On Thu, 17 Jan 2008, Tom Lane wrote:

BTW, is a symlink's atime changed by accessing it?

It seems so in the cases I've tried or researched, but it's complicated. After burning through a bunch of time looking into this I wanted to drop some notes so nobody else has to wander down the specific dead-ends I just followed.

I figured I'd just run some experiments to figure this out for my Linux system, but that didn't go so well. The process of running anything that shows the atime:

  ls -l --time=atime <file>
  ls -lu <file>
  stat <file>

actually updates the atime to right now along the way. I hacked up something with perl that directly calls lstat() and it did the same thing.

Mystified, I found this thread suggesting the same thing is true on Mac OS X: http://lists.apple.com/archives/darwin-kernel/2006/Dec/msg00054.html

The point made in there is that how symlinks are encoded varies not just from OS to OS but from filesystem to filesystem, and that encoding changes how things like atime work. On Linux with ext2, I found this note:

"Symbolic links are also filesystem objects with inodes. They deserve special mention because the data for them is stored within the inode itself if the symlink is less than 60 bytes long. It uses the fields which would normally be used to store the pointers to data blocks."

So what I think is happening is: the process of doing anything at all with a Linux symlink references the inode that has the link. That updates the atime on that inode. But since there's no actual data underneath that lookup in cases where the link is less than 60 bytes, the inode atime is the link atime, so that just updated the link's atime to right now as well. I have no idea how any tmp cleaner could ever find a short symlink it can delete if I'm understanding this correctly.

I left behind the link I was just playing with and I'll see if I can get tmpwatch to eat it tomorrow, that seems like the most appropriate test here.

* Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD

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