Extending Filesystems | User Mode Linux

Sometimes you might set up a filesystem for a UML instance that sub-sequently turns out to be too small. For the different types of file-systems we have covered in this chapter, there are different options.

By default, the space available in a hostfs mount is the same as in the host filesystem in which the data resides. Increasing this requires either deleting files to increase the amount of free space or increasing the size of the filesystem somehow. If the filesystem resides on a logical volume, a free disk partition can be added to the corresponding volume group. Otherwise, you will need to move the hostfs data to a different partition or repartition the disk to increase the size of the existing partition.

Another option is to control the space consumption on hostfs mounts by using quotas on the host. By running different UML instances as different UIDs and assigning disk quotas to those UIDs, you can control the disk consumption independently of the space that's actually available on the host filesystem. In this case, increasing the space available to a UML instance on a hostfs mount is a matter of adjusting its disk quota on the host.

As we saw earlier, you can change the size of a humfs mount by changing the value on the total line in the superblock file.

The situation with a ubd block device is more complicated. Increasing the size of the host file is simple:

host% dd if=/dev/zero of=root_fs bs=1024 \     seek=$[ 2 * 1024 * 1024 ] count=1

This increases the size of the root_fs file to 2GB. A more complicated problem is making that extra space available within the UML filesystem. Some but not all filesystems support being resized without making a backup and recreating the filesystem from scratch. Fewer support being resized without unmounting the filesystem. One that does is ext2 (and ext3 since it has a nearly identical on-disk format). By default, ext2online resizes the filesystem to fill the disk that it resides on, which is what you almost always want:

UML# ext2online /dev/ubda

You can also specify the mount point rather than the block device, which may be more intuitive and less error prone:

UML# ext2online /

With other filesystems, you may have to unmount the filesystem before resizing it to fill the device. If the filesystem in question is the UML instance's root filesystem, you will likely need to halt the instance and resize the filesystem on the host.

For filesystems that don't support resizing at all, you have to copy the data to someplace else and recreate the filesystem from scratch using mkfs. Then you can copy your data back into it. Again, if this is the root filesystem of the UML instance, you will need to shut it down and then recreate the filesystem on the host.