pmirror: Add option --noatime

[donald]

Open source file with O_NOATIME if option --noatime is given.

If pmirror is used for backup purpose, we don't want to trigger metadata
writes on the source filesystem. This needs to be optional, because
O_NOATIME only works for the owner of a file or for a privileged user.

As --cksum is implemented by an external command, we can't do this
without potentially touching the access time.

Related: #123

[pmenzel]

Nice. Looks good to me.

I’d use (present tense and more common word):

Closes: #123

[wwwutz]

Whatever. I consider atime as access time. Whoever accesses a file. It's the access time and if pmirror read()s the file: it does exactly that. This is just plain wrong not to set it if mount has enabled it. You fell for a stupid request in my opinion. But as I said: whatever. I'd vote for noatime mount option anyway which would obsolete it. As would a different file system.

I just don't understand what problems you would like to solve.

whatever.

[donald]

Just adding the feature --noatime to pmirror can't be too wrong, because its optional and off by default.

IMO backup software should not modify the source filesystem in a user visible way, just as defragmentation software, virus checker, filesystem checks or raid verifyer shouldn't do that, although these tools also read data from the files.

So adding this feature might make it more suitable to be used for backup (as we do). So there might be a pull request to add "--noatime" no mxmirror and that might be another and maybe more appropriate place to argue about whether we want or don't want out backup processes to touch metadata on the source filesystem.