As I have written before snapshots is a extremely fast and convenient way of perfoming live backups which can be mounted and handled like any other filesystem. But I also told you that LVM snaphots could render themselves as unusable when they run out of free blocks. Today I hacked up a quick script which automagically increments sizes of snapshots logical volumes. Just drop the following snippet into /etc/cron.hourly and relax:

#! /bin/bash

THRESHOLD="80"
INCREMENT="15"

set -e
IFS=':'

/sbin/lvs --noheadings --units M --separator : |   
while read lv vg attr lsize origin snapp move log copyp
do
  # Check whether this is a snapshot or not
  [ "${snapp}" ] || continue

  snapp=${snapp%.*}
  lv=${lv// /}

  # Check whether the thing needs resizing
  [ "${snapp}" -ge "${THRESHOLD}" ] || continue

  lsize=${lsize%.*}
  isize=$(( INCREMENT * lsize / 100 ))
  echo "lvresize -L +${isize}M ${vg}/${lv}"
  /sbin/lvresize -L "+${isize}M" "${vg}/${lv}"
done

You can change the following settings:

• THRESHOLD is the percent of usage which triggers resizing. When actual usage is greater than this value snapshots will grow.
• INCREMENT is used to calculate how much size is added to the volume, it is a percent of the current volume size.

Of course the script could be improved (i.e. it could check whether there are space for growing in the volume group), but this naïve implementation is enough to make me happy and not to worry about checking status of my snapshots periodically :D