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

More comments for the WAL package.

Signed-off-by: Krasi Georgiev <kgeorgie@redhat.com>
This commit is contained in:
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.
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 {
alloc 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.
if clear {
p.alloc = pageSize // write till end of page
p.alloc = pageSize // Write till end of page.
w.pageCompletions.Inc()
}
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
// 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 {
// 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.
// This ensures that records do not cross segment boundaries.
left := w.page.remaining() - recordHeaderSize // Active pages.
left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages.
left := w.page.remaining() - recordHeaderSize // Free space in the active page.
left += (pageSize - recordHeaderSize) * (w.pagesPerSegment() - w.donePages - 1) // Free pages in the active segment.
if len(rec) > left {
if err := w.nextSegment(); err != nil {
@ -511,7 +516,9 @@ func (w *WAL) log(rec []byte, final bool) error {
copy(buf[recordHeaderSize:], part)
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.
if final || typ != recFull || w.page.full() {
if err := w.flushPage(false); err != nil {