disk space grows by >500% while preparing a backup

when preparing a backup of a database with a large amount (100,000+) of small tables using innodb_file_per_table, each table grows to at least 1MB in size. This can result in a large increase in disk usage.

The problem appears to be caused by a call to _fil_io in fil0fil.c to load block_offset 0 of a tablespace that hasn't been loaded yet. Before the tablespace is loaded, space->size is 0 as well. This causes this chunk of code:

 if (space->size <= block_offset) {
  ulint actual_size;

  mutex_exit(&fil_system->mutex);
  fil_extend_space_to_desired_size(&actual_size, space->id,
       ((block_offset + 1) / 64 + 1) * 64);
  mutex_enter(&fil_system->mutex);
  /* should retry? but it may safe for xtrabackup for now. */
 }

to think this is a request past the end of the file and to grow the file to the 1MB boundary.

A simple fix is to also test if block_offset != 0 before growing the file. You could also check to see if space->size == 0 (tablespace hasn't been loaded). But that would be unnecessary because if space->size > 0, a request for block_offset 0 would fail the space->size <= block_offset test