FWIW LGTM.

Maybe at this point we could consider creating a struct:

type appendOpts struct {
    chunkDiskMapper *chunks.ChunkDiskMapper
    chunkRange      int64
    samplesPerChunk int
}

Because those 3 are passed down through a lot of methods and it would make them more readable IMO:

-func (s *memSeries) append(t int64, v float64, appendID uint64, chunkDiskMapper *chunks.ChunkDiskMapper, chunkRange int64, samplesPerChunk int) (sampleInOrder, chunkCreated bool)
+func (s *memSeries) append(t int64, v float64, appendID uint64, opts appendOpts) (sampleInOrder, chunkCreated bool)

Edit: maybe chunkOpts:

type chunkOpts struct {
    diskMapper *chunks.ChunkDiskMapper
    range      int64
    samples    int
}

Because those 3 are passed down through a lot of methods and it would make them more readable IMO:

-func (s *memSeries) append(t int64, v float64, appendID uint64, chunkDiskMapper *chunks.ChunkDiskMapper, chunkRange int64, samplesPerChunk int) (sampleInOrder, chunkCreated bool)
+func (s *memSeries) append(t int64, v float64, appendID uint64, chunk chunkOpts) (sampleInOrder, chunkCreated bool)

LGTM nice catch @colega and work @leizor

I'm going to approve and merge this one for now because it brings the optimization now() and also because I leave for a short vacation tomorrow. That said, I also like the idea of grouping the arguments in a struct to only pass that struct down. So lets see if we can have this done in a separate PR :)