Hi, On Sat, 06 Aug 2011 09:38:26 +0200, Evert Mouw wrote: > 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. If memory serves me right, ext3cow allocates an inode every time its file is changed and a pointer to the previous inode is stored in the new inode. At present, nilfs does not have such per-file pointer to past versions, so actually it's not optimal for file history lookups. We may be able to do indirectly by adding a new field to on-disk inode to keep a checkpoint number in which the inode lastly changed. However, the new field must be updated every time garbage collector thins out the previous version, -- this may complicate things unacceptably because the current nilfs is designed so that it never overwrites in-use blocks except super blocks. Apart from these implementation issues, I don't feel the "@" symbol semantics is odd, though it can come under fire from related kernel developers as a violation of namespace semantics. With regards, Ryusuke Konishi > Thanks for your cool stuff! > > Evert -- To unsubscribe from this list: send the line "unsubscribe linux-nilfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
