Write optimization: avoid copying unused portions of pages.

libraries/libmdb/mdb.c

@@ -1328,6 +1328,27 @@ none:
 	return np;
 }
 
+/** Copy a page: avoid copying unused portions of the page.
+ * @param[in] dst page to copy into
+ * @param[in] src page to copy from
+ */
+static void
+mdb_page_copy(MDB_page *dst, MDB_page *src, unsigned int psize)
+{
+	dst->mp_flags = src->mp_flags | P_DIRTY;
+	dst->mp_pages = src->mp_pages;
+
+	if (IS_LEAF2(src)) {
+		memcpy(dst->mp_ptrs, src->mp_ptrs, psize - PAGEHDRSZ - SIZELEFT(src));
+	} else {
+		unsigned int i, nkeys = NUMKEYS(src);
+		for (i=0; i<nkeys; i++)
+			dst->mp_ptrs[i] = src->mp_ptrs[i];
+		memcpy((char *)dst+src->mp_upper, (char *)src+src->mp_upper,
+			psize - src->mp_upper);
+	}
+}
+
 /** Touch a page: make it dirty and re-insert into tree with updated pgno.
  * @param[in] mc cursor pointing to the page to be touched
  * @return 0 on success, non-zero on failure.
@@ -1345,11 +1366,16 @@ mdb_page_touch(MDB_cursor *mc)
 		DPRINTF("touched db %u page %zu -> %zu", mc->mc_dbi, mp->mp_pgno, np->mp_pgno);
 		assert(mp->mp_pgno != np->mp_pgno);
 		mdb_midl_append(&mc->mc_txn->mt_free_pgs, mp->mp_pgno);
+		if (SIZELEFT(mp)) {
+			/* If page isn't full, just copy the used portion */
+			mdb_page_copy(np, mp, mc->mc_txn->mt_env->me_psize);
+		} else {
 			pgno = np->mp_pgno;
 			memcpy(np, mp, mc->mc_txn->mt_env->me_psize);
+			np->mp_pgno = pgno;
+			np->mp_flags |= P_DIRTY;
+		}
 		mp = np;
-		mp->mp_pgno = pgno;
-		mp->mp_flags |= P_DIRTY;
 
 finish:
 		/* Adjust other cursors pointing to mp */