Para crear una instantánea del sistema de archivos, básicamente se requiere que el sistema de archivos de estar en un volumen lógico (LV) y un poco de espacio libre en el grupo de volúmenes subyacente (VG).

El simple script (python) de Benjamin Schweizer facilita la creación de instantáneas de un LV especificando unicamente la ruta de un directorio contenido dentro de él. Esto permite no tener que preocuparse de los volúmenes lógicos reales, puntos de montaje y rutas.

Algo que se debe recalcar es lo siguiente. Si el directorio que se especifica pertenece a un volumen lógico, el cual contiene a su vez otras carpetas, al restaurar la snapshot los cambios sobre los otros directorios también son restaurados. LVM trabaja a nivel de bloque y no soporta instantáneas de directorios específicos. Si por ejemplo se hace una instantánea de “/var/log”, pero ese directorio pertenece al LV de root, de restaurarse la instantánea creada con dnapshot todo el sistema raíz sería también restaurado (después de reinicio del sistema ya que es la raíz y no puede ser desmontada en caliente).

Leer: LVM / dnsapshot.

Sintaxis dsnapshot.

usage:
    dsnapshot --test|--create|--remove <PATH> [--size=<SIZE>]

    --test      test lvm
    --create    create and mount snapshot of <PATH>
    --remove    umount and remove snapshot where <PATH> resides in
    --size      maxium size of snapshot volume, defaults to 8192M

examples:
    dsnapshot --test /var/log
    dsnapshot --create /var/log
    dsnapshot --create /var/log --size=2G
    dsnapshot --remove /var/lib/dsnapshot/var-87hj32/...

Test

dsnapshot --test /boot/
path is not on a logical volume

dsnapshot --test /var/log/
success, found /dev/vg_es/lv_root:var/log/

dsnapshot --create /var/log/ --size=1G
/var/lib/dsnapshot/lv_root-67f4f769/var/log/

dsnapshot --remove /var/lib/dsnapshot/lv_root-67f4f769/

Dsnapshot Código fuente: https://github.com/cxcv/dsnapshot

dsnapshot

#!/usr/bin/env python
#
# Copyright (c) 2008 Steinle Solution-Factory GmbH. All rights reserved.
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are
# met:
#
#   * Redistributions of source code must retain the above copyright
#     notice, this list of conditions and the following disclaimer.
#   * Redistributions in binary form must reproduce the above copyright
#     notice, this list of conditions and the following disclaimer in the
#     documentation and/or other materials provided with the
#     distribution.
#   * Neither the name of Steinle Solution-Factory nor the names of its
#     contributors may be used to endorse or promote products derived
#     from this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
# IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
# TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
# PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#
# Abstract
# ~~~~~~~~
# The dsnapshot script provides a high-level interface to the Linux
# Logical Volume Manager. It uses its block-level snapshot support to
# create directory snapshots. In contrast to block-level snapshots,
# directory snapshots resemble the file system layer. Thus, you can
# snapshot any directory that is on a logical volume and you don't have
# to worry about the actual logical volumes, mount points and paths.
#
# Authors
# ~~~~~~~
# Benjamin Schweizer <benjamin at steinle dot net>
#
# Changes
# ~~~~~~~
# 2008-09-15, benjamin: initial release.
#
# Todo
# ~~~~
# - before removal, check if the logical_volume is a snapshot device
# - more expressive error messages
# - recursive snapshots accross filesystem boundaries?
#
 
import os
import sys
import subprocess
import random
 
MOUNT_ROOT = '/var/lib/dsnapshot'
SNAPSHOT_SIZE = '8192M'
 
def usage():
    print """dsnapshot/2008-09-15
 
usage:
    dsnapshot --test|--create|--remove <PATH> [--size=<SIZE>]
 
    --test      test lvm
    --create    create and mount snapshot of <PATH>
    --remove    umount and remove snapshot where <PATH> resides in
    --size      maxium size of snapshot volume, defaults to 8192M
 
examples:
    dsnapshot --test /var/log
    dsnapshot --create /var/log
    dsnapshot --create /var/log --size=2G
    dsnapshot --remove /var/lib/dsnapshot/var-87hj32/...
 
