Struct PageHeader 

pub struct PageHeader {
    pub shard: u32,
    pub page_addr: u64,
    pub used: u32,
    pub dead_bytes: u32,
    pub epoch: u32,
}

The header at the front of a log page.

Fields

shard: u32

Which shard owns this page. Two shards never touch each other’s pages, so this is a check rather than a lookup, and it is the check that catches a segment allocated to two shards at once.

page_addr: u64

The log address of byte zero of the payload, meaning of byte 32 of the page. Log addresses are logically infinite, so this is what maps a physical segment back to a position in the log.

used: u32

Bytes of the payload that have been written, header excluded.

dead_bytes: u32

Of those, how many belong to records the index no longer points at. Compaction reads this and nothing else to decide what to do (06 section 5).

epoch: u32

The epoch the page was last written in.

Implementations

impl PageHeader

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

Writes the header and its checksum into the first 32 bytes of page.

Panics

If page is shorter than PAGE_HEADER_LEN.

pub fn decode(page: &[u8]) -> Result<PageHeader>

Reads the header back, checking the magic and the checksum.

pub fn dead_fraction(&self) -> f64

The fraction of this page that is dead, between 0 and 1.

An empty page is 0 rather than a division by zero, because an empty page is not worth compacting and that is the only question this answers.

pub fn wants_compaction(&self) -> bool

Whether this page is worth compacting, at 06 section 5’s threshold.

Trait Implementations

impl Clone for PageHeader

fn clone(&self) -> PageHeader

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Copy for PageHeader

impl Debug for PageHeader

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

Formats the value using the given formatter.

impl Default for PageHeader

fn default() -> PageHeader

Returns the “default value” for a type.

impl Eq for PageHeader

impl PartialEq for PageHeader

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

Equality operator ==.

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

Inequality operator !=.

impl StructuralPartialEq for PageHeader