For snapshot filesystems and in some cases where it is expected to do tree rebalancing it would be desirable to allow a "whiteout" for an extent. That means the extent would be present in the tree, and would explicitly list the data blocks as a "hole" (i.e. ee_start == 0). This is useful because it avoids the need to search all of the "backing" inodes to see if there is allocated data, and it also handles the case where the new file is truncated and extended (leaving a hole) but there is still data in the backing file. Since block == 0 is not a valid data for ext3 it should never happen, and if it ever did happen it would be better to return all zeros (and not allow writing to the extent) instead of overwriting the superblock). This would need some special casing in the extent code, but it probably would not be much different than what is currently the case for preallocated extents, and it would be a good time to add this in along with the preallocated extents. Cheers, Andreas -- Andreas Dilger Principal Software Engineer Cluster File Systems, Inc. - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
