Writes to the virtual block device are not immediately written to the
host's disk; this may cause problems if, for example, the User-Mode
Linux 'Virtual Machine' uses a journalling filesystem and the host
computer crashes.

Synchronous operation (i.e. always writing data to the host's disk
immediately) is configurable on a per-UBD basis by using a special
kernel command line option. Alternatively, you can say Y here to
turn on synchronous operation by default for all block devices.

If you're running a journalling file system (like reiserfs, for
example) in your virtual machine, you will want to say Y here. If
you care for the safety of the data in your virtual machine, Y is a
wise choice too. In all other cases (for example, if you're just
playing around with User-Mode Linux) you can choose N.