ext4 - getting at birth time (file create time) and getting/setting nanosecond time stamps and utime

Björn Jacke bj at SerNet.DE
Wed Oct 21 17:03:36 MDT 2009

On 2009-10-21 at 10:36 -0500 Steve French sent off:
> On Wed, Oct 21, 2009 at 6:59 AM, Henrik Nordstrom
> <henrik at henriknordstrom.net> wrote:
> > utimen -> nanosecond  (struct timespec) (1/1000000000), same granularity
> > as current stat(), clock_gettime() and friends.
> Yes.   I have opened bug 6836 in samba bugzilla to track having smbd
> use utimensat (but not sure whether they will be able to detect when
> the file system can support nanosecond timestamps through - even with
> use of utimensat - so there may be questions about how to do runtime
> detection of whether the filesystem is one like ext4, xfs, jfs etc.
> that can store such timestamps, if not they probably still have to
> store 100 nanosecond "DCE time" granulariy timestamps in xattrs)

up to these days Samba just supports the granularity the undelying filesystem
supports and I think there is no need to change this. The utimensat
documentation says: "The file's relevant timestamp shall be set to the greatest
value supported by the file system that is not greater than the specified
time." So just using this call (where available) is all Samba will have to do -
I see nothing more to care about.

Unfortunately the designers of utimensat() didn't think of setting of birth
times on systems that have it. It would have been nice if this brand new call
had covered this, too. The FreeBSD folks thought about adding a system call to
set birth time.  All they currently have is a rather hackish[1] way to modify
it timewise in one direction.


[1] http://fuse4bsd.creo.hu/localcgi/man-cgi.cgi?utimes+2
    ... For file systems that support file birth (creation) times (such as UFS2),
    the birth time will be set to the value of the second element if the second
    element is older than the currently set birth time.

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