Re: [PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]

[Benny Halevy <bhalevy@panasas.com>]

On Jul. 22, 2010, 20:24 +0300, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Thu, Jul 22, 2010 at 10:03 AM, Jan Engelhardt <jengelh@medozas.de> wrote:
>>
>> I beg to differ. ctime is not completely useless. It reflects changes on
>> the inode for when you don't you change the content.
> 
> Uh. Yes. Except that why is file metadata really different from file
> data? Most people really don't care. And a lot of people have asked
> for creation dates - and I seriously doubt that Windows people
> complain a lot about the fact that there you have mtime for metadata
> changes too.
> 
> The point being that Unix ctime semantics certainly have well-defined
> semantics, but they are in no way "better" than having a real creation
> time, and are often worse.

Yeah, having create time would be important.
That said, having a non user-settable modify timestamp is crucial
for quickly determining whether a file has changed.

Benny

> 
> Just imagine what you could do as an MIS person if you actually had a
> creation time you could somewhat trust? You talk about seeing somebody
> change the permissions of /etc/passwd, but realistically, absent
> preexisting semantics, who would really ask for that? The only reason
> you mention that as an example of what you can do with ctime is that
> that is indeed pretty much the _only_ thing you can do with ctime, and
> it really isn't that useful.
> 
> In contrast, with a creation date, you see the difference between
> people overwriting files by writing to them, or overwriting files by
> creating a new one and moving it over the old one. At a guess, that
> would be quite as useful to a sysadmin as ctime is now (my gut feel is
> that it would be more so, but whatever).
> 
> IOW, there really isn't anything magically good about UNIX ctime
> semantics, and in fact they are totally broken in the presence of
> extended attributes (that's file data, but it only changes ctime? WTF
> is up with that? Yes, I know why it happens, and it makes sense within
> the insane unix ctime rules, but no way does it make sense in a bigger
> picture unless you are in total denial and try to claim that xattrs
> are just metadata despite having contents).
> 
> And yes, I am also sure that there are applications that do depend on
> ctime semantics. Trond mentioned NFS serving, and that's unfortunate.
> I bet there are others. That's inevitable when you have 40 years of
> history. So I'm not claiming that re-using ctime is painfree, but for
> somebody that cares about samba a lot, I bet it's a _lot_ better than
> adding a new time that almost nobody actually supports as things stand
> now.
> 
> Of people can just use xattrs and do it all entirely in user space. I
> assume that's what samba does now, even outside of birthtime.
> 
>                      Linus
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