A hard link is created with the ln target link command. The link file is then a new name for the target file. Hard links can only be created on the same filesystem, while symbolic links are not subject to this limitation.
The available hard drive space prohibits implementation of a complete daily backup. As such, the rsync command is preceded by a duplication of the content of the previous backup with hard links, which prevents usage of too much hard drive space. The rsync process then only replaces files that have been modified since the last backup. With this mechanism a great number of backups can be kept in a small amount of space. Since all backups are immediately available and accessible (for example, in different directories of a given share on the network), you can quickly make comparisons between two given dates.
This backup mechanism is easily implemented with the dirvish program. It uses a backup storage space (“bank” in its vocabulary) in which it places timestamped copies of sets of backup files (these sets are called “vaults” in the dirvish documentation).
The main configuration is in the /etc/dirvish/master.conf file. It defines the location of the backup storage space, the list of “vaults” to manage, and default values for expiration of the backups. The rest of the configuration is located in the bank/vault/dirvish/default.conf files and contains the specific configuration for the corresponding set of files.
Example 9.3. The /etc/dirvish/master.conf file
bank:
/backup
exclude:
lost+found/
core
*~
Runalclass="underline"
root 22:00
expire-default: +15 days
expire-rule:
# MIN HR DOM MON DOW STRFTIME_FMT
* * * * 1 +3 months
* * 1-7 * 1 +1 year
* * 1-7 1,4,7,10 1
The bank setting indicates the directory in which the backups are stored. The exclude setting allows you to indicate files (or file types) to exclude from the backup. The Runall is a list of file sets to backup with a time-stamp for each set, which allows you to assign the correct date to the copy, in case the backup is not triggered at precisely the assigned time. You have to indicate a time just before the actual execution time (which is, by default, 10:04 pm in Debian, according to /etc/cron.d/dirvish). Finally, the expire-default and expire-rule settings define the expiration policy for backups. The above example keeps forever backups that are generated on the first Sunday of each quarter, deletes after one year those from the first Sunday of each month, and after 3 months those from other Sundays. Other daily backups are kept for 15 days. The order of the rules does matter, Dirvish uses the last matching rule, or the expire-default one if no other expire-rule matches.
IN PRACTICE Scheduled expiration
The expiration rules are not used by dirvish-expire to do its job. In reality, the expiration rules are applied when creating a new backup copy to define the expiration date associated with that copy. dirvish-expire simply peruses the stored copies and deletes those for which the expiration date has passed.
Example 9.4. The /backup/root/dirvish/default.conf file
client: rivendell.falcot.com
tree: /
xdev: 1
index: gzip
image-default: %Y%m%d
exclude:
/var/cache/apt/archives/*.deb
/var/cache/man/**
/tmp/**
/var/tmp/**
*.bak
The above example specifies the set of files to back up: these are files on the machine rivendell.falcot.com (for local data backup, simply specify the name of the local machine as indicated by hostname), especially those in the root tree (tree: /), except those listed in exclude. The backup will be limited to the contents of one filesystem (xdev: 1). It will not include files from other mount points. An index of saved files will be generated (index: gzip), and the image will be named according to the current date (image-default: %Y%m%d).
There are many options available, all documented in the dirvish.conf(5) manual page. Once these configuration files are setup, you have to initialize each file set with the dirvish --vault vault --init command. From there on the daily invocation of dirvish-runall will automatically create a new backup copy just after having deleted those that expired.
IN PRACTICE Remote backup over SSH
When dirvish needs to save data to a remote machine, it will use ssh to connect to it, and will start rsync as a server. This requires the root user to be able to automatically connect to it. The use of an SSH authentication key allows precisely that (see Section 9.2.2.1, “Key-Based Authentication”).
9.10.2. Restoring Machines without Backups
Desktop computers, which are not backed up, will be easy to regenerate from CD-ROMs made by the mondo program. These bootable CD-ROMs allow complete re-installation of the machine's system. But beware: files that are not part of the system or the user's home directory will not, themselves, be backed up by mondo. This includes, for example, users' local crontabs, as well as any changes made to system configuration since the preparation of the CD-ROM.
The Falcot Corp administrators are aware of the limits in their backup policy. Since they can't protect the backup server as well as a tape in a fireproof safe, they have installed it in a separate room so that a disaster such as a fire in the server room won't destroy backups along with everything else. Furthermore, they do an incremental backup on DVD-ROM once per week — only files that have been modified since the last backup are included.
GOING FURTHER Backing up SQL and LDAP services
Many services (such as SQL or LDAP databases) can not be backed up by simply copying their files (unnless they are properly interrupted during creation of the backups, which is frequently problematic, since they are intended to be available at all times). As such, it is necessary to use an “export” mechanism to create a “data dump” that can be safely backed up. These are often quite large, but they compress well. To reduce the storage space required, you will only store a complete text file per week, and a diff each day, which is created with a command of the type diff file_from_yesterday file_from_today. The xdelta program produces incremental differences from binary dumps.
CULTURE TAR, the standard for tape backups