s3: pin small ranged GETs to range reads

A ranged GET whose first read lands within SeqTolerance of offset 0 is
judged sequential immediately, and even a far-starting range could flip
back mid-request; either way readChunkSliceAt downloads each covered
chunk in full, multiplying disk reads for small ranged reads (measured
~7x). Pin random mode for ranged requests no larger than SeqTolerance so
all of the request's buffer reads stay range fetches. Larger ranges keep
the dynamic pattern, where whole-chunk fetches amortize.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/filer/reader_at.go

@@ -180,6 +180,12 @@ func (c *ChunkReadAt) ReleaseStream() {
 	c.readerCache.releaseStream(&c.stream)
 }
 
+// PinRandomMode pins this reader to range fetches; used by callers that
+// already know the request is a partial read, e.g. a small ranged GET.
+func (c *ChunkReadAt) PinRandomMode() {
+	c.readerPattern.PinRandomMode()
+}
+
 func (c *ChunkReadAt) ReadAt(p []byte, offset int64) (n int, err error) {
 
 	c.readerPattern.MonitorReadAt(offset, len(p))

weed/filer/reader_pattern.go

@@ -7,6 +7,7 @@ import (
 type ReaderPattern struct {
 	isSequentialCounter int64
 	readFrontier        int64 // highest (offset+size) observed across reads
+	pinnedRandom        int64 // set when the caller already knows the reads are a partial range
 }
 
 const ModeChangeLimit = 3
@@ -65,5 +66,12 @@ func (rp *ReaderPattern) MonitorReadAt(offset int64, size int) {
 }
 
 func (rp *ReaderPattern) IsRandomMode() bool {
-	return atomic.LoadInt64(&rp.isSequentialCounter) < 0
+	return atomic.LoadInt64(&rp.pinnedRandom) != 0 || atomic.LoadInt64(&rp.isSequentialCounter) < 0
+}
+
+// PinRandomMode pins the pattern to random for readers that already know
+// they serve a partial range, e.g. a small ranged GET: contiguous buffer
+// reads within the request must not escalate into whole-chunk fetches.
+func (rp *ReaderPattern) PinRandomMode() {
+	atomic.StoreInt64(&rp.pinnedRandom, 1)
 }

weed/filer/reader_pattern_test.go

@@ -123,3 +123,16 @@ func TestReaderPatternRangedReadStaysRandom(t *testing.T) {
 	}
 }
 
+
+// A reader pinned random (a known small ranged request) must stay random
+// for every buffer of the request, near or far.
+func TestReaderPatternPinnedRandom(t *testing.T) {
+	rp := NewReaderPattern()
+	rp.PinRandomMode()
+	for i := int64(0); i < 10; i++ {
+		rp.MonitorReadAt(i*256*1024, 256*1024) // contiguous from the start
+		if !rp.IsRandomMode() {
+			t.Fatalf("pinned reader flipped to sequential at read %d", i+1)
+		}
+	}
+}

weed/s3api/s3api_object_handlers.go

@@ -1104,6 +1104,12 @@ func (s3a *S3ApiServer) streamFromVolumeServers(w http.ResponseWriter, r *http.R
 	chunkViews := filer.ViewFromVisibleIntervals(visibleIntervals, offset, size)
 	reader := filer.NewChunkReaderAtFromClient(ctx, s3a.readerCache, chunkViews, totalSize, filer.DefaultPrefetchCount)
 	defer reader.ReleaseStream()
+	// A small ranged request reads back less than a whole chunk; pin random
+	// mode so every buffer fetch stays a range read instead of downloading
+	// each covered chunk in full.
+	if isRangeRequest && size <= filer.SeqTolerance {
+		reader.PinRandomMode()
+	}
 	streamPrepTime = time.Since(tStreamPrep)
 
 	// A cached chunk whose volume server is down, or whose needle was evicted and