Read-only compatible features
Read-only compatible features are required to be supported in order to alter data, but not needed to correctly read data. Obviously, if your operating system does not support one or more of these features, you should not run fsck against the filesystem. Read-only compatible features are summarized in Table 7.4.
Table 7.4. Read-only Compatible Features.
| Bit | Name | Description |
|---|---|---|
| 0x1 | Sparse Super | Sparse superblocks (only in BG 0 or power of 3, 5, or 7) |
| 0x2 | Large File | File(s) larger than 2GB exist on the filesystem |
| 0x4 | Btree Dir | Btrees are used in directories (not common) |
| 0x8 | Huge File | File sizes are represented in logical blocks, not sectors |
| 0x10 | Gdt Csum | Group descriptor tables have checksums |
| 0x20 | Dir Nlink | Subdirectories are not limited to 32k entries |
| 0x40 | Extra Isize | Indicates large inodes are present on the filesystem |
| 0x80 | Has Snapshot | Filesystem has a snapshot |
| 0x100 | Quota | Disk quotas are being used on the filesystem |
| 0x200 | BigAlloc | File extents are tracked in multi-block clusters |
| 0x400 | Metadata Csum | Checksums are used on metadata items |
| 0x800 | Replica | The filesystem supports replicas |
| 0x1000 | ReadOnly | Should only be mounted as read-only |
When the Sparse Superblock feature is in use, the superblock is only found in block group 0 or in block groups that are a power of 3, 5, or 7. If there is at least one file greater than 2 gigabytes on the filesystem, the Large File feature flag will be set. The Huge File feature flag indicates at least one huge file is present. Huge files have their sizes specified in clusters (the size of which is stored in the superblock) instead of data blocks.
The Btree Directory feature allows large directories to be stored in binary-trees. This is not common. Another feature related to large directories is Directory Nlink. When the Directory Nlink flag is set, subdirectories are not limited to 32,768 entries as in previous versions of ext3.
The GDT Checksum feature allows checksums to be stored in the group descriptor tables. The Extra Isize feature flag indicates that large inodes are present on the filesystem. If a filesystem snapshot is present, the Has Snapshot flag will be set. Disk quota use is indicated by the Quota feature flag.
The Big Alloc feature is useful if most of the files on the filesystem are huge. When this is in use, file extents (discussed later in this chapter) and other filesystem structures use multi-block clusters for units. The Metadata Checksum flag indicates that checksums are stored for the metadata items in inodes, etc. If a filesystem supports replicas, the Replica flag will be set. A filesystem with the Read Only flag set should only be mounted as readonly. This flag can be set to prevent others from modifying the filesystem’s contents.
Only two features in the read-only compatible set affect the filesystem layout: Sparse Super Blocks and Extra Isize. Sparse super blocks affect which block groups have superblock backups. Like 64-bit mode, the Extra Isize feature affects the layout indirectly by changing the inode size.