This requires changing the generator so it can handle passing optional arguments all thr way through to the daemon, changing the protocol (see previous patch set), and implementing the new mkfs-opts call. At the moment there is just an optional blocksize argument, thus mimicking what you can already do with 'mkfs-b'. But this change will allow us in future to encode much of the rest of the /sbin/mkfs command line. Here is how it works from guestfish (with verbose & trace): mkfs_opts "ext2" "/dev/vda1" "blocksize:1024"send_to_daemon: 0x2393bc0 g->state = 3, n = 72 proc 210 (part_disk) took 0.18 seconds recv_from_daemon: 0x2393bc0 g->state = 3, size_rtn = 0x7fffd24c565c, buf_rtn = 0x7fffd24c5650 mkfs -t ext2 -b 1024 /dev/vda1 mke2fs 1.41.12 (17-May-2010) = 0 mkfs_opts "ext2" "/dev/vda1"send_to_daemon: 0x2393bc0 g->state = 3, n = 72 proc 278 (mkfs_opts) took 0.19 seconds recv_from_daemon: 0x2393bc0 g->state = 3, size_rtn = 0x7fffd24c565c, buf_rtn = 0x7fffd24c5650 mkfs -t ext2 /dev/vda1 mke2fs 1.41.12 (17-May-2010) = 0 mkfs_opts "ext2" "/dev/vda1" "blocksize:4096"send_to_daemon: 0x2393bc0 g->state = 3, n = 72 proc 278 (mkfs_opts) took 0.30 seconds recv_from_daemon: 0x2393bc0 g->state = 3, size_rtn = 0x7fffd24c565c, buf_rtn = 0x7fffd24c5650 mkfs -t ext2 -b 4096 /dev/vda1 mke2fs 1.41.12 (17-May-2010) = 0 mkfs-opts should have 2-3 parameter(s) type 'help mkfs-opts' for help on mkfs-opts mkfs_opts "vfat" "/dev/vda1" "blocksize:32768"send_to_daemon: 0x2393bc0 g->state = 3, n = 72 proc 278 (mkfs_opts) took 0.30 seconds recv_from_daemon: 0x2393bc0 g->state = 3, size_rtn = 0x7fffd24c565c, buf_rtn = 0x7fffd24c5650 blockdev --getss /dev/vda1 mkfs -t vfat -s 64 /dev/vda1 = 0 Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://et.redhat.com/~rjones/libguestfs/ See what it can do: http://et.redhat.com/~rjones/libguestfs/recipes.html