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    <p>You can probably skip step 1 completely. The file can't be deleted while it's open, and choosing appropriate locking flags when opening it can prevent it from being modified as well. For example, the <a href="http://msdn.microsoft.com/en-us/library/aa363858%28v=vs.85%29.aspx" rel="nofollow">CreateFile</a> function takes a dwShareMode argument.</p> <p>You need to compute the entire rdiff signature before you can start creating the rdiff delta. You can avoid reading the entire file by computing signatures and then deltas for each (say) 100 MB block of the file at a time. You will lose some compression efficiency this way*. You might also consider switching from rdiff to <a href="http://xdelta.org/" rel="nofollow">xdelta</a>, which can create a delta file in a single pass over the input.</p> <p>Compression and encryption can be done in parallell with computing the delta. If the compression and encryption is done by separate programs, they often allow reading from standard input and writing to standard output. This can be used easiest by pipes in a batch file, for example:</p> <pre><code>rdiff signature oldfile oldfile.sig rdiff delta oldfile.sig newfile | gzip -c | gpg -e -r ... &gt; compressed_encrypted_delta </code></pre> <p>If you use libraries for compression/encryption in your program, you will need to choose libraries that support streaming operation.</p> <p>*or lose a lot of efficiency if data is moved around in the file. If someone prepends 100 MB to a 10 GB file, rdiff will produce a delta file of about 100MB. rdiff done in blocks of 100 MB or less at a time will produce about 10 GB of delta. Blocks of 200 MB will produce about 5 GB of delta, since only half the data in each block is from the corresponding block of the old version of the file.</p>
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    1. POStream rdiff - delta differencing?
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    1. COWhat do you mean, the "appropriate locking flags"? How do I accomplish that? Also, you said "Compression and encryption can be done in parallell with computing the delta", how is this accomplished? I just don't see (after hours of trying to implement this) how it is possible. What would you do?
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    2. COUpdated answer.
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