pub enum SegmentAllocation {
OnWrite,
Preallocate,
}Expand description
How a new segment’s .log file gets its disk blocks: Kafka’s
preallocate.
Variants§
OnWrite
preallocate=false, Kafka’s default: the file allocates blocks as
appends grow it.
Preallocate
preallocate=true: reserve a segment’s disk blocks ahead of its
appends, a quarter of segment_size at a time (at least 1 MiB), so
appends do not allocate as they grow it and a broker with many
partitions does not commit a whole segment per partition up front,
and write its batches through O_DIRECT. A truncate gives the
reservation up, and the segment takes it again.
The O_DIRECT writes bypass the page cache. An append returns once
its write is queued, and shared write threads put the queued writes on
the disk, several at a time, so a produce request is acknowledged
before its bytes reach the disk, as it is through the page cache. The
newest LogConfig::tail_cache_size of the segment, and every byte
that is not on the disk yet, stays in memory to serve reads. A write
that fails fails every later append and sync of the segment. A direct
write covers whole blocks, so the active segment’s file runs up to a
block past its last batch until it is sealed. Where
the kernel does not report the alignment O_DIRECT needs (before Linux
6.1, or on tmpfs), the segment writes through the page cache.
Kafka sets the file’s length to segment.bytes and trims it back when
the segment closes. krabka reserves the blocks without changing the
length (fallocate(FALLOC_FL_KEEP_SIZE)), so a segment’s file is
never longer than the batches in it, and gives back what is left of
the reservation when the segment is sealed. The bytes on disk are the
same either way. Off Linux, or on a filesystem that cannot reserve
blocks, a segment allocates as it is written.
Trait Implementations§
Source§impl Clone for SegmentAllocation
impl Clone for SegmentAllocation
Source§fn clone(&self) -> SegmentAllocation
fn clone(&self) -> SegmentAllocation
1.0.0 (const: unstable) · Source§fn clone_from(&mut self, source: &Self)
fn clone_from(&mut self, source: &Self)
source. Read moreimpl Copy for SegmentAllocation
Source§impl Debug for SegmentAllocation
impl Debug for SegmentAllocation
Source§impl Default for SegmentAllocation
impl Default for SegmentAllocation
Source§fn default() -> SegmentAllocation
fn default() -> SegmentAllocation
impl Eq for SegmentAllocation
Source§impl PartialEq for SegmentAllocation
impl PartialEq for SegmentAllocation
impl StructuralPartialEq for SegmentAllocation
Auto Trait Implementations§
impl Freeze for SegmentAllocation
impl RefUnwindSafe for SegmentAllocation
impl Send for SegmentAllocation
impl Sync for SegmentAllocation
impl Unpin for SegmentAllocation
impl UnsafeUnpin for SegmentAllocation
impl UnwindSafe for SegmentAllocation
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