lib/{storage,mergeset}: do not allow setting dataFlushInterval to values smaller than pending{Items,Rows}FlushInterval

Pending rows and items unconditionally remain in memory for up to pending{Items,Rows}FlushInterval,
so there is no any sense in setting dataFlushInterval (the interval for guaranteed flush of in-memory data to disk)
to values smaller than pending{Items,Rows}FlushInterval, since this doesn't affect the interval
for flushing pending rows and items from memory to disk.

This is a follow-up for 4c80b17027

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6221

lib/mergeset/table.go

@@ -47,8 +47,7 @@ const maxPartSize = 400e9
 // The interval for flushing buffered data to parts, so it becomes visible to search.
 const pendingItemsFlushInterval = time.Second
 
-// The interval for guaranteed flush of recently ingested data from memory to on-disk parts,
+// The interval for guaranteed flush of recently ingested data from memory to on-disk parts so they survive process crash.
-// so they survive process crash.
 var dataFlushInterval = 5 * time.Second
 
 // SetDataFlushInterval sets the interval for guaranteed flush of recently ingested data from memory to disk.
@@ -57,9 +56,13 @@ var dataFlushInterval = 5 * time.Second
 //
 // This function must be called before initializing the indexdb.
 func SetDataFlushInterval(d time.Duration) {
-	if d >= time.Second {
+	if d < pendingItemsFlushInterval {
-		dataFlushInterval = d
+		// There is no sense in setting dataFlushInterval to values smaller than pendingItemsFlushInterval,
+		// since pending rows unconditionally remain in memory for up to pendingItemsFlushInterval.
+		d = pendingItemsFlushInterval
 	}
+
+	dataFlushInterval = d
 }
 
 // maxItemsPerCachedPart is the maximum items per created part by the merge,

lib/storage/partition.go

@@ -48,8 +48,7 @@ var rawRowsShardsPerPartition = cgroup.AvailableCPUs()
 // The interval for flushing buffered rows into parts, so they become visible to search.
 const pendingRowsFlushInterval = 2 * time.Second
 
-// The interval for guaranteed flush of recently ingested data from memory to on-disk parts,
+// The interval for guaranteed flush of recently ingested data from memory to on-disk parts, so they survive process crash.
-// so they survive process crash.
 var dataFlushInterval = 5 * time.Second
 
 // SetDataFlushInterval sets the interval for guaranteed flush of recently ingested data from memory to disk.
@@ -58,11 +57,15 @@ var dataFlushInterval = 5 * time.Second
 //
 // This function must be called before initializing the storage.
 func SetDataFlushInterval(d time.Duration) {
-	if d >= time.Second {
+	if d < pendingRowsFlushInterval {
+		// There is no sense in setting dataFlushInterval to values smaller than pendingRowsFlushInterval,
+		// since pending rows unconditionally remain in memory for up to pendingRowsFlushInterval.
+		d = pendingRowsFlushInterval
+	}
+
 	dataFlushInterval = d
 	mergeset.SetDataFlushInterval(d)
 }
-}
 
 // The maximum number of rawRow items in rawRowsShard.
 //