FileBack PC uses the time stamp of files to determine if
they have changed. It compares the modification date and time of the files
in the source folder with the modification date and time of the files in the
destination folder, and if there is a difference, it considers this a change
in the file and performs another backup. Normally this works perfectly,
except when the time stamps of the files change due to a change in daylight
saving time.The underlying problem is actually caused by a
difference in the way that time stamps are handled with the different file
systems used by Windows (NTFS and FAT32). The time stamp stored by NTFS uses
an absolute time (GMT) that is converted to your local time whenever you
access a file on an NTFS disk. FAT32, on the other hand, stores the time
stamp in local time. So what happens when your computer changes in our out
of daylight time (or you change the time zone setting) is that the time stamps
on all files on NTFS drives appear to change, whereas time the time stamps
on FAT32 drives do not.
Why does FileBack PC appear to behave erratically?
If a backup job in FileBack PC accesses files using two different file
systems, NTFS on the source, and FAT32 on the destination, for example, this
causes the date stamps to be out of sync after a time change. So to bring them back into sync
FileBack PC has to do one of two things, depending on what version of the
product you are running.
The problem has been analyzed here at Maximum Output Software and we have
determined that there isn't a perfect way to handle this
situation. There are too many possible scenarios to properly address them
all (a few examples: How do we determine what time zone the computers
containing the source and destination folders are in? -- there is no
official way to
determine the time zone of a remote or networked computer. Are they
currently observing daylight time? -- same issue. Which file system
contains the "correct" date/time for a file?). So we recommend that (1) the same file system be used on all
disks that FileBack PC is set to use, or (2) allow this backup to take place
twice per year whenever going into or out of daylight time.
FileBack PC Releases Since July 2008
In the July 2008 release of FileBack PC some intelligence was added to better
handle this situation than it has been addressed in the past. FileBack PC
will compare the dates and times of the files in the source and destination, and
if they differ by exactly one hour (usually as a result of a change into our out
of daylight saving time), the date and time of the files in the destination will
be adjusted to match the source without recopying the files. This
technique only will be employed, however, if the file systems used by the source
and destination are different (one uses NTFS, the other FAT32, for example), to
minimize the chance that a file legitimately modified one hour after its
previous version is excluded during a backup.
This logic will not work in all situations, however. For example, some
Network Attached Storage (NAS) devices and Samba servers report that they are
using the NTFS file system (when they are not) and report file dates and times
incorrectly as a result. In these cases files must be re-copied to ensure
that they really have not been modified. Since NAS devices attempt to make
themselves transparent, they are difficult to detect, and it is even harder to
determine what quirks they exhibit. It is not feasible to add workarounds
for all improper device behavior.
FileBack PC Releases Before July 2008
FileBack PC must re-copy or resynchronize all files to make sure that the
file dates and times are the same in both the source and destination.
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