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This chapter discusses the various aspects of making backups with Obnam.
Your first backup
Let's make a backup! To walk through the examples in this directory,
you need to have some live data to backup. The examples use specific
filenames for this. You'll need to adapt the examples to your own
files. The examples assume your home directory is
that you have a directory called
Documents in your home directory
for your documents. Further, it assumes you have a USB drive mounted
/media/backups, and that you will be using a directory
tomjon-repo on that drive as the backup repository.
With those assumptions, here's how you would backup your documents:
obnam backup -r /media/backups/tomjon-repo ~/Documents
That's all. It will take a little while, if you have a lot of documents, but eventually it'll look something like this:
Backed up 11 files (of 11 found), uploaded 97.7 KiB in 0s at 647.2 KiB/s average speed
(In reality, the above text will be all on one line, but that didn't fit in this manual's line width.)
This tells you that Obnam found a total of eleven files, of which it backed up all eleven. The files contained a total of about a hundred kilobytes of data, and that the upload speed for that data was over six hundred kilobytes per second. The actual units are using IEC prefixes, which are base-2, to avoid ambiguity. See Wikipedia on kibibytes for more information.
Your first backup run should probably be quite small to see that all settings are right without having to wait a long time. You may want to choose a small directory to start with, instead of your entire home directory.
Your second backup
Once you've run your first backup, you'll want to run a second one. It's done the same way:
obnam backup -r /media/backups/tomjon-repo ~/Documents
Note that you don't need to tell Obnam whether you want a full backup or an incremental backup. Obnam makes each backup generation be a snapshot of the data at the time of the backup, and doesn't make a difference between full and incremental backups. Each backup generation is equal to each other backup generation. This doesn't mean that each generation will store all the data separately. Obnam makes sure each new generation only backs up data that isn't already in the repository. Obnam finds that data in any file in any previous generation, amongst all the clients sharing the same repository.
We'll later cover how to remove backup generations, and you'll learn that Obnam can remove any generation, even if it shares some of the data with other generations, without those other generations losing any data.
After you've your second backup generation, you'll want to see the generations you have:
$ obnam generations -r /media/backups/tomjon-repo 2 2014-02-05 23:13:50 .. 2014-02-05 23:13:50 (14 files, 100000 bytes) 5 2014-02-05 23:42:08 .. 2014-02-05 23:42:08 (14 files, 100000 bytes)
This lists two generations, which have the identifiers 2 and 5. Note that generation identifiers are not necessarily a simple sequence like 1, 2, 3. This is due to how some of the internal data structures of Obnam are implemented, and not because its author in any way thinks it's fun to confuse people.
The two time stamps for each generation are when the backup run started and when it ended. In addition, for each generation is a count of files in that generation (total, not just new or changed files), and the total number of bytes of file content data they have.
Choosing what to backup, and what not to backup
Obnam needs to be told what to back up, by giving it a list of
directories, known as backup roots. In the examples in this chapter so
far, we've used the directory
~/Documents (that is, the directory
Documents in your home directory) as the backup root. There can be
multiple backup roots:
obnam -r /media/backups/tomjon-repo ~/Documents ~/Photos
Everything in the backup root directories gets backed up -- unless it's explicitly excluded. There are several ways to exclude things from backups:
--excludesetting uses regular expressions that match the full pathname of each file or directory: if the pathname matches, the file or directory is not backed up. In fact, Obnam pretends it doesn't exist. If a directory matches, then any files and sub-directories also get excluded. This can be used, for example, to exclude all MP3 files (
--exclude-cachessetting excludes directories that contain a special "cache tag" file called
CACHEDIR.TAG, that starts with a specific sequence of bytes. Such a tag file can be created in, for example, a Firefox or other web browser cache directory. Those files are usually not important to back up, and tagging the directory can be easier than constructing a regular expression for
--one-file-systemsetting excludes any mount points and the contents of the mounted filesystem. This is useful for skipping, for example, virtual filesystems such as
/proc, remote filesystems mounted over NFS, and Obnam repositories mounted with
obnam mount(which we'll cover in the next chapter).
In general it is better to back up too much rather than too little.
You should also make sure you know what is and isn't backed up. The
--pretend option tells Obnam to run a backup, except it doesn't
change anything in the backup repository, so it's quite fast. This way
you can see what would be backed up, and tweak exclusions as needed.