"""
 
def get_logical_volume_path(real_path):
    """returns device path where path resides on"""
    st = os.stat(real_path)
    major = os.major(st.st_dev)
    minor = os.minor(st.st_dev)
 
    try:
        sp = subprocess.Popen(['lvdisplay', '--colon'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        returncode = sp.wait()
    except OSError, (errno, errstr):
        raise Exception('lvdisplay-call failed with exit code %d:\n%s' % (errno, errstr))
    if returncode:
        raise Exception('lvdisplay-call failed with exit code %d:\n%s' % (returncode, sp.stderr.read()))
 
    line = sp.stdout.readline()
    if line[:-1] == '  No volume groups found':
        raise Exception('lvdisplay-call failed: No volume groups found.')
 
    while line:
        # "  /dev/storage/usr:storage:3:1:-1:1:8388608:128:-1:0:0:253:1"
        _logical_volume_path = line.split(':')[0][2:]
        _major = int(line.split(':')[11])
        _minor = int(line.split(':')[12])
        if _major == major and _minor == minor:
            return _logical_volume_path
        line = sp.stdout.readline()
 
    raise Exception('path is not on a logical volume')
 
def get_relative_path(real_path):
    """returns relative path from mount point"""
    head = real_path
    relative_path = ''
 
    while not os.path.ismount(head):
        head, tail = os.path.split(head)
        relative_path = os.path.join(tail, relative_path)
 
    return relative_path
 
def create_and_mount_snapshot(original_logical_volume_path):
    """creates and mounts a lvm2 snapshot"""
    snapshot_logical_volume_path = "%s-%08x" % (original_logical_volume_path, random.randint(0,0xffffffff))
    snapshot_mount_path = os.path.join(MOUNT_ROOT, os.path.basename(snapshot_logical_volume_path))
    os.stat(original_logical_volume_path)
 
    # create block level snapshot
    sp = subprocess.Popen(['lvcreate', '--snapshot', '--size', SNAPSHOT_SIZE, original_logical_volume_path, '--name', snapshot_logical_volume_path], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    returncode = sp.wait()
    if returncode:
        raise Exception('lvcreate-call failed with exit code %d:\n%s' % (returncode, sp.stderr.read()))
 
    # mount snapshot
    os.mkdir(snapshot_mount_path, 0700)
    sp = subprocess.Popen(['mount', snapshot_logical_volume_path, snapshot_mount_path, '-o', 'ro'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    returncode = sp.wait()
    if returncode:
        raise Exception('mount-call failed with exit code %d:\n%s' % (returncode, sp.stderr.read()))
 
    return snapshot_mount_path
 
def umount_and_remove_snapshot(snapshot_logical_volume_path, real_path):
    """umounts and removes a lvm2 snapshot"""
    # check if real_snapshot_mount_path was created by this script
    snapshot_mount_path = real_path
    while not os.path.ismount(snapshot_mount_path):
        snapshot_mount_path, tail = os.path.split(snapshot_mount_path)
 
    snapshot_mount_path2 = os.path.join(MOUNT_ROOT, os.path.basename(snapshot_logical_volume_path))
 
    if snapshot_mount_path != snapshot_mount_path2:
        raise Exception('destruction refused; looks not like my footprint')
 
    # todo: check if snapshot_logical_volume is a snapshot
    #   requires parsing of lvdisplay verbose output
 
    # umount snapshot
    sp = subprocess.Popen(['umount', snapshot_mount_path], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    returncode = sp.wait()
    if returncode:
        raise Exception('umount-call failed with exit code %d:\n%s' % (returncode, sp.stderr.read()))
    os.rmdir(snapshot_mount_path)
 
    # remove block level snapshot
    sp = subprocess.Popen(['lvremove', '--force', snapshot_logical_volume_path], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    returncode = sp.wait()
    if returncode:
        raise Exception('lvremove-call failed with exit code %d:\n%s' % (returncode, sp.stderr.read()))
 
if __name__ == '__main__':
    if len(sys.argv) < 3:
        usage()
        sys.exit(0)
 
    mode = sys.argv[1]
    real_path = os.path.realpath(sys.argv[2])
    if len(sys.argv) == 4 and sys.argv[3].split('=')[0] == '--size':
        SNAPSHOT_SIZE=sys.argv[3].split('=')[1]
 
    # implicit checks
    try:
        os.stat(real_path)
        logical_volume_path = get_logical_volume_path(real_path)
        relative_path = get_relative_path(real_path)
        try:
            os.stat(MOUNT_ROOT)
        except OSError:
            os.mkdir(MOUNT_ROOT, 0700)
    except Exception, e:
        raise SystemExit(e)
 
    if mode == '--test':
        print "success, found %s:%s" % (logical_volume_path, relative_path)
    elif mode == '--create':
        try:
            snapshot_mount_path = create_and_mount_snapshot(logical_volume_path)
            print os.path.join(snapshot_mount_path, relative_path)
        except Exception, e:
            raise SystemExit(e)
    elif mode == '--remove':
        try:
            umount_and_remove_snapshot(logical_volume_path, real_path)
        except Exception, e:
            raise SystemExit(e)
    else:
        usage()
        raise SystemExit
 
# vim: ts=4 sw=4 sts=4 et
# eof.