diff --git a/test_suite/scripts/modules/80_lib_rw.sh b/test_suite/scripts/modules/80_lib_rw.sh
index 1c529780..68ba9d38 100644
--- a/test_suite/scripts/modules/80_lib_rw.sh
+++ b/test_suite/scripts/modules/80_lib_rw.sh
@@ -131,6 +131,8 @@ function lib_rw_cksum
     eval $varname_cksum='('${my_cksum_out[0]}' '${my_cksum_out[1]}')'
 }
 
+# if the size to compare equals 0 we take the mars size of the
+# data devices
 function lib_rw_compare_checksums
 {
     [ $# -eq 6 ] || lib_exit 1 "wrong number $# of arguments (args = $*)"
@@ -139,23 +141,39 @@ function lib_rw_compare_checksums
     local dev=$(lv_config_get_lv_device $res)
     local host role primary_cksum_out secondary_cksum_out cksum_out
     local cksum_dev=$dev
+    local dd_bsize=4096 dd_count
                                             
     for role in "primary" "secondary"; do
+        local dummy_file
         eval host='$'${role}_host
+        dummy_file=$main_mars_directory/dummy-$host
         marsadm_do_cmd $host "down" $res || lib_exit 1
-        if [ $cmp_size -ne 0 ]; then
-            local dummy_file=$main_mars_directory/dummy-$host
-            lib_vmsg "  dumping $cmp_size G of $dev to $dummy_file"
-            lib_remote_idfile $host \
-                "dd if=$dev of=$dummy_file bs=1024 count=$(($cmp_size * 1024 * 1024))" || lib_exit 1
-            lib_remote_idfile $host "ls -l $dummy_file"
-            cksum_dev=$dummy_file
+        if [ $cmp_size -eq 0 ]; then
+            local link_value
+            local link="${resource_dir_list[$res]}/size"
+            lib_vmsg "  reading link $host:$link"
+            link_value=$(lib_remote_idfile $primary_host "readlink $link") || \
+                                                                    lib_exit 1
+            if ! expr "$link_value" : '^[0-9][0-9]*$' >/dev/null; then
+                lib_exit 1 "  $link_value is not a numeric value"
+            fi
+            if [ $((($link_value / $dd_bsize) * $dd_bsize)) -ne $link_value ]
+            then
+                lib_exit 1 "value $link_value not divsible by $dd_bsize"
+            fi
+            dd_count=$(($link_value / $dd_bsize))
+        else
+            dd_count=$((($cmp_size * 1024 * 1024 * 1024) / $dd_bsize))
         fi
+        lib_vmsg "  dumping $(($dd_count * $dd_bsize)) bytes of $dev to $dummy_file"
+        lib_remote_idfile $host \
+            "dd if=$dev of=$dummy_file bs=$dd_bsize count=$dd_count" || \
+                                                                    lib_exit 1
+        lib_remote_idfile $host "ls -l $dummy_file"
+        cksum_dev=$dummy_file
         lib_rw_cksum $host $cksum_dev "cksum_out"
         eval ${role}_cksum_out='"${cksum_out[*]}"'
-        if [ $cmp_size -ne 0 ]; then
-            lib_remote_idfile $host "rm -f $dummy_file" || lib_exit 1
-        fi
+        lib_remote_idfile $host "rm -f $dummy_file" || lib_exit 1
         marsadm_do_cmd $host "up" $res || lib_exit 1
     done
     if [ "$primary_cksum_out" != "$secondary_cksum_out" ]; then