Differences

This shows you the differences between two versions of the page.

--- sneaker_net_sync [2013/06/12 04:51] – [The Workaround] stephen
+++ sneaker_net_sync [2017/01/01 20:05] (current) – external edit 127.0.0.1
@@ Line 87: / Line 87: @@
 Click ''Start'' to apply the delta file.
+===== Edge Cases =====
+There is a possible situation that would prevent Sneaker Net Sync from detecting that a file has changed. When a computer program modifies the contents of a file the operating system (Windows) automatically updates the modified date of the file. However, a program can also directly set the modified date to anything. A program could theoretically read the modified date of a file, change the contents of the it and then set the modified date back to its original value. This would be very strange behaviour indeed. If the length of the file did not change, Sneaker Net Sync would not detect the change and would not ever re-sync the file.
+This edge case is very unlikely to happen, but if it was a concern, hash values could be used. This would prevent the edge case, but using hash values dramatically slows down the indexing and delta creation process.
+Well... almost. Hash values do not //fully// eliminate the edge case. While it is incredibly unlikely that two files with different content would have the same hash value, it is technically //possible//. Sneaker Net Sync uses a 128-bit hash, which means that there are approximately 3.402 × 10<sup>38</sup> possible unique hash values. Assuming truly random distribution, the chances of two files having the same hash value is about the same as throwing three pins at the Earth from space such that they land at some random point on the Earth's surface, and all three land within 1 mm of each other (the Earth's surface area is approximately 5.1 × 10<sup>20</sup>mm<sup>2</sup>).
+If this possibility is unacceptable, the only option would be to fully recopy **all** the files every time a re-sync is done. Even this would not absolutely and completely guarantee that the files are synced - there could be read or write errors while copying the files to and from the transport media.
 ===== Daylight Saving Issues =====
@@ Line 135: / Line 145: @@
   - A file is modified on the master computer.
   - Sneaker Net Sync creates a delta file.
-  - The same is modified, without changing its length, one hour (+/- 3 seconds) after it was originally modified.
+  - The same file is modified, without changing its length, one hour (+/- 3 seconds) after it was originally modified.
 Because the modification date of the file will have increased by one hour and the length remained the same, Sneaker Net Sync will either ignore the change or update the modified date without updating the contents.
 If this edge case is a concern, the Time Zone Options can be set to ''None'' or hash values can be used.