linux-kernel - Re: Nanosecond fs timestamp support: sad

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Date:	Fri, 22 Jul 2011 19:06:12 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	NeilBrown <neilb@...e.de>
Cc:	Andi Kleen <andi@...stfloor.org>, Matt Mackall <mpm@...enic.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Nanosecond fs timestamp support: sad

On Sat, Jul 23, 2011 at 08:59:15AM +1000, NeilBrown wrote:
> On Fri, 22 Jul 2011 18:31:58 -0400 "J. Bruce Fields" <bfields@...ldses.org>
> wrote:
> 
> > On Fri, Jul 22, 2011 at 06:10:39PM -0400, bfields wrote:
> > > On Fri, Jul 22, 2011 at 11:47:32PM +0200, Andi Kleen wrote:
> > > > On Fri, Jul 22, 2011 at 04:11:42PM -0500, Matt Mackall wrote:
> > > > > On Fri, 2011-07-22 at 22:59 +0200, Andi Kleen wrote:
> > > > > > > Indeed. Only usefully exists on ext4 and requires extra system calls.
> > > > > > 
> > > > > > Not sure what you mean?  It's in stat(2), just like the timestamps.
> > > > > 
> > > > > I don't see anything that looks like a version or generation number in
> > > > > either the man pages, the asm-generic/stat.h, or glibc's asm/stat.h.
> > > > > Pointer?
> > > > 
> > > > Hmm you're right. I thought it was in there, but apparently not.
> > > > I think it should be added there though. We still have some unused 
> > > > fields.
> > > 
> > > But last I checked I thought it was only ext4 that actually incremented
> > > the i_version on IO, and even then only when given a (non-default) mount
> > > option.
> > > 
> > > My notes on what needs to be done there:
> > > 
> > > 	- collect data to determine whether turning on i_version causes
> > > 	  any significant performance regressions.
> > > 		- Last I talked to him, Ted Tso recommended running
> > > 		  Bonnie on a local disk, since it does a lot of little
> > > 		  writes, which is somewhat of a worst case, as it will
> > > 		  generate extra metadata updates for each write.
> > > 		  Compare total wall-clock time, number of iops, and
> > > 		  number of bytes (using some kind of block tracing).
> > > 	- If there aren't any problems, turn it on by default, and we're
> > > 	  done.
> > 
> > (Well,and talk the other filesystem implementors into doing it.)
> > 
> 
> But does anyone apart from NFSv4 actually *want* i_version as opposed to the
> more-generally-useful precise timestamps?

It *seems* like a generally useful idea, but I don't know of any other
users.

> If not, we probably should tell NFSv4 to use timestamps and focus on making
> them work well.
> ??

Well, sure, I couldn't complain about that if that proved possible.

--b.

> 
> The timestamp used doesn't need to update ever nanosecond.  I think if it
> were just updated on every userspace->kernel transition  (or effective
> equivalents inside kernel threads) that would be enough capture all
> causality.  I wonder how that would be achieved..  I wonder if RCU machinery
> could help - doesn't it keep track of when threads schedule ... or something?
> 
> NeilBrown
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