Method gc

tsdb/head.go:1760–1798

gc removes data before the minimum timestamp from the head. It returns * The actual min times of the chunks present in the Head. * The min OOO time seen during the GC. * Min mmap file number seen in the series (in-order and out-of-order) after gc'ing the series.

()

// * The min OOO time seen during the GC.
// * Min mmap file number seen in the series (in-order and out-of-order) after gc'ing the series.
func (h *Head) gc() (actualInOrderMint, minOOOTime int64, minMmapFile int) {
	// Only data strictly lower than this timestamp must be deleted.
	mint := h.MinTime()
	// Only ooo m-map chunks strictly lower than or equal to this ref
	// must be deleted.
	minOOOMmapRef := chunks.ChunkDiskMapperRef(h.minOOOMmapRef.Load())

	// Drop old chunks and remember series IDs and hashes if they can be
	// deleted entirely.
	deleted, affected, chunksRemoved, staleSeriesDeleted, actualInOrderMint, minOOOTime, minMmapFile := h.series.gc(mint, minOOOMmapRef)
	seriesRemoved := len(deleted)

	h.metrics.seriesRemoved.Add(float64(seriesRemoved))
	h.metrics.chunksRemoved.Add(float64(chunksRemoved))
	h.metrics.chunks.Sub(float64(chunksRemoved))
	h.numSeries.Sub(uint64(seriesRemoved))
	h.numStaleSeries.Sub(uint64(staleSeriesDeleted))

	// Remove deleted series IDs from the postings lists.
	h.postings.Delete(deleted, affected)

	// Remove tombstones referring to the deleted series.
	h.tombstones.DeleteTombstones(deleted)
	h.tombstones.TruncateBefore(mint)

	if h.wal != nil {
		h.walExpiriesMtx.Lock()
		// Samples for deleted series are likely still in the WAL, so flag that the deleted series records should be kept during
		// WAL checkpointing while the WAL contains data through actualInOrderMint.
		// If we didn't keep these series records then on start up when we replay the WAL, or any other code that reads the WAL,
		// wouldn't be able to use those samples since we would have no labels for that ref ID.
		for ref := range deleted {
			h.walExpiries[chunks.HeadSeriesRef(ref)] = actualInOrderMint
		}
		h.walExpiriesMtx.Unlock()
	}

	return actualInOrderMint, minOOOTime, minMmapFile
}

// Tombstones returns a new reader over the head's tombstones.
func (h *Head) Tombstones() (tombstones.Reader, error) {