Simplify Cache.Snapshot

This simplifies the cache.Snapshot func to swap the hot cache to
the snapshot cache instead of copy and appending entries.  This
reduces the amount of time the cache is write locked which should
reduce cache contention for the read only code paths.

tsdb/engine/tsm1/cache.go

@@ -345,27 +345,19 @@ func (c *Cache) Snapshot() (*Cache, error) {
 		}
 	}
 
-	// Append the current cache values to the snapshot. Because we're accessing
-	// the Cache we need to call f on each partition in serial.
-	if err := c.store.applySerial(func(k string, e *entry) error {
-		e.mu.RLock()
-		defer e.mu.RUnlock()
-		snapshotEntry, ok := c.snapshot.store.entry(k)
-		if ok {
-			if err := snapshotEntry.add(e.values); err != nil {
-				return err
-			}
-		} else {
-			c.snapshot.store.add(k, e)
-			snapshotEntry = e
-		}
-		atomic.AddUint64(&c.snapshotSize, uint64(Values(e.values).Size()))
-		return nil
-	}); err != nil {
-		return nil, err
+	// Did a prior snapshot exist that failed?  If so, return the existing
+	// snapshot to retry.
+	if c.snapshot.Size() > 0 {
+		return c.snapshot, nil
 	}
 
-	snapshotSize := c.Size() // record the number of bytes written into a snapshot
+	c.snapshot.store, c.store = c.store, c.snapshot.store
+	snapshotSize := c.Size()
+
+	// Save the size of the snapshot on the snapshot cache
+	c.snapshot.size = snapshotSize
+	// Save the size of the snapshot on the live cache
+	c.snapshotSize = snapshotSize
 
 	// Reset the cache's store.
 	c.store.reset()

tsdb/engine/tsm1/cache_test.go

@@ -425,7 +425,7 @@ func TestCache_CacheWriteMemoryExceeded(t *testing.T) {
 		t.Fatalf("cache keys incorrect after writes, exp %v, got %v", exp, keys)
 	}
 	if err := c.Write("bar", Values{v1}); err == nil || !strings.Contains(err.Error(), "cache-max-memory-size") {
-		t.Fatalf("wrong error writing key bar to cache")
+		t.Fatalf("wrong error writing key bar to cache: %v", err)
 	}
 
 	// Grab snapshot, write should still fail since we're still using the memory.
@@ -434,7 +434,7 @@ func TestCache_CacheWriteMemoryExceeded(t *testing.T) {
 		t.Fatalf("failed to snapshot cache: %v", err)
 	}
 	if err := c.Write("bar", Values{v1}); err == nil || !strings.Contains(err.Error(), "cache-max-memory-size") {
-		t.Fatalf("wrong error writing key bar to cache")
+		t.Fatalf("wrong error writing key bar to cache: %v", err)
 	}
 
 	// Clear the snapshot and the write should now succeed.