add some context

CHANGELOG.md

@@ -10,7 +10,7 @@ If you were at `firehose-core` version `1.0.0` and are bumping to `1.1.0`, you s
 
 ## Unreleased
 
-* Added `--merger-delete-threads` to customize the number of threads the merger will use to delete files.
+* Added `--merger-delete-threads` to customize the number of threads the merger will use to delete files. It's recommended to increase this when using Ceph as S3 storage provider to 25 or higher (due to performance issues with deletes the merger might otherwise not be able to delete one-block files fast enough).
 
 ## v1.2.5
 

cmd/apps/merger.go

@@ -23,7 +23,7 @@ func RegisterMergerApp(rootLog *zap.Logger) {
 			cmd.Flags().Uint64("merger-stop-block", 0, "If non-zero, merger will trigger shutdown when blocks have been merged up to this block")
 			cmd.Flags().Duration("merger-time-between-store-lookups", 1*time.Second, "Delay between source store polling (should be higher for remote storage)")
 			cmd.Flags().Duration("merger-time-between-store-pruning", time.Minute, "Delay between source store pruning loops")
-			cmd.Flags().Int("merger-delete-threads", 8, "Number of threads for deleting files in parallel.")
+			cmd.Flags().Int("merger-delete-threads", 8, "Number of threads for deleting files in parallel (increase this in case the merger isn't able to keep up with deleting one-block files).")
 			return nil
 		},
 		FactoryFunc: func(runtime *launcher.Runtime) (launcher.App, error) {