diff --git a/db/commitlog/commitlog.cc b/db/commitlog/commitlog.cc
index 9da70efaf7..c30e7033e4 100644
--- a/db/commitlog/commitlog.cc
+++ b/db/commitlog/commitlog.cc
@@ -587,8 +587,8 @@ public:
     /**
      * Add a "mutation" to the segment.
      */
-    future<replay_position> allocate(const cf_id_type& id, size_t size,
-            serializer_func func) {
+    future<replay_position> allocate(const cf_id_type& id, shared_ptr<entry_writer> writer) {
+        const auto size = writer->size(*this);
         const auto s = size + entry_overhead_size; // total size
         if (s > _segment_manager->max_mutation_size) {
             return make_exception_future<replay_position>(
@@ -602,8 +602,8 @@ public:
             if (position() + s > _segment_manager->max_size) {
                 // do this in next segment instead.
                 return finish_and_get_new().then(
-                        [id, size, func = std::move(func)](auto new_seg) {
-                            return new_seg->allocate(id, size, func);
+                        [id, writer = std::move(writer)] (auto new_seg) mutable {
+                            return new_seg->allocate(id, std::move(writer));
                         });
             }
             // enough data?
@@ -634,7 +634,7 @@ public:
         out.write(crc.checksum());
 
         // actual data
-        func(out);
+        writer->write(*this, out);
 
         crc.process_bytes(p + 2 * sizeof(uint32_t), size);
 
@@ -1128,8 +1128,21 @@ void db::commitlog::segment_manager::release_buffer(buffer_type&& b) {
  */
 future<db::replay_position> db::commitlog::add(const cf_id_type& id,
         size_t size, serializer_func func) {
-    return _segment_manager->active_segment().then([=](auto s) {
-        return s->allocate(id, size, std::move(func));
+    class serializer_func_entry_writer final : public entry_writer {
+        serializer_func _func;
+        size_t _size;
+    public:
+        serializer_func_entry_writer(size_t sz, serializer_func func)
+            : _func(std::move(func)), _size(sz)
+        { }
+        virtual size_t size(segment&) override { return _size; }
+        virtual void write(segment&, output& out) override {
+            _func(out);
+        }
+    };
+    auto writer = ::make_shared<serializer_func_entry_writer>(size, std::move(func));
+    return _segment_manager->active_segment().then([id, writer] (auto s) {
+        return s->allocate(id, writer);
     });
 }
 
diff --git a/db/commitlog/commitlog.hh b/db/commitlog/commitlog.hh
index 5bd048d6a5..b277e2f4d5 100644
--- a/db/commitlog/commitlog.hh
+++ b/db/commitlog/commitlog.hh
@@ -283,6 +283,11 @@ public:
             const sstring&, commit_load_reader_func, position_type = 0);
 private:
     commitlog(config);
+
+    struct entry_writer {
+        virtual size_t size(segment&) = 0;
+        virtual void write(segment&, output&) = 0;
+    };
 };
 
 }