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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 15 Mar 2018 21:38:29 +0100
Message-ID: <CAK8P3a2JgVVRsiBzTuUACwmCoVFkkJbF9zGfk4Yu0KzzRV=81Q@mail.gmail.com>
Subject: Re: [RFC v2 03/83] Add super.h.
To:     Andiry Xu <jix024@eng.ucsd.edu>
Cc:     "Darrick J. Wong" <darrick.wong@oracle.com>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>,
        Dan Williams <dan.j.williams@intel.com>,
        "Rudoff, Andy" <andy.rudoff@intel.com>, coughlan@redhat.com,
        Steven Swanson <swanson@cs.ucsd.edu>,
        Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.com>,
        swhiteho@redhat.com, miklos@szeredi.hu,
        Jian Xu <andiry.xu@gmail.com>, Andiry Xu <jix024@cs.ucsd.edu>
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On Thu, Mar 15, 2018 at 6:51 PM, Andiry Xu <jix024@eng.ucsd.edu> wrote:
> On Thu, Mar 15, 2018 at 2:05 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Thu, Mar 15, 2018 at 7:11 AM, Andiry Xu <jix024@eng.ucsd.edu> wrote:
>
> Superblock mtime is not a big problem as it is updated rarely. 64-bit
> seconds and 32-bit nanoseconds make the inode and log entry bigger,
> and updating file->atime cannot be done with a single 64bit update.
> That may be annoying and needs to use journaling.

If this is a big concern, you could use a format similar to what ext4 has:
30 bits of nanoseconds, and 34 bits of seconds, where the upper two
bits count the epoch. That gives you a time range from years 1902 to
2446.

You could also have a resolution of less than a nanosecond. Note
that today, the file time stamps generated by the kernel are in
jiffies resolution, so at best one millisecond. However, most modern
file systems go with the 64+32 bit timestamps because it's not all
that expensive.

      Arnd