zsnap.sh

#!/bin/sh
############################################################ CONFIGURATION

#
# Create snapshot if delta becomes at least this many bytes
#
MIN_SIZE=$(( 1024 * 1024 * 10 )) # 10MB

#
# Create snapshot if previous is at least this old
#
MIN_AGE=$(( 3600 * 24 )) # 24 hours

#
# Snapshot name
#
SNAPNAME_FORMAT="zsnap-%Y-%m-%d_%H.%M.%S"

############################################################ MAIN

set -e # make any/all errors fatal

#
# Get current epoch and pre-create snapname
#
now=$( date +%s )
newsnapname=$( date -r $now +"$SNAPNAME_FORMAT" )

#
# Operate on the datasets
#
for dataset in $( zfs list -Ht filesystem | awk '$0=$1' ); do
    #
    # Get the date/time and name of the most recent snapshot for dataset
    #
    read -r creation snapname <<-EOF
    $( zfs get -pHrt snapshot creation "$dataset" 2> /dev/null |
        awk -v dataset="$dataset" '
        BEGIN { last_epoch = 0 }
        (ds = $1) sub(/@.*/, "", ds) && ds == dataset {
            if ((epoch = $--NF) < last_epoch) next
            last_epoch = epoch
            last_snapshot = $1
            sub(/^[^@]*@/, "", last_snapshot)
        }
        END { print last_epoch, last_snapshot }
    ' | sort -nr )
    EOF

    #
    # Skip filesystem if still young and few bytes written
    #
    age=$(( $now - $creation ))
    written=$( zfs get -Hp written "$dataset" | awk '{print $--NF}' )
    [ $age -lt $MIN_AGE -a $written -lt $MIN_SIZE ] && continue

    #
    # Create new snapshot
    #
    echo "zfs snapshot $dataset@$newsnapname"
done

################################################################################
# END
################################################################################