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add FAQ entry about unexpected 'A' status for unchanged file(s), fixes #403

This commit is contained in:
Thomas Waldmann 2015-11-16 16:03:55 +01:00
parent 38472900af
commit 559c8908c1

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@ -165,6 +165,32 @@ Yes, if you want to detect accidental data damage (like bit rot), use the
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!?
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The files cache (which is used to determine whether |project_name| already
"knows" / has backed up a file and if so, to skip the file from chunking)
does intentionally *not* contain files that:
- have >= 10 as "entry age" (|project_name| 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 |project_name| 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.
Why was Borg forked from Attic?
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