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add warnings on repository copies to avoid future problem like #4272 (#4285)

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docs: add warnings about repo copies to avoid problems like #4272
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anarcat 2019-02-10 14:23:59 -05:00 committed by TW
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docs/faq.rst
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@ -26,29 +26,53 @@ or deleting archives, which may make *simultaneous* backups fail.
Can I copy or synchronize my repo to another location?
------------------------------------------------------
Yes, you could just copy all the files. Make sure you do that while no
backup is running. If you copy a repository while a backup is running,
the lock held will be present in the copy. Thus, before using borg on the copy
from a different host, you need to use :ref:`break-lock` on the copied
repository, because Borg is cautious and does not automatically remove
stale locks made by a different host.
So what you get here is this:
- client machine ---borg create---> repo1
- repo1 ---copy---> repo2
There is no special borg command to do the copying, just use cp or rsync if
you want to do that.
But think about whether that is really what you want. If something goes
wrong in repo1, you will have the same issue in repo2 after the copy.
If you want to have 2 independent backups, it is better to do it like this:
If you want to have redundant backup repositories (preferably at separate
locations), the recommended way to do that is like this:
- ``borg init repo1``
- ``borg init repo2``
- client machine ---borg create---> repo1
- client machine ---borg create---> repo2
This will create distinct repositories (separate repo ID, separate
keys) and nothing bad happening in repo1 will influence repo2.
Some people decide against above recommendation and create identical
copies of a repo (using some copy / sync / clone tool).
While this might be better than having no redundancy at all, you have
to be very careful about how you do that and what you may / must not
do with the result (if you decide against our recommendation).
What you would get with this is:
- client machine ---borg create---> repo
- repo ---copy/sync---> copy-of-repo
There is no special borg command to do the copying, you could just
use any reliable tool that creates an identical copy (cp, rsync, rclone
might be options).
But think about whether that is really what you want. If something goes
wrong in repo, you will have the same issue in copy-of-repo.
Make sure you do the copy/sync while no backup is running, see
:ref:`borg_with-lock` about how to do that.
Also, you must not run borg against multiple instances of the same repo
(like repo and copy-of-repo) as that would create severe issues:
- Data loss: they have the same repository ID, so the borg client will
think they are identical and e.g. use the same local cache for them
(which is an issue if they happen to be not the same).
See :issue:`4272` for an example.
- Encryption security issues if you would update repo and copy-of-repo
independently, due to AES counter reuse.
There is also a similar encryption security issue for the disaster case:
If you lose repo and the borg client-side config/cache and you restore
the repo from an older copy-of-repo, you also run into AES counter reuse.
Which file types, attributes, etc. are *not* preserved?
-------------------------------------------------------