` -
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+ 32768. Therefore, the maximum length of a initialized extent is 32768
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+ blocks, and the maximum length of an uninitialized extent is 32767.
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+ * - 0x6
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+ - \_\_le16
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+ - ee\_start\_hi
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+ - Upper 16-bits of the block number to which this extent points.
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+ * - 0x8
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+ - \_\_le32
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+ - ee\_start\_lo
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+ - Lower 32-bits of the block number to which this extent points.
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+
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+Prior to the introduction of metadata checksums, the extent header +
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+extent entries always left at least 4 bytes of unallocated space at the
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+end of each extent tree data block (because (2^x % 12) >= 4). Therefore,
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+the 32-bit checksum is inserted into this space. The 4 extents in the
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+inode do not need checksumming, since the inode is already checksummed.
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+The checksum is calculated against the FS UUID, the inode number, the
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+inode generation, and the entire extent block leading up to (but not
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+including) the checksum itself.
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+
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+``struct ext4_extent_tail`` is 4 bytes long:
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+
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+.. list-table::
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+ :widths: 1 1 1 77
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+ :header-rows: 1
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+
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+ * - Offset
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+ - Size
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+ - Name
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+ - Description
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+ * - 0x0
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+ - \_\_le32
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+ - eb\_checksum
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+ - Checksum of the extent block, crc32c(uuid+inum+igeneration+extentblock)
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+
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+Inline Data
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+~~~~~~~~~~~
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+
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+If the inline data feature is enabled for the filesystem and the flag is
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+set for the inode, it is possible that the first 60 bytes of the file
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+data are stored here.
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Darrick J. Wong