Struct Superblock 

pub struct Superblock {

    pub format_version: u32,
    pub min_reader_version: u32,
    pub page_size: u32,
    pub shard_count: u32,
    pub seq: u64,
    pub file_size: u64,
    pub file_uuid: [u8; 16],
    pub created_unix_ms: u64,
    pub checkpoint_unix_ms: u64,
    pub db_count: u32,
    pub flags: u32,
    pub replid_hi: u64,
    pub replid_lo: u64,
    pub replid_ext: u32,
    pub shard_table_off: u32,
    pub shard_table_len: u16,
    pub catalog_addr: u64,
    pub free_list_addr: u64,
    pub archival_root: u64,
}

The header half of a superblock slot, decoded.

The shard table and the checkpoint entries are not in here on purpose. They are variable length, they live further into the same 16 KiB, and a caller that only wants to know the page size should not pay to decode 16384 slot assignments to find out.

Fields

format_version: u32

The format this slot was written by.

min_reader_version: u32

The lowest reader version that may read it.

page_size: u32

Segment size, fixed at creation.

shard_count: u32

Shard count, fixed at creation.

seq: u64

Monotonic. The higher of the two valid slots wins.

file_size: u64

Segments times page_size plus DATA_START.

file_uuid: [u8; 16]

Identifies this database across copies of the file.

created_unix_ms: u64

When the file was created.

checkpoint_unix_ms: u64

When this checkpoint was taken.

db_count: u32

Logical databases, default 16.

flags: u32

See superblock_flags.

replid_hi: u64

The replication id, high 64 bits of the 40 hex characters.

replid_lo: u64

The replication id, next 64 bits.

replid_ext: u32

The replication id, last 32 bits.

shard_table_off: u32

Offset within the slot of the run length encoded shard table.

shard_table_len: u16

Length in bytes of that encoding.

catalog_addr: u64

Address of the collection catalogue, or 0 if there are no collections.

free_list_addr: u64

Address of the free segment list.

archival_root: u64

Address of the archival root, or 0.

Implementations

impl Superblock

pub fn encode(&self, slot: &mut [u8])

Writes the header fields into the first 160 bytes of slot.

Leaves the checksum alone, because the bytes it covers have not all been written yet. Call seal once the table and the entries are in.

Panics

If slot is not exactly SUPERBLOCK_LEN bytes. That is a programmer error rather than a corrupt file, so it is not an Err.

pub fn decode(slot: &[u8]) -> Result<Superblock>

Reads the header fields, checking everything that makes the file ours.

In order: the length, the magic, the checksum, then the version. The checksum comes before the version check so that a corrupt slot is reported as corrupt rather than as a version from the future, which is what happens if a flipped bit lands in the version field.

pub const fn checkpoint_off(&self) -> usize

Where the checkpoint entries begin, which is right after the table.

pub fn checkpoints_fit(&self) -> bool

Whether every checkpoint entry fits inside the slot.

A file whose shard count and table length do not leave room for the entries is corrupt in a way that would otherwise show up as a checkpoint full of zeroes, which reads as a valid empty database.

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.

impl Default for Superblock

fn default() -> Self

Returns the “default value” for a type.

impl Eq for Superblock

impl PartialEq for Superblock

fn eq(&self, other: &Superblock) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for Superblock