Here are misc. notes about topics that are maybe not covered in enough detail in the usage section.
--chunker-params
¶The chunker params influence how input files are cut into pieces (chunks) which are then considered for deduplication. They also have a big impact on resource usage (RAM and disk space) as the amount of resources needed is (also) determined by the total amount of chunks in the repository (see Indexes / Caches memory usage for details).
--chunker-params=buzhash,10,23,16,4095
results in a fine-grained deduplication|
and creates a big amount of chunks and thus uses a lot of resources to manage
them. This is good for relatively small data volumes and if the machine has a
good amount of free RAM and disk space.
--chunker-params=buzhash,19,23,21,4095
(default) results in a coarse-grained
deduplication and creates a much smaller amount of chunks and thus uses less
resources. This is good for relatively big data volumes and if the machine has
a relatively low amount of free RAM and disk space.
--chunker-params=fixed,4194304
results in fixed 4MiB sized block
deduplication and is more efficient than the previous example when used for
for block devices (like disks, partitions, LVM LVs) or raw disk image files.
--chunker-params=fixed,4096,512
results in fixed 4kiB sized blocks,
but the first header block will only be 512B long. This might be useful to
dedup files with 1 header + N fixed size data blocks. Be careful not to
produce a too big amount of chunks (like using small block size for huge
files).
If you already have made some archives in a repository and you then change chunker params, this of course impacts deduplication as the chunks will be cut differently.
In the worst case (all files are big and were touched in between backups), this will store all content into the repository again.
Usually, it is not that bad though:
usually most files are not touched, so it will just re-use the old chunks it already has in the repo
files smaller than the (both old and new) minimum chunksize result in only one chunk anyway, so the resulting chunks are same and deduplication will apply
If you switch chunker params to save resources for an existing repo that already has some backup archives, you will see an increasing effect over time, when more and more files have been touched and stored again using the bigger chunksize and all references to the smaller older chunks have been removed (by deleting / pruning archives).
If you want to see an immediate big effect on resource usage, you better start a new repository when changing chunker params.
For more details, see Chunks.
--noatime / --noctime
¶You can use these borg create
options not to store the respective timestamp
into the archive, in case you do not really need it.
Besides saving a little space for the not archived timestamp, it might also affect metadata stream deduplication: if only this timestamp changes between backups and is stored into the metadata stream, the metadata stream chunks won’t deduplicate just because of that.
--nobsdflags / --noflags
¶You can use this not to query and store (or not extract and set) flags - in case you don’t need them or if they are broken somehow for your fs.
On Linux, dealing with the flags needs some additional syscalls. Especially when dealing with lots of small files, this causes a noticeable overhead, so you can use this option also for speeding up operations.
--umask
¶borg uses a safe default umask of 077 (that means the files borg creates have only permissions for owner, but no permissions for group and others) - so there should rarely be a need to change the default behaviour.
This option only affects the process to which it is given. Thus, when you run
borg in client/server mode and you want to change the behaviour on the server
side, you need to use borg serve --umask=XXX ...
as a ssh forced command
in authorized_keys
. The --umask
value given on the client side is
not transferred to the server side.
Also, if you choose to use the --umask
option, always be consistent and use
the same umask value so you do not create a mixup of permissions in a borg
repository or with other files borg creates.
--read-special
¶The --read-special
option is special - you do not want to use it for normal
full-filesystem backups, but rather after carefully picking some targets for it.
The option --read-special
triggers special treatment for block and char
device files as well as FIFOs. Instead of storing them as such a device (or
FIFO), they will get opened, their content will be read and in the backup
archive they will show up like a regular file.
Symlinks will also get special treatment if (and only if) they point to such a special file: instead of storing them as a symlink, the target special file will get processed as described above.
One intended use case of this is backing up the contents of one or multiple block devices, like e.g. LVM snapshots or inactive LVs or disk partitions.
You need to be careful about what you include when using --read-special
,
e.g. if you include /dev/zero
, your backup will never terminate.
Restoring such files’ content is currently only supported one at a time via
--stdout
option (and you have to redirect stdout to where ever it shall go,
maybe directly into an existing device file of your choice or indirectly via
dd
).
To some extent, mounting a backup archive with the backups of special files
via borg mount
and then loop-mounting the image files from inside the mount
point will work. If you plan to access a lot of data in there, it likely will
scale and perform better if you do not work via the FUSE mount.
Imagine you have made some snapshots of logical volumes (LVs) you want to back up.
Note
For some scenarios, this is a good method to get “crash-like” consistency (I call it crash-like because it is the same as you would get if you just hit the reset button or your machine would abruptly and completely crash). This is better than no consistency at all and a good method for some use cases, but likely not good enough if you have databases running.
Then you create a backup archive of all these snapshots. The backup process will see a “frozen” state of the logical volumes, while the processes working in the original volumes continue changing the data stored there.
You also add the output of lvdisplay
to your backup, so you can see the LV
sizes in case you ever need to recreate and restore them.
After the backup has completed, you remove the snapshots again.
$ # create snapshots here
$ lvdisplay > lvdisplay.txt
$ borg create --read-special arch lvdisplay.txt /dev/vg0/*-snapshot
$ # remove snapshots here
Now, let’s see how to restore some LVs from such a backup.
$ borg extract arch lvdisplay.txt
$ # create empty LVs with correct sizes here (look into lvdisplay.txt).
$ # we assume that you created an empty root and home LV and overwrite it now:
$ borg extract --stdout arch dev/vg0/root-snapshot > /dev/vg0/root
$ borg extract --stdout arch dev/vg0/home-snapshot > /dev/vg0/home
Borg does not auto-compact the segment files in the repository at commit time (at the end of each repository-writing command) any more (since borg 1.2.0).
This has some notable consequences:
repository space is not freed immediately when deleting / pruning archives
commands finish quicker
repository is more robust and might be easier to recover after damages (as
it contains data in a more sequential manner, historic manifests, multiple
commits - until you run borg compact
)
user can choose when to run compaction (it should be done regularly, but not necessarily after each single borg command)
user can choose from where to invoke borg compact
to do the compaction
(from client or from server, it does not need a key)
less repo sync data traffic in case you create a copy of your repository by using a sync tool (like rsync, rclone, …)
You can manually run compaction by invoking the borg compact
command.
When running Borg using an automated script, ssh
might still ask for a password,
even if there is an SSH key for the target server. Use this to make scripts more robust:
export BORG_RSH='ssh -oBatchMode=yes'