Difference between revisions of "Live-backup"
Revision as of 23:39, 28 March 2009
FSArchiver can be used to backup linux operating systems when they are running. In other words, if you have linux installed on your hard disk, and it's currently running, you can make a backup of that disk using fsarchiver. It's called a
live backup or
hot backup. All you need is an fsarchiver binary, an another filesystem where to save the archive. It can be on another partition on the hard-disk, or a network file-system such as Samba of NFS.
Backup with a snapshot
If the partitions you want to save are LVM (Logical volume Manager) Logical-Volumes, and it you have free extents in the Volume-Group, then you can make a filesystem snapshot of these Logical-Volumes. A snapshot if a frozen copy of a Logical-Volume made at a given time. After that time, the original partition can still be modified normally, and the snapshot provides a consistent filesystem that can be backed up. To create an LVM snapshot, you have to use lvcreate with option
--snapshot, and then the snapshot can be mounted read-only on a directory. By default, all the partitions except /boot are configured as Logical-Volumes with recent Redhat-Enterprise and Fedora distributions. You can have the list of all your Logical-Volumes with the command called
lvdisplay. If this command is not installed on your system, you are probably not using LVM.
# lvdisplay -c /dev/vgmain/distfiles:vgmain:3:1:-1:1:12582912:1536:-1:0:-1:251:21 /dev/vgmain/misc:vgmain:3:1:-1:1:50331648:6144:-1:0:-1:251:29 /dev/vgmain/tftpboot:vgmain:3:1:-1:1:1048576:128:-1:0:-1:251:30 /dev/vgmain/vdisk:vgmain:3:1:-1:1:209715200:25600:-1:0:-1:251:31 /dev/vgmain/chrooti386:vgmain:3:1:-1:2:1572864:192:-1:0:-1:251:32
Backup with no snapshot
If your partition are not LVM Logical-Volumes, you can't make a snapshot. If the partition are not used, it's recommended to remount it as read-only, with the following command:
mount -o remount,ro /dev/xxx
If the partition cannot be remounted read-only (which is the case of the root filesystem in general), it's still possible to use fsarchiver to make a backup, but you will have to take extra care. By default, fsarchiver complains if you try to save a filesystem which is mounted in read-write mode. This is because in cannot guarantee that the data will be consistent because files may change during the backup of the filesystem. This is the reason why it shows the following error message, and stops:
# fsarchiver savefs /mnt/archives/gentoo-backup-20090328-01.fsa /dev/sda2 -v create.c#0642,filesystem_mount_partition(): partition [/dev/sda2] is mounted read/write. please mount it read-only and then try again. you can do "mount -o remount,ro /dev/sda2". you can also run fsarchiver with option '-A' if you know what you are doing. removing /mnt/archives/gentoo-backup-20090328-01.fsa
When a filesystem is writeable during the backup, it means changes can be done in files during that time, and there may be inconsistencies in the data. For instance, if you are backing up a web server which is running both Apache and Mysql, the Mysql database refers to files that can be uploaded in the Apache directory from the website. In that case the backup could contains a reference in the database but not the referred file because these files have been backed up already. So you have to know whether or not your system may have such inconsistencies.
If there is no risk of inconsistency, then you can use fsarchiver with option
-A to continue the backup of a filesystem which is mounted in read-write mode.
If you have a problem on your filesystems, you may want to restore the live-backup you made. You cannot restore a filesystem which is mounted, so it's necessary to restore from a Linux-Rescue system. We recommend that you use SystemRescueCd for multiple reasons: