Struct Superblock 

pub struct Superblock {

    pub inodes_count: u32,
    pub blocks_count: u64,
    pub free_blocks_count: u64,
    pub free_inodes_count: u32,
    pub r_blocks_count: u64,
    pub first_data_block: u32,
    pub log_block_size: u32,
    pub blocks_per_group: u32,
    pub inodes_per_group: u32,
    pub magic: u16,
    pub state: u16,
    pub errors_behavior: u16,
    pub minor_rev_level: u16,
    pub rev_level: u32,
    pub inode_size: u16,
    pub first_inode: u32,
    pub feature_compat: u32,
    pub feature_incompat: u32,
    pub feature_ro_compat: u32,
    pub uuid: [u8; 16],
    pub volume_name: String,
    pub last_mounted: String,
    pub desc_size: u16,
    pub reserved_gdt_blocks: u16,
    pub backup_bgs: [u32; 2],
    pub hash_seed: [u32; 4],
    pub default_hash_version: u8,
    pub checksum_seed: u32,
    pub journal_inode: u32,
    pub last_orphan: u32,
    pub mtime: u32,
    pub wtime: u32,
    pub mnt_count: u16,
    pub max_mnt_count: u16,
    pub lastcheck: u32,
    pub checkinterval: u32,
    pub creator_os: u32,
    pub def_resuid: u16,
    pub def_resgid: u16,
    pub raw: Vec<u8>,
}

Parsed in-memory representation of the ext4 superblock. Field names mirror the kernel’s struct ext4_super_block (s_ prefix dropped).

Fields

inodes_count: u32

blocks_count: u64

free_blocks_count: u64

free_inodes_count: u32

r_blocks_count: u64

s_r_blocks_count (lo at 0x08, hi at 0x154 — 64-bit on INCOMPAT_64BIT volumes). Blocks reserved for the superuser — explains the gap between free_blocks and what df reports as available to a normal user.

first_data_block: u32

log_block_size: u32

blocks_per_group: u32

inodes_per_group: u32

magic: u16

state: u16

s_state (0x3A). EXT4_VALID_FS = cleanly unmounted. Any other value means the FS was mounted and not cleanly unmounted (dirty) or that the kernel marked the FS as having errors.

errors_behavior: u16

s_errors (0x3C). Kernel error policy: 1=continue, 2=remount-ro, 3=panic. Informational from a Swift host’s POV but useful in diagnostics output.

minor_rev_level: u16

s_minor_rev_level (0x3E). Bumped by filesystem admin tools for minor format tweaks within a major rev_level.

rev_level: u32

inode_size: u16

first_inode: u32

s_first_ino (0x54, dynamic-rev only). First non-reserved inode number; defaults to 11 on rev_level=1+ filesystems.

feature_compat: u32

feature_incompat: u32

feature_ro_compat: u32

uuid: [u8; 16]

volume_name: String

last_mounted: String

s_last_mounted (0x88, 64 bytes). Last directory the FS was mounted at — handy for diagnostics (“when was this disk last in another machine?”).

desc_size: u16

reserved_gdt_blocks: u16

s_reserved_gdt_blocks — blocks held back after the group descriptor table so the filesystem can be grown online.

They sit between the GDT and the block bitmap in every group that carries a superblock backup, and they are not free space: anything that rebuilds a block bitmap has to mark them used or the next allocation writes over the filesystem’s own room to expand.

backup_bgs: [u32; 2]

s_backup_bgs — the only two groups carrying backups when SPARSE_SUPER2 is set. Meaningless without that feature.

hash_seed: [u32; 4]

default_hash_version: u8

checksum_seed: u32

journal_inode: u32

last_orphan: u32

s_last_orphan (0xE8). Head of the orphan-inode list — inodes whose link count reached zero while still open. The kernel inserts unlink-while-open targets here so that, on the next mount, recovery can reclaim them. Each inode’s i_dtime field is overloaded to point at the next orphan in the chain; the chain terminates with a zero dtime.

mtime: u32

s_mtime (0x2C). Timestamp the FS was last mounted.

wtime: u32

s_wtime (0x30). Timestamp the FS was last written to.

mnt_count: u16

s_mnt_count (0x34). Mounts since last fsck.

max_mnt_count: u16

s_max_mnt_count (0x36). Forced fsck after this many mounts; 0 disables.

lastcheck: u32

s_lastcheck (0x40). Timestamp of the last fsck pass.

checkinterval: u32

s_checkinterval (0x44). Seconds between forced fscks; 0 disables time-based forced fsck.

creator_os: u32

s_creator_os (0x48). 0=Linux, 1=Hurd, 2=Masix, 3=FreeBSD, 4=Lites.

def_resuid: u16

s_def_resuid (0x50). UID with access to reserved blocks.

def_resgid: u16

s_def_resgid (0x52). GID with access to reserved blocks.

raw: Vec<u8>

Implementations

impl Superblock

pub fn read<D: BlockDevice + ?Sized>(dev: &D) -> Result<Self>

Read and parse the superblock from a block device.

pub fn parse(raw: Vec<u8>) -> Result<Self>

pub fn is_clean(&self) -> bool

Whether the filesystem was cleanly unmounted. false here means the FS was not cleanly unmounted and a journal replay (or fsck) is required before writes are safe. Read-only consumers can still mount a dirty FS; callers that intend to write should surface this to the user and either run fsck or refuse to mount read-write.

pub fn group_has_super(&self, g: u64) -> bool

Whether block group g carries a superblock and group-descriptor backup.

Three layouts, and the filesystem’s own flags decide which:

• SPARSE_SUPER2: group 0, and the two groups named by s_backup_bgs. Nothing else.
• SPARSE_SUPER clear: every group. This is the old ext2 layout, and the one that costs most to get wrong — assuming the sparse rule leaves real backups looking like free space.
• otherwise: the classic sparse rule — groups 0 and 1, and every power of 3, 5 or 7.

Answering unconditionally with the classic rule is wrong in both of the first two cases, and wrong in the direction that matters: on a filesystem without SPARSE_SUPER it reports “no backup here” for groups that have one, so a rebuilt bitmap offers the backup superblock and its descriptor table as free blocks.

pub fn block_size(&self) -> u32

Block size in bytes: 1024 << log_block_size.

pub fn block_group_count(&self) -> u64

Number of block groups.

pub fn is_64bit(&self) -> bool

Whether the 64BIT incompat feature is enabled.

Trait Implementations

impl Clone for Superblock

fn clone(&self) -> Superblock

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Superblock

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.