Frequently asked questions¶
Can I backup VM disk images?¶
Yes, the deduplication technique used by Borg makes sure only the modified parts of the file are stored. Also, we have optional simple sparse file support for extract.
Can I backup from multiple servers into a single repository?¶
Yes, but in order for the deduplication used by Borg to work, it
needs to keep a local cache containing checksums of all file
chunks already stored in the repository. This cache is stored in
~/.cache/borg/. If Borg detects that a repository has been
modified since the local cache was updated it will need to rebuild
the cache. This rebuild can be quite time consuming.
So, yes it’s possible. But it will be most efficient if a single repository is only modified from one place. Also keep in mind that Borg will keep an exclusive lock on the repository while creating or deleting archives, which may make simultaneous backups fail.
Which file types, attributes, etc. are preserved?¶
- Regular files
- Hardlinks (considering all files in the same archive)
- Symlinks (stored as symlink, the symlink is not followed)
- Character and block device files
- FIFOs (“named pipes”)
- Time of last modification (nanosecond precision with Python >= 3.3)
- IDs of owning user and owning group
- Names of owning user and owning group (if the IDs can be resolved)
- Unix Mode/Permissions (u/g/o permissions, suid, sgid, sticky)
- Extended Attributes (xattrs) on Linux, OS X and FreeBSD
- Access Control Lists (ACL) on Linux, OS X and FreeBSD
- BSD flags on OS X and FreeBSD
Which file types, attributes, etc. are not preserved?¶
- UNIX domain sockets (because it does not make sense - they are meaningless without the running process that created them and the process needs to recreate them in any case). So, don’t panic if your backup misses a UDS!
- The precise on-disk representation of the holes in a sparse file. Archive creation has no special support for sparse files, holes are backed up as (deduplicated and compressed) runs of zero bytes. Archive extraction has optional support to extract all-zero chunks as holes in a sparse file.
Why is my backup bigger than with attic? Why doesn’t Borg do compression by default?¶
Attic was rather unflexible when it comes to compression, it always compressed using zlib level 6 (no way to switch compression off or adjust the level or algorithm).
Borg offers a lot of different compression algorithms and levels. Which of them is the best for you pretty much depends on your use case, your data, your hardware – so you need to do an informed decision about whether you want to use compression, which algorithm and which level you want to use. This is why compression defaults to none.
How can I specify the encryption passphrase programmatically?¶
The encryption passphrase can be specified programmatically using the BORG_PASSPHRASE environment variable. This is convenient when setting up automated encrypted backups. Another option is to use key file based encryption with a blank passphrase. See Repository encryption for more details.
Be careful how you set the environment; using the
system() call or using inline shell scripts
might expose the credentials in the process list directly
and they will be readable to all users on a system. Using
export in a shell script file should be safe, however, as
the environment of a process is accessible only to that
When backing up to remote encrypted repos, is encryption done locally?¶
Yes, file and directory metadata and data is locally encrypted, before leaving the local machine. We do not mean the transport layer encryption by that, but the data/metadata itself. Transport layer encryption (e.g. when ssh is used as a transport) applies additionally.
When backing up to remote servers, do I have to trust the remote server?¶
Yes and No.
No, as far as data confidentiality is concerned - if you use encryption, all your files/dirs data and metadata are stored in their encrypted form into the repository.
Yes, as an attacker with access to the remote server could delete (or otherwise make unavailable) all your backups.
The borg cache eats way too much disk space, what can I do?¶
There is a temporary (but maybe long lived) hack to avoid using lots of disk space for chunks.archive.d (see issue #235 for details):
# this assumes you are working with the same user as the backup cd ~/.cache/borg/<REPOID> rm -rf chunks.archive.d ; touch chunks.archive.d
This deletes all the cached archive chunk indexes and replaces the directory that kept them with a file, so borg won’t be able to store anything “in” there in future.
This has some pros and cons, though:
- much less disk space needs for ~/.cache/borg.
- chunk cache resyncs will be slower as it will have to transfer chunk usage metadata for all archives from the repository (which might be slow if your repo connection is slow) and it will also have to build the hashtables from that data. chunk cache resyncs happen e.g. if your repo was written to by another machine (if you share same backup repo between multiple machines) or if your local chunks cache was lost somehow.
The long term plan to improve this is called “borgception”, see ticket #474.
If a backup stops mid-way, does the already-backed-up data stay there?¶
Yes, Borg supports resuming backups.
During a backup a special checkpoint archive named
is saved every checkpoint interval (the default value for this is 5
minutes) containing all the data backed-up until that point. This means
that at most <checkpoint interval> worth of data needs to be retransmitted
if a backup needs to be restarted.
Once your backup has finished successfully, you can delete all
If it crashes with a UnicodeError, what can I do?¶
Check if your encoding is set correctly. For most POSIX-like systems, try:
export LANG=en_US.UTF-8 # or similar, important is correct charset
I can’t extract non-ascii filenames by giving them on the commandline!?¶
This might be due to different ways to represent some characters in unicode or due to other non-ascii encoding issues.
If you run into that, try this:
- avoid the non-ascii characters on the commandline by e.g. extracting the parent directory (or even everything)
- mount the repo using FUSE and use some file manager
Can Borg add redundancy to the backup data to deal with hardware malfunction?¶
No, it can’t. While that at first sounds like a good idea to defend against some defect HDD sectors or SSD flash blocks, dealing with this in a reliable way needs a lot of low-level storage layout information and control which we do not have (and also can’t get, even if we wanted).
So, if you need that, consider RAID or a filesystem that offers redundant storage or just make backups to different locations / different hardware.
See also ticket 225.
Can Borg verify data integrity of a backup archive?¶
Yes, if you want to detect accidental data damage (like bit rot), use the
check operation. It will notice corruption using CRCs and hashes.
If you want to be able to detect malicious tampering also, use a encrypted
repo. It will then be able to check using CRCs and HMACs.
I am seeing ‘A’ (added) status for a unchanged file!?¶
The files cache is used to determine whether Borg already “knows” / has backed up a file and if so, to skip the file from chunking. It does intentionally not contain files that:
- have >= 10 as “entry age” (Borg has not seen this file for a while)
- have a modification time (mtime) same as the newest mtime in the created archive
So, if you see an ‘A’ status for unchanged file(s), they are likely the files with the most recent mtime in that archive.
This is expected: it is to avoid data loss with files that are backed up from a snapshot and that are immediately changed after the snapshot (but within mtime granularity time, so the mtime would not change). Without the code that removes these files from the files cache, the change that happened right after the snapshot would not be contained in the next backup as Borg would think the file is unchanged.
This does not affect deduplication, the file will be chunked, but as the chunks will often be the same and already stored in the repo (except in the above mentioned rare condition), it will just re-use them as usual and not store new data chunks.
Since only the files cache is used in the display of files status, those files are reported as being added when, really, chunks are already used.
Why was Borg forked from Attic?¶
Borg was created in May 2015 in response to the difficulty of getting new code or larger changes incorporated into Attic and establishing a bigger developer community / more open development.
More details can be found in ticket 217 that led to the fork.
Borg intends to be:
- as simple as possible, but no simpler
- do the right thing by default, but offer options
- welcome feature requests
- accept pull requests of good quality and coding style
- give feedback on PRs that can’t be accepted “as is”
- discuss openly, don’t work in the dark
- Borg is not compatible with Attic
- do not break compatibility accidentally, without a good reason or without warning. allow compatibility breaking for other cases.
- if major version number changes, it may have incompatible changes