tsdb: commit data periodically in CreateBlock (#10788)

To avoid building up data in memory, commit and make a new appender
periodically.

The number `commitAfter = 10000` was chosen arbitrarily; testing with
10x more or less gives slightly worse results.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/tsdbblockutil.go

@@ -44,6 +44,8 @@ func CreateBlock(series []storage.Series, dir string, chunkRange int64, logger l
 		}
 	}()
 
+	sampleCount := 0
+	const commitAfter = 10000
 	ctx := context.Background()
 	app := w.Appender(ctx)
 
@@ -57,10 +59,19 @@ func CreateBlock(series []storage.Series, dir string, chunkRange int64, logger l
 			if err != nil {
 				return "", err
 			}
+			sampleCount++
 		}
 		if it.Err() != nil {
 			return "", it.Err()
 		}
+		// Commit and make a new appender periodically, to avoid building up data in memory.
+		if sampleCount > commitAfter {
+			if err = app.Commit(); err != nil {
+				return "", err
+			}
+			app = w.Appender(ctx)
+			sampleCount = 0
+		}
 	}
 
 	if err = app.Commit(); err != nil {