Don't attempt to share filesystems simply by booting two UMLs from the same file. That's the same thing as booting two physical machines from a shared disk. It will result in filesystem corruption.

The way to share a filesystem between two virtual machines is to use the copy-on-write (COW) layering capability of the ubd block driver. As of 2.4.6-2um, the driver supports layering a read-write private device over a read-only shared device. A machine's writes are stored in the private device, while reads come from either device - the private one if the requested block is valid in it, the shared one if not. Using this scheme, the majority of data which is unchanged is shared between an arbitrary number of virtual machines, each of which has a much smaller file containing the changes that it has made. With a large number of UMLs booting from a large root filesystem, this leads to a huge disk space saving. It will also help performance, since the host will be able to cache the shared data using a much smaller amount of memory, so UML disk requests will be served from the host's memory rather than its disks.

To add a copy-on-write layer to an existing block device file, simply add the name of the COW file to the appropriate ubd switch:

ubd0=root_fs_cow,root_fs_debian_22

where 'root_fs_cow' is the private COW file and 'root_fs_debian_22' is the existing shared filesystem. The COW file need not exist. If it doesn't, the driver will create and initialize it. Once the COW file has been initialized, it can be used on its own on the command line:

ubd0=root_fs_cow

The name of the backing file is stored in the COW file header, so it would be redundant to continue specifying it on the command line.

When checking the size of the COW file in order to see the gobs of space that you're saving, make sure you use 'ls -ls' to see the actual disk consumption rather than the length of the file. The COW file is sparse, so the length will be very different from the disk usage. Here is a 'ls -l' of a COW file and backing file from one boot and shutdown:

                
host% ls -l cow.debian debian2.2
-rw-r--r--    1 jdike    jdike    492504064 Aug  6 21:16 cow.debian
-rwxrw-rw-    1 jdike    jdike    537919488 Aug  6 20:42 debian2.2

              

Doesn't look like much saved space, does it? Well, here's 'ls -ls':

                
host% ls -ls cow.debian debian2.2
   880 -rw-r--r--    1 jdike    jdike    492504064 Aug  6 21:16 cow.debian
525832 -rwxrw-rw-    1 jdike    jdike    537919488 Aug  6 20:42 debian2.2

              

Now, you can see that the COW file has less than a meg of disk, rather than 492 meg.

Once a filesystem is being used as a readonly backing file for a COW file, do not boot directly from it or modify it in any way. Doing so will invalidate any COW files that are using it. The mtime and size of the backing file are stored in the COW file header at its creation, and they must continue to match. If they don't, the driver will refuse to use the COW file.

If you attempt to evade this restriction by changing either the backing file or the COW header by hand, you will get a corrupted filesystem.

Among other things, this means that upgrading the distribution in a backing file and expecting that all of the COW files using it will see the upgrade will not work.

Because UML stores the backing file name and its mtime in the COW header, if you move the backing file, that information becomes invalid. So, the procedure for moving a backing file is

• Move it in a way that preserves timestamps. Usually, this is a "-p" switch. "cp -a" works because "-a" implies "-p".
• Update the COW header by booting UML on it, specifying both the COW file and the new location of the backing file

  host% ubd0=COW file,new backing file location

  UML will notice the mismatch between the command line and COW header, check the size and mtime of the new backing file path, and update the COW header to reflect it if it checks out.

If you forget to preserve the timestamps when you move the backing file, you can fix the mtime by hand as follows

host% mtime=whatever UML says mtime should be ; \ touch --date="`date -d 1970-01-01\ UTC\ $mtime\ seconds`" backing file

Note that if you do this on a backing file that has truly been changed, and not just moved, then you will get file corruption and you will lose the filesystem.