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few more comments to explain the WAL workflow (#430)

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More comments for the WAL package.

Signed-off-by: Krasi Georgiev <kgeorgie@redhat.com>
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Krasi Georgiev 2018-11-08 10:27:16 +02:00 committed by GitHub
parent 3a08a71d86
commit a9470dd8d5
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1 changed files with 13 additions and 6 deletions
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19
wal/wal.go
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@ -46,6 +46,10 @@ const (
// before. // before.
var castagnoliTable = crc32.MakeTable(crc32.Castagnoli) var castagnoliTable = crc32.MakeTable(crc32.Castagnoli)
// page is an in memory buffer used to batch disk writes.
// Records bigger than the page size are split and flushed separately.
// A flush is triggered when a single records doesn't fit the page size or
// when the next record can't fit in the remaining free page space.
type page struct { type page struct {
alloc int alloc int
flushed int flushed int
@ -397,7 +401,7 @@ func (w *WAL) flushPage(clear bool) error {
// No more data will fit into the page. Enqueue and clear it. // No more data will fit into the page. Enqueue and clear it.
if clear { if clear {
p.alloc = pageSize // write till end of page p.alloc = pageSize // Write till end of page.
w.pageCompletions.Inc() w.pageCompletions.Inc()
} }
n, err := w.segment.Write(p.buf[p.flushed:p.alloc]) n, err := w.segment.Write(p.buf[p.flushed:p.alloc])
@ -465,13 +469,14 @@ func (w *WAL) Log(recs ...[]byte) error {
} }
// log writes rec to the log and forces a flush of the current page if its // log writes rec to the log and forces a flush of the current page if its
// the final record of a batch. // the final record of a batch, the record is bigger than the page size or
// the current page is full.
func (w *WAL) log(rec []byte, final bool) error { func (w *WAL) log(rec []byte, final bool) error {
// If the record is too big to fit within pages in the current // If the record is too big to fit within the active page in the current
// segment, terminate the active segment and advance to the next one. // segment, terminate the active segment and advance to the next one.
// This ensures that records do not cross segment boundaries. // This ensures that records do not cross segment boundaries.
left := w.page.remaining() - recordHeaderSize // Active pages. left := w.page.remaining() - recordHeaderSize // Free space in the active page.
left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages. left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages in the active segment.
if len(rec) > left { if len(rec) > left {
if err := w.nextSegment(); err != nil { if err := w.nextSegment(); err != nil {
@ -511,7 +516,9 @@ func (w *WAL) log(rec []byte, final bool) error {
copy(buf[recordHeaderSize:], part) copy(buf[recordHeaderSize:], part)
p.alloc += len(part) + recordHeaderSize p.alloc += len(part) + recordHeaderSize
// If we wrote a full record, we can fit more records of the batch // By definition when a record is split it means its size is bigger than
// the page boundary so the current page would be full and needs to be flushed.
// On contrary if we wrote a full record, we can fit more records of the batch
// into the page before flushing it. // into the page before flushing it.
if final || typ != recFull || w.page.full() { if final || typ != recFull || w.page.full() {
if err := w.flushPage(false); err != nil { if err := w.flushPage(false); err != nil {