fix: permit small TSI series caches to grow

tsdb/index/tsi1/cache.go

@@ -582,7 +582,21 @@ func decideResize(hits, misses, lastHits, lastMisses, capacity, maxCapacity, min
 	if gets == 0 {
 		return capacity, gets, rate, false
 	}
-	if gets < minSamples {
+	// Clamp the sample floor to capacity: the grow path fires once per
+	// `capacity` forced evictions, and each forced eviction follows a Get miss,
+	// so a thrashing cache (low hit rate — precisely the workload growth exists
+	// to fix) produces only ~capacity Gets per window. Demanding more than
+	// capacity samples would therefore gate growth off entirely for any cache
+	// configured smaller than minSamples, which cannot physically clear the
+	// floor. Clamping lets a sub-minSamples cache grow on the evidence one full
+	// window provides, while leaving minSamples in force for caches large enough
+	// to supply it. minSamples may be 0, and capacity is always >= 1 here, so the
+	// clamp never lowers the floor below what the gets == 0 check already gated.
+	effMinSamples := minSamples
+	if effMinSamples > capacity {
+		effMinSamples = capacity
+	}
+	if gets < effMinSamples {
 		return capacity, gets, rate, false
 	}
 

tsdb/index/tsi1/cache_test.go

@@ -443,6 +443,26 @@ func TestDecideResize_PolicyTable(t *testing.T) {
 			minSamples: 100, target: target,
 			wantNewCap: initialCap * 2, wantGrow: true,
 		},
+		{
+			// Regression: a cache configured smaller than minSamples still grows.
+			// Its window is only ~capacity Gets long under thrash, so the raw
+			// minSamples floor could never be cleared; the floor is clamped to
+			// capacity. Here capacity 50 < minSamples 100, gets == capacity, rate
+			// 0 (< target), so growth must fire despite gets < minSamples.
+			name: "sub-minSamples cache grows on a full-window thrash",
+			hits: 0, misses: 50, capacity: 50, maxCap: maxCap,
+			minSamples: 100, target: target,
+			wantNewCap: 100, wantGrow: true,
+		},
+		{
+			// The clamp lowers the floor to capacity, not to zero: a sub-minSamples
+			// cache with fewer than capacity Gets still has too little evidence and
+			// must not grow. capacity 50, gets 30 < 50 → gated.
+			name: "sub-minSamples cache below its clamped floor does not grow",
+			hits: 0, misses: 30, capacity: 50, maxCap: maxCap,
+			minSamples: 100, target: target,
+			wantNewCap: 50, wantGrow: false,
+		},
 	}
 
 	for _, tt := range tests {