add swap hook for ganeti-deboostrap-instance

Second post about ganeti. This time I’ll talk about adding a swap partition to an image added with ganeti-deboostrap-instance. Browsing the web, it seems that an old version of the ganeti debostrap script allowed for the creation of a swap partition from the command line. The actual version in sid does not, so, if you want to add a swap partition, you need to write a small hook in /etc/ganeti/instance-debootstrap/hooks/.

Part of the code below is taken from the instance-debootstrap script.

if [ $DISK_COUNT -lt 2 -o -z "$DISK_1_PATH" ]; then
    log_error "Skip swap creation"
    exit 0
fi

swapdev=$DISK_1_PATH

# Make sure we're not working on the root directory
if [ -z "$TARGET" -o "$TARGET" = "/" ]; then
    echo "Invalid target directory '$TARGET', aborting." 1>&2
    exit 1
fi

if [ "$(mountpoint -d /)" = "$(mountpoint -d "$TARGET")" ]; then
    echo "The target directory seems to be the root dir, aborting."  1>&2
    exit 1
fi

if [ -f /sbin/blkid -a -x /sbin/blkid ]; then
  VOL_ID="/sbin/blkid -o value -s UUID"
  VOL_TYPE="/sbin/blkid -o value -s TYPE"
else
  for dir in /lib/udev /sbin; do
    if [ -f $dir/vol_id -a -x $dir/vol_id ]; then
      VOL_ID="$dir/vol_id -u"
      VOL_TYPE="$dir/vol_id -t"
    fi
  done
fi

if [ -z "$VOL_ID" ]; then
  log_error "vol_id or blkid not found, please install udev or util-linux"
  exit 1
fi

if [ -n "$swapdev" ]; then
  mkswap $swapdev
  swap_uuid=$($VOL_ID $swapdev || true )
fi

[ -n "$swapdev" -a -n "$swap_uuid" ] && cat >> $TARGET/etc/fstab <<EOF
UUID=$swap_uuid   swap            swap    defaults        0       0
EOF

This script does two things: first it checks if the user passed a second disk argument to the gnt-instance add call. I decided arbitrarily that the second disk is going to be used a swap disk. Second it figures out the vol-id of this disk, create the swap partition and write an entry in the fstab. All in all it’s a straightforward procedure, but I love when I can cut and paste easy scripts :)

The call to create the instance is as follows, using a disk of 5G for the system and a disk of 1G for the swap.

gnt-instance add -t plain --disk 0:size=5G --disk 1:size=1G -B memory=1024 -o debootstrap+unstable --no-ip-check --no-name-check node1 ```