journal: serialize to chunks usually smaller than 4 pages...

...in order to mitigate LMDB free space fragmentation to some degree

src/knot/journal/journal_basic.h

@@ -27,7 +27,8 @@ typedef struct {
 	void *conf; // needed only for journal write operations
 } zone_journal_t;
 
-#define JOURNAL_CHUNK_MAX (70 * 1024)
+#define JOURNAL_CHUNK_MAX (70 * 1024) // must be at least 64k + 6B
+#define JOURNAL_CHUNK_THRESH (15 * 1024)
 #define JOURNAL_HEADER_SIZE (32)
 
 /*! \brief Convert journal_mode to LMDB environment flags. */

src/knot/journal/journal_write.c

@@ -27,7 +27,8 @@ static void journal_write_serialize(knot_lmdb_txn_t *txn, serialize_ctx_t *ser,
 	MDB_val chunk;
 	uint32_t i = 0;
 	while (serialize_unfinished(ser) && txn->ret == KNOT_EOK) {
-		serialize_prepare(ser, JOURNAL_CHUNK_MAX - JOURNAL_HEADER_SIZE, &chunk.mv_size);
+		serialize_prepare(ser, JOURNAL_CHUNK_THRESH - JOURNAL_HEADER_SIZE,
+		                  JOURNAL_CHUNK_MAX - JOURNAL_HEADER_SIZE, &chunk.mv_size);
 		if (chunk.mv_size == 0) {
 			break; // beware! If this is ommited, it creates empty chunk => EMALF when reading.
 		}

src/knot/journal/serialization.c

@@ -143,7 +143,8 @@ static knot_rrset_t get_next_rrset(serialize_ctx_t *ctx)
 	}
 }
 
-void serialize_prepare(serialize_ctx_t *ctx, size_t max_size, size_t *realsize)
+void serialize_prepare(serialize_ctx_t *ctx, size_t thresh_size,
+                       size_t max_size, size_t *realsize)
 {
 	*realsize = 0;
 
@@ -185,6 +186,9 @@ void serialize_prepare(serialize_ctx_t *ctx, size_t max_size, size_t *realsize)
 			return;
 		}
 		*realsize = candidate;
+		if (candidate >= thresh_size) {
+			return;
+		}
 		tmp_phase++;
 	}
 }

src/knot/journal/serialization.h

@@ -46,11 +46,13 @@ serialize_ctx_t *serialize_zone_init(const zone_contents_t *z);
  *
  * \note This MUST be called before each serialize_chunk() !
  *
- * \param ctx      Serializing context.
- * \param max_size Maximum size of next chunk.
- * \param realsize Output: real exact size of next chunk.
+ * \param ctx           Serializing context.
+ * \param thresh_size   Optimal size of next chunk.
+ * \param max_size      Maximum size of next chunk.
+ * \param realsize      Output: real exact size of next chunk.
  */
-void serialize_prepare(serialize_ctx_t *ctx, size_t max_size, size_t *realsize);
+void serialize_prepare(serialize_ctx_t *ctx, size_t thresh_size,
+                       size_t max_size, size_t *realsize);
 
 /*!
  * \brief Perform one step of serializiation: fill one chunk.