Storing backups remotely
You probably want to store at least one backup remotely, or "off site". Obnam can make backups over a network, using the SFTP protocol (part of SSH). You need the following to achieve this:
A server that you can access over SFTP. This can be a server you own, a virtual machine ("VPS") you rent, or some other arrangement. You could, for example, have a machine at a friends' place, in exchange for having one of their machines at your place, so that you both can backup remotely.
An ssh key for logging into the server. Obnam does not currently support logging in via passwords.
Enough disk space on the server to hold your backups.
Obnam only uses the SFTP part of the SSH connection to access the server. To test whether it will work, you can run this command:
USER@SERVER to be your actual user and address for your
server. This should say something like
Connected to localhost. and
you should be able to run the
ls -la command to see a directory list
of files on the remote side.
Once all of that is set up correctly, to get Obnam to use the server as a backup repository, you only need to give an SFTP URL:
obnam -r sftp://USER@SERVER/~/my-precious-backups
For details on SFTP URLs, see the next section.
Whenever obnam accepts a URL, it can be either a local pathname, or an SFTP URL. An SFTP URL has the following form:
domain is a normal Internet domain name for a server,
is your username on that server,
port is an optional numeric port
path is a pathname on the server side. Like bzr(1),
but unlike the SFTP URL standard, the pathname is absolute, unless it
/~/ in which case it is relative to the user's home
directory on the server.
sftp://email@example.com/~/backup-repois the directory
backup-repoin the home directory of the user
liwon the server
backups.pieni.net. Note that we don't need to know the absolute path of the home directory.
sftp://firstname.lastname@example.org/homeis the directory
/home(note absolute path) on the server
my.server.example.com, and the
rootuser is used to access the server.
sftp://foo.example.com:12765/anti-clown-societyis the directory
/anti-clown-societyon the server
foo.example.com, accessed via the port 12765.
You can use SFTP URLs for the repository, or the live data (
but note that due to limitations in the protocol, and its
implementation in the paramiko library, some things will not work very
well for accessing live data over SFTP. Most importantly, the handling
of of hardlinks is rather suboptimal. For live data access, you should
not end the URL with /~/ and should append a dot at the end in this
Obnam can also access the live data over SFTP, instead of via the local filesystem. This means you can run Obnam on, say, your desktop machine to backup your server, or on your laptop to backup your phone (assuming you can get an SSH server installed on your phone). Sometimes it is not possible to install Obnam on the machine where the live data resides, and then it is useful to do a pull backup instead: you run Obnam on a different machine, and read the live data over the SFTP protocol.
To do this, you specify the live data location (the
root setting, or
as a command line argument to
obnam backup) using an SFTP URL. You
should also set the client name explicitly. Otherwise Obnam will use
the hostname of the machine on which it runs as the name, and this can
be highly confusing: the client name is
my-laptop and the server is
down-with-clowns and Obnam will store the backups as if the data
It gets worse if you backup your laptop as well to the same backup repository. Then Obnam will store both the server and the laptop backups using the same client name, resulting in much confusion to everyone.
obnam backup -r /mnt/backups sftp://server.example.com/home \ --client-name=server.example.com
Configuration files: a quick intro
By this time you may have noticed that Obnam has a number of configurable settings you can tweak in a number of ways. Doing it on the command line is always possible, but then you get quite long command lines. You can also put them into a configuration file.
Every command line option Obnam knows can be set in a configuration file. Later in this manual there is a whole chapter that covers all the details of configuration files, and all the various settings you can use. For now, we'll give a quick introduction.
An Obnam configuration looks like this:
[config] repository = /media/backup/tomjon-repo root = /home/liw/Documents, /home/liw/Photos exclude = \.mp3$ exclude-caches = yes one-file-system = no
This form of configuration file is commonly known as an "INI file",
from Microsoft Windows
.INI files. All the Obnam settings go into a
[config], and each setting has the same name as the
command line option, but without the double dash prefix. Thus, it's
--exclude on the command line and
exclude in the configuration
Some settings can have multiple values, such as
The values are comma separated. If there's a lot of values, you can
split them on multiple lines, where the second and later lines are
indented by space or TAB characters.
That should get you started, and you can reference the "Obnam configuration files and settings" chapter for all the details.
