On Nov 19, 2007 7:47 AM, David Brown <git@xxxxxxxxxx> wrote: > On Sun, Nov 18, 2007 at 10:34:55PM +1300, Martin Langhoff wrote: > > >I do hope anyone doing those things is _very_ aware that the mtime > >metadata has a specific meaning -- when did this specific file in this > >filesystem last change -- and is used by many tools in that sense. You > >are trying to use it for something else. Lots of things will break. > > > >Like incremental backups, for example. > > 'mtime' does _not_ have the specific meaning of 'when did this specific > file last change'. That is the 'ctime' field. 'mtime' is also updated > when a file is modified, but can be changed by the user. Many utilities > restore mtime to older values, including tar. Hmmm. After a bit of googling I've found conflicting descriptions of the mtime/ctime semantics (I thought - for 10 years now - that ctime was "creation time", it is "changed time"). Some people think that anything that updates mtime also updates ctime, and others say the opposite. Wikipedia says (at http://en.wikipedia.org/wiki/MAC_times and http://en.wikipedia.org/wiki/Stat_%28Unix%29 ) that mtime is about the content, and ctime about metadata (owner, permissions, moved inode, etc). Changes in content "touch" mtime + ctime. With that in mind, I think it makes sense for things like make and amanda to read mtime as referring to a real change of that concrete file. The abstract notion of the file having changed in the big DSCM in the sky is useful, but putting that data in mtime messes things up. > However, it will make 'make' very confusing, since it uses the mtime to > determine if files are out of date. If moving to an older version of a > file causes the file to become older, make won't recompile. This is > arguably a defect in make, but that is how it works. It's not a bug in make. mtime has a definite meaning, and make is using that meaning. Same with amanda. cheers, m - To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
