[Solved] Reflect all changes to a filesystem in another filesystem

Question

I'm looking for a solution to the following problem:
I want to setup an external harddisk so that two different partitions on it behave in the following way:
Given an ext4 partition mounted rw at /mnt/ext and a NTFS partition mounted also rw at /mnt/ntfs, any changes I do under /mnt/ext should be reflected exactly under /mnt/ntfs minus everything ext4 specific. The NTFS partition should work as an exact mirror of the ext4 partition, reflecting all writes, deletes, and changes in the directory structure.
The purpose of this is having a Linux native FS with all the journaling and stuff for normal use, while being able to go around in a Windows/Mac infested  environment and pass on data from the NTFS partition to random users without having to maintain/sync everything to the NTFS partition beforehand.
I have looked into union mounts, especially aufs3 as a seemingly up-to-date and actively maintained project, but got a little bit lost in the documentation because of its complexity.
From the LWN series on unioning filesystems (a little old) at http://lwn.net/Articles/396020 (under the section "background") I learned that union FSs are mostly used to stack one/or multiple read-write FS onto a read-only FS.
At least for aufs, its design documentation https://github.com/sfjro/aufs3-linux/bl … 1intro.txt states that it supports "multiple writable branches", but "several policies to select one among multiple writable branches", indicating that a certain change can be written only to one of several branches at the same time, which, as explained above, is not my intention.
Does somebody know whether such a setup can be realised using aufs or any other working union fs solution, or could point me towards a more simple and better solution? (Or at least tell me that it is impossible.)
Thanks in advance.
Last edited by 2ion (2013-08-19 22:35:57)

Answer 1

WonderWoofy wrote:
Yeah, aufs won't actually mirror the filesystem like you want here.  Honestly, I'm not entirely sure why you don't just use the NTFS filesystem to store these files anyway.  I mean, unless you have some sort of performance requriements for the files stored on the ext4 side, this seems unnnecessary.
But if this setup is a requirement for whatever you want to do here (for example if the NTFS is on removable media and the ext4 is internal) then I would just set up an rsync on some kind of cronjob or systemd.timer.  This of course would mean that you would have a kind of delay for when the actual changes occurred, but with a sufficiently low interval, I think it would be fine (not too low or you might end up with multiple rsync processes occuring at the same time).
Maybe someone else might have a better idea...
Yes, the reason I'm uncomfortable with using NTFS is that in the past, I once ended up with a damaged NTFS file system (invalid data read, errors on write) which to repair I found impossible using just the NTFS tools available on Linux. The internet basically said to go and use Windows to fix it, but I at the time didn't have a Windows system, and possibly won't have one in the future – thus I can't rely on NTFS. And I'd rather not run a Windows system utility under WINE.