When your precious data is very large
When your precious data is very large, the first backup may a very long time. Ditto, if you get a lot of new precious data for a later backup. In these cases, you may need to be very patient, and just let the backup take its time, or you may choose to start small and add to the backups a bit at a time.
The patient option is easy: you tell Obnam to backup everything, set
it running, and wait until it's done, even if it takes hours or days.
If the backup terminates prematurely, e.g., because of a network link
going down, you won't have to start from scratch thanks to Obnam's
checkpoint support. Every gigabyte or so (by default) Obnam stops a
backup run to create a checkpoint generation. If the backup later
crashes, you can just re-run Obnam and it will pick up from the latest
checkpoint. This is all fully automatic, you don't need to do anything
for it to happen. See the
--checkpoint setting for choosing how
often the checkpoints should happen.
Note that if Obnam doesn't get to finish, and you have to re-start it, the scanning starts over from the beginning. The checkpoint generation does not contain enough state for Obnam to continue scanning from the latest file in the checkpoint: it'd be very complicated state, and easily invalidated by filesystem changes. Instead, Obnam scans all files, but most files will hopefully not have changed since the checkpoint was made, so the scanning should be relatively fast.
The only problem with the patient option is that your most precious data doesn't get backed up while all your large, but less precious data is being backed up. For example, you may have a large amount of downloaded videos of conference presentations, which are nice, but not hugely important. While those get backed up, your own documents do not get backed up.
You can work around this by initially excluding everything except the most precious data. When that is backed up, you gradually reduce the excludes, re-running the backup, until you've backed up everything. As an example, your first backup might have the following configuration:
obnam backup -r /media/backups/tomjon-repo ~ \ --exclude ~/Downloads
This would exclude all downloaded files. The next backup run might exclude only video files:
obnam backup -r /media/backups/tomjon-repo ~ \ --exclude ~/Downloads/'.*\.mp4$'
After this, you might reduce excludes to allow a few videos, such as those whose name starts with a specific letter:
obnam backup -r /media/backups/tomjon-repo ~ \ --exclude ~/Downloads/'[^b-zB-Z].*\.mp4$'
Continue allowing more and more videos until they've all been backed up.
Obnam de-duplicates the data it backs up, across all files in all generations for all clients sharing the repository. It does this by breaking up all file data into bits called chunks. Every time Obnam reads a file and gets a chunk together, it looks into the backup repository to see if an identical chunk already exists. If it does, Obnam doesn't need to upload the chunk, saving space, bandwidth, and time.
De-duplication in Obnam is useful in several situations:
- When you have two identical files, obviously. They might have different names, and be in different directories, but contain the same data.
- When a file keeps growing, but all the new data is added at the end. This is typical for log files, for example. If the leading chunks are unmodified, only the new data needs to be backed up.
- When a file or directory is renamed or moved. If you decide that the
English name for the
Photosdirectory is annoying and you want to use the Finnish
Valokuvatinstead, you can rename that in an instant. However, without de-duplication, you then have to backup all your photos again.
- When all a team works on the same things, and everyone has copies of the same files, the backup repository only needs one copy of each file, rather than one per team member.
De-duplication in Obnam isn't perfect. The granularity of finding
duplicate data is quite coarse (see the
--chunk-size setting), and
so Obnam often doesn't find duplication when it exists, when the
changes are small.
De-duplication isn't useful in the following scenarios:
A file changes such that things move around within the file. The (current) Obnam de-duplication is based on non-overlapping chunks from the beginning of a file. If some data is inserted, Obnam won't notice that the chunks have shifted around. This can happen, for example, for disk or ISO images.
Files with duplicate data that is not on a chunk boundary. For example, emails with large attachments. Each email recipient gets different
Receivedheaders, which shifts the body and attachments by different amounts. As a result, Obnam won't notice the duplication.
Data in compressed files, such as
.tar.xzfiles. Obnam doesn't know about the file compression, and only sees the compressed version of the data. Thus, Obnam won'd de-duplicate it.
A future version of Obnam will hopefully improve the de-duplication algorithms. If you see this optimistic paragraph in a version of Obnam released in 2017 or later, please notify the maintainers. Thank you.
De-duplication and safety against checksum collisions
This is a bit of a scary topic, but it would be dishonest to not discuss it at all. Feel free to come back to this section later.
Obnam uses the MD5 checksum algorithm for recognising duplicate data chunks. MD5 has a reputation for being unsafe: people have constructed files that are different, but result in the same MD5 checksum. This is true. MD5 is not considered safe for security critical applications.
