time-shifting

[Evert Mouw <post-yfx6isLFVcpeoWH0uzbU5w@public.gmane.org>]

Hi,

Yesterday I played a bit with NILFS2 on ubuntu 11.04. It was quite easy 
to use, but finding previous versions of a file is still a bit 
complicated from a user point of view.

I read a bit about ext3cow, which is currently not actively maintained 
anymore. In one of the papers on ext3cow, I read the following:

Other disk file systems provide coarse-grained access to versions through
the creation of namespace tunnels [Hitz et al. 1994] or via mounting 
separate logical volumes
[Strunk et al. 2000; Soules et al. 2003]. Some disk file systems provide 
no interface
to versions, restricting versioning to internal use only [Rosenblum and 
Ousterhout 1992;
Seltzer et al. 1993].

In time-shifting, ext3cow introduces an interface to versioning that 
presents a continuous
view of time. Users or applications specify a file name and any point in 
time, which
ext3cow scopes to the appropriate snapshot or file version. The 
time-shifting interface
allows user-space tools to create snapshots and access versions.

Ext3cow: A Time-Shifting File System for Regulatory Compliance
ZACHARY N. J. PETERSON and RANDAL BURNS
ACM Transactions on Storage, 1(2), May, 2005
http://www.ext3cow.com/ext3cow/Publications.html

A usage scenario is given in a screenshot:
http://www.ext3cow.com/ext3cow/Welcome_files/example1.jpg

I don't think using the "@" symbol for such thinks is a great idea (I 
sometimes use it in my filenames) but the ability to use regular tools 
to access old versions of files is great.

I think it would be really useful to have access to previous versions of 
files without having to go through checkpoints or snapshots of the whole 
filesystem / partition. So having something like lshistory or lsversions 
for specific files would be great. It could output the versions, the 
checkpoints where those versions are available, etc.  Maybe direct 
retrieval of old versions (cathistory?) could also be possible using 
such a path.

Of course the architecture of NILFS is different from ext3cow so I don't 
know whether this is even possible. Something like a data structure 
linking the different verions of some inode must exist before this can 
happen. Just an idea, though.

Thanks for your cool stuff!

Evert

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