| .. SPDX-License-Identifier: GPL-2.0 |
| |
| Inline Data |
| ----------- |
| |
| The inline data feature was designed to handle the case that a file's |
| data is so tiny that it readily fits inside the inode, which |
| (theoretically) reduces disk block consumption and reduces seeks. If the |
| file is smaller than 60 bytes, then the data are stored inline in |
| ``inode.i_block``. If the rest of the file would fit inside the extended |
| attribute space, then it might be found as an extended attribute |
| “system.data” within the inode body (“ibody EA”). This of course |
| constrains the amount of extended attributes one can attach to an inode. |
| If the data size increases beyond i_block + ibody EA, a regular block |
| is allocated and the contents moved to that block. |
| |
| Pending a change to compact the extended attribute key used to store |
| inline data, one ought to be able to store 160 bytes of data in a |
| 256-byte inode (as of June 2015, when i_extra_isize is 28). Prior to |
| that, the limit was 156 bytes due to inefficient use of inode space. |
| |
| The inline data feature requires the presence of an extended attribute |
| for “system.data”, even if the attribute value is zero length. |
| |
| Inline Directories |
| ~~~~~~~~~~~~~~~~~~ |
| |
| The first four bytes of i_block are the inode number of the parent |
| directory. Following that is a 56-byte space for an array of directory |
| entries; see ``struct ext4_dir_entry``. If there is a “system.data” |
| attribute in the inode body, the EA value is an array of |
| ``struct ext4_dir_entry`` as well. Note that for inline directories, the |
| i_block and EA space are treated as separate dirent blocks; directory |
| entries cannot span the two. |
| |
| Inline directory entries are not checksummed, as the inode checksum |
| should protect all inline data contents. |
