minor tweaks in purge

- allocate once
- log purge count

blockstore/splitstore/splitstore.go

@@ -1139,9 +1139,13 @@ func (s *SplitStore) purgeBatch(cids []cid.Cid, deleteBatch func([]cid.Cid) erro
 }
 
 func (s *SplitStore) purge(curTs *types.TipSet, cids []cid.Cid) error {
+	deadCids := make([]cid.Cid, 0, batchSize)
+	purgeCnt := 0
+	defer log.Infof("purged %d objects", purgeCnt)
+
 	return s.purgeBatch(cids,
 		func(cids []cid.Cid) error {
-			deadCids := make([]cid.Cid, 0, len(cids))
+			deadCids := deadCids[:0]
 
 			s.txnLk.Lock()
 			defer s.txnLk.Unlock()
@@ -1170,6 +1174,8 @@ func (s *SplitStore) purge(curTs *types.TipSet, cids []cid.Cid) error {
 				return xerrors.Errorf("error purging cold objects: %w", err)
 			}
 
+			purgeCnt += len(deadCids)
+
 			return nil
 		})
 }