Every checksum algorithm can have collisions. Changing Obnam to use, say, SHA1, SHA2, or the as new SHA3 algorithm would not remove the chance of collisions. It would reduce the chance of accidental collisions, but the chance of those is already so small with MD5 that it can be disregarded. Or put in another way, if you care about the chance of accidental MD5 collisions, you should be caring about accidental SHA1, SHA2, or SHA3 collisions as well.
Apart from accidental collisions, there are two cases where you should worry about checksum collisions (regardless of algorithm).
First, if you have an enemy who wishes to corrupt your backed up data, they may replace some of the backed up data with other data that has the same checksum. This way, when you restore, your data is corrupted without Obnam noticing.
Second, if you're into researching checksum collisions, you're likely to have files that cause checksum collisions, and in that case, if you restore after a catastrophe, you probably want to get the files back intact, rather having Obnam confuse one with the other.
To deal with these situations, Obnam has three de-duplication modes,
set using the
- The default mode,
fatalist, assumes checksum collisions do not happen. This is a reasonable compromise between performance, safety, and security for most people.
verifymode assumes checksum collisions do happen, and verifies that the already backed up chunk is identical to the chunk to be backed up, by comparing the actual data. Doing this requires downloading the chunk from the backup repository, which can be quite slow, since checksums will often match. This is a useful mode if you have very fast access to the backup repository, and want to de-duplicate, such as when the backup repository is on a locally connected hard drive.
nevermode turns off de-duplication completely. This is useful if you're worried about checksum collisions, and do not require de-duplication.
There is, unfortunately, no way to get both de-duplication that is invulnerable to checksum collision and is fast even when accessing the backup repository is slow. The only way to be invulnerable is to compare the data, and if downloading the data from the repository is slow, then the comparison will take significant time.
Multiple clients can share a repository, and to prevent them from trampling on each other, they lock parts of the repository while working. The "Sharing a repository between multiple clients" chapter will discuss this in more detail.
If Obnam terminates abruptly, even if there's only one client ever using the repository, the lock may stay around and prevent that one client for making new backups. The termination may be due to the network connection breaking, or due to a bug in Obnam. It can also happen if Obnam is interrupted by the user before it's finished.
The Obnam command
force-lock deals with this situation. It is
dangerous, though. If you force open a lock that is in active use by
any running Obnam instance, on any client machine using that
repository, there will likely to be some stepping of toes. The result
may, in extreme cases, even result in repository corruption. So be
If you've decided you can safely do it, this is an example of how to do it:
obnam -r /media/backups/tomjon-repo force-lock
It is not currently possibly to only break locks related to one client.
Consistency of live data
Making a backup can take a good while. While the backup is running, the filesystem may change. This leads to the snapshot of data Obnam presents as a backup generation being internally inconsistent. For example, before a backup you might have two files, A and B, which need to be kept in sync. You run a backup, and while it's happening, you change A, and then B. However, you're unlucky, and Obnam manages to backup A before you save your changes, and B after you save changes to that. The backup generation now has versions of A and B that are not synchronised. This is bad.
This can be dealt with in various ways, depending on the circumstances. Here's a few:
- The Logical Volume Manager (LVM) provides snapshots. You can set up your backups so that they first create a snapshot of each logical volume to be backed up, run the backup, and delete the snapshot afterwards. This prevents anyone from modifying the files in the snapshot, but allows normal use to continue while the backup happens.
- A similar thing can be done using the btrfs filesystem and its subvolumes.
- You can shut down the system, reboot it into single user mode, and run the backup, before rebooting back into normal mode. This is not a good way to do it, but it is the safest way to get a consistent snapshot of the filesystem.
Note that filesystem level snapshots can't really guarantee a consistent view of the live data. An application may be in the middle of writing a file, or set of files, when the snapshot is being made. To some extent this indicates an application bug, but knowing that doesn't let you sleep better.
Usually, though, most systems have enough idle time that a consistent backup snapshot can happen during that time. For a laptop, for example, a backup can be run while the user is elsewhere, instead of actively using the machine.
Part of your backup verification suite should check that the data in a backup generation is internally consistent, if that can be done. Otherwise, you'll either have to analyse the applications you use, or trust they're not too buggy.
If you didn't understand this section, don't worry and be happy and sleep well.