Differences

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

--- sneaker_net_sync [2013/05/22 23:59] – [Daylight Saving Issues] 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 113: / Line 123: @@
 ''None'': Nothing special is done. **All** files will be resynced when going into daylight saving time and back to standard time. Typically, this will take a long time and will create a huge delta file and/or may require multiple resyncs to 'catch up'. Once the files are resynced, the file modified dates will match between the master and secondary computers.
-Only use this option if Sneaker Net Sync is not affected by the daylight saving issue, or there a concern about the [[#the_edge_case|edge case]].
+Only use this option if Sneaker Net Sync is not affected by the daylight saving issue, or there a concern about the [[#the_daylight_saving_edge_case|edge case]].
 ''Ignore time differences'': If the name, file size and hash (if used) match and the modified date is different by the amount specified by the ''Time Difference'' setting (+/- 3 seconds) the difference will be ignored and the file will be considered to be 'in sync'. Nothing special will happen when going into daylight saving time or back to standard time. Files added or modified during daylight saving time will have modified dates that do not match the master computer's files. These files will also have modified dates that are incorrect relative to the files that were //not// modified during daylight saving time. The incorrect modified dates will never resync.
@@ Line 121: / Line 131: @@
 ''Correct time differences'': If the name, file size and hash (if used) match and the modified date is different by the amount specified by the ''Time Difference'' setting (+/- 3 seconds) the modified will be resynced, but not the contents of the file. When going into daylight saving time or back to standard time, **all** the files will have their modified resynced. Typically, this does not take too long and does not result in a very large delta file. Once the modified dates are resynced, they will remain synced.
-Use this option if it is important that the modified date remain synced.
+Use this option if it is important that the modified date remain synced as much as possible.
 ''Correct time ... ignore time...'': When it is daylight saving time, modified dates are ignored as described above. When it is standard time, modified dates corrected as described above. When going into daylight saving time, nothing special will happen. When going back to standard time, incorrect modified dates will be resynced. This will only affect files that were added or updated during the daylight saving period so will typically be much faster than the ''Correct time differences'' option. Modified dates will be synced during standard time. During daylight saving time, the modified dates of new or updated files will be correct.
@@ Line 129: / Line 139: @@
 ''Time Difference'': The amount of time that should be ignored or corrected.
-==== The Edge Case ====
+==== The Daylight Saving Edge Case ====
+If the Time Zone Options is set to anything other than ''None'', there is a known edge case that could potentially cause Sneaker Net Sync to fail recognise that a file has changed - although the chances of this occurring are very remote. It would have to happen like this:
+  - A file is modified on the master computer.
+  - Sneaker Net Sync creates a delta file.
+  - 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.