1*4882a593Smuzhiyun.. SPDX-License-Identifier: GPL-2.0 2*4882a593Smuzhiyun 3*4882a593SmuzhiyunAbout this Book 4*4882a593Smuzhiyun=============== 5*4882a593Smuzhiyun 6*4882a593SmuzhiyunThis document attempts to describe the on-disk format for ext4 7*4882a593Smuzhiyunfilesystems. The same general ideas should apply to ext2/3 filesystems 8*4882a593Smuzhiyunas well, though they do not support all the features that ext4 supports, 9*4882a593Smuzhiyunand the fields will be shorter. 10*4882a593Smuzhiyun 11*4882a593Smuzhiyun**NOTE**: This is a work in progress, based on notes that the author 12*4882a593Smuzhiyun(djwong) made while picking apart a filesystem by hand. The data 13*4882a593Smuzhiyunstructure definitions should be current as of Linux 4.18 and 14*4882a593Smuzhiyune2fsprogs-1.44. All comments and corrections are welcome, since there is 15*4882a593Smuzhiyunundoubtedly plenty of lore that might not be reflected in freshly 16*4882a593Smuzhiyuncreated demonstration filesystems. 17*4882a593Smuzhiyun 18*4882a593SmuzhiyunLicense 19*4882a593Smuzhiyun------- 20*4882a593SmuzhiyunThis book is licensed under the terms of the GNU Public License, v2. 21*4882a593Smuzhiyun 22*4882a593SmuzhiyunTerminology 23*4882a593Smuzhiyun----------- 24*4882a593Smuzhiyun 25*4882a593Smuzhiyunext4 divides a storage device into an array of logical blocks both to 26*4882a593Smuzhiyunreduce bookkeeping overhead and to increase throughput by forcing larger 27*4882a593Smuzhiyuntransfer sizes. Generally, the block size will be 4KiB (the same size as 28*4882a593Smuzhiyunpages on x86 and the block layer's default block size), though the 29*4882a593Smuzhiyunactual size is calculated as 2 ^ (10 + ``sb.s_log_block_size``) bytes. 30*4882a593SmuzhiyunThroughout this document, disk locations are given in terms of these 31*4882a593Smuzhiyunlogical blocks, not raw LBAs, and not 1024-byte blocks. For the sake of 32*4882a593Smuzhiyunconvenience, the logical block size will be referred to as 33*4882a593Smuzhiyun``$block_size`` throughout the rest of the document. 34*4882a593Smuzhiyun 35*4882a593SmuzhiyunWhen referenced in ``preformatted text`` blocks, ``sb`` refers to fields 36*4882a593Smuzhiyunin the super block, and ``inode`` refers to fields in an inode table 37*4882a593Smuzhiyunentry. 38*4882a593Smuzhiyun 39*4882a593SmuzhiyunOther References 40*4882a593Smuzhiyun---------------- 41*4882a593Smuzhiyun 42*4882a593SmuzhiyunAlso see https://www.nongnu.org/ext2-doc/ for quite a collection of 43*4882a593Smuzhiyuninformation about ext2/3. Here's another old reference: 44*4882a593Smuzhiyunhttp://wiki.osdev.org/Ext2 45