diff --git a/db/commitlog/commitlog.cc b/db/commitlog/commitlog.cc
index b740eed402..dba8986adc 100644
--- a/db/commitlog/commitlog.cc
+++ b/db/commitlog/commitlog.cc
@@ -1671,9 +1671,9 @@ future<db::commitlog::segment_manager::sseg_ptr> db::commitlog::segment_manager:
 
         align = f.disk_write_dma_alignment();
         auto is_overwrite = false;
+        auto existing_size = f.known_size();
 
         if ((flags & open_flags::dsync) != open_flags{}) {
-            auto existing_size = f.known_size();
             is_overwrite = true;
             // would be super nice if we just could mmap(/dev/zero) and do sendto
             // instead of this, but for now we must do explicit buffer writes.
@@ -1683,8 +1683,6 @@ future<db::commitlog::segment_manager::sseg_ptr> db::commitlog::segment_manager:
             if (existing_size > max_size) {
                 co_await f.truncate(max_size);
             } else if (existing_size < max_size) {
-                totals.total_size_on_disk += (max_size - existing_size);
-
                 clogger.trace("Pre-writing {} of {} KB to segment {}", (max_size - existing_size)/1024, max_size/1024, filename);
 
                 // re-open without o_dsync for pre-alloc. The reason/rationale
@@ -1732,6 +1730,12 @@ future<db::commitlog::segment_manager::sseg_ptr> db::commitlog::segment_manager:
             co_await f.truncate(max_size);
         }
 
+        // #12810 - we did not update total_size_on_disk unless o_dsync was 
+        // on. So kept running with total == 0 -> free for all in creating new segment.
+        // Always update total_size_on_disk. Will wrap-around iff existing_size > max_size. 
+        // That is ok.
+        totals.total_size_on_disk += (max_size - existing_size);
+
         if (cfg.extensions && !cfg.extensions->commitlog_file_extensions().empty()) {
             for (auto * ext : cfg.extensions->commitlog_file_extensions()) {
                 auto nf = co_await ext->wrap_file(filename, f, flags);