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Re: [PATCH] xfsrestore: use utimensat() to provide atime/mtime with ns r

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] xfsrestore: use utimensat() to provide atime/mtime with ns resolution
From: Brian Foster <bfoster@xxxxxxxxxx>
Date: Fri, 5 Sep 2014 07:02:12 -0400
Cc: Eric Sandeen <sandeen@xxxxxxxxxxx>, xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140905012404.GV20518@dastard>
References: <1409848708-42666-1-git-send-email-bfoster@xxxxxxxxxx> <20140905004501.GU20518@dastard> <54090C33.2060102@xxxxxxxxxxx> <20140905012404.GV20518@dastard>
User-agent: Mutt/1.5.23 (2014-03-12)
On Fri, Sep 05, 2014 at 11:24:04AM +1000, Dave Chinner wrote:
> On Thu, Sep 04, 2014 at 08:04:51PM -0500, Eric Sandeen wrote:
> > On 9/4/14, 7:45 PM, Dave Chinner wrote:
> > >On Thu, Sep 04, 2014 at 12:38:28PM -0400, Brian Foster wrote:
> > >>xfsdump encodes and stores the full atime and mtime for each file with
> > >>nanosecond resolution. xfsrestore uses utime() to set the times of each
> > >>file that is restored. The latter supports resolution of 1 second, thus
> > >>sub-second timestamp data is lost on restore.
> > >
> > >That doesn't seem like a big deal. What sort of problems does this
> > >actually cause?
> > >
> > >FYI, many linux filesystems only have second resolution timestamps
> > >and hence applications can't rely on sub-second timestamp resolution
> > >to actually mean anything useful....
> > 
> > But why not restore the same resolution as is actually stored in the dump?
> > Throwing it away seems odd, and restoring it looks easy enough.
> 
> Comes from a time when we couldn't restore what was in the dump. :/
> 
> > In any case, there was a user who noticed & complained.  Seems like a
> > very reasonable thing to fix, to me.
> 
> Sure, but we don't make changes with the justification "just
> because". xfsrestore has had this behaviour since dump/restore was
> first introduced, so first we need to understand what the actual
> problem is. Was the user complaining because they noticed they were
> "different" in passing, or was it noticed because the difference is
> the root cause of some other problem?
> 

No problems that I'm aware of. As Eric mentioned, it was noticed during
an evaluation of possible data transfer mechanisms for a glusterfs
setup. The user had to evaluate whether it would lead to any issues (a
geo-replication tracking thing I suspect) for a customer, but I hadn't
heard anything that suggested it was. The utime() call appears to be
obsolete as well, for whatever that's worth.

Brian

> Cheers,
> 
> Dave.
> -- 
> Dave Chinner
> david@xxxxxxxxxxxxx

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