195 lines
6.3 KiB
ReStructuredText
195 lines
6.3 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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The Contents of inode.i_block
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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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Symbolic Links
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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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Direct/Indirect Block Addressing
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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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.. ifconfig:: builder != 'latex'
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.. include:: blockmap.rst
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.. ifconfig:: builder == 'latex'
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[Table omitted because LaTeX doesn't support nested tables.]
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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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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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Extent Tree
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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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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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The extent tree header is recorded in ``struct ext4_extent_header``,
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which is 12 bytes long:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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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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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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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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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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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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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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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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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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``struct ext4_extent_tail`` is 4 bytes long:
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.. list-table::
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:widths: 8 8 24 40
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:header-rows: 1
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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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Inline Data
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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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