Re: Nanosecond fs timestamp support: sad

From: J. Bruce Fields
Date: Fri Jul 22 2011 - 18:10:49 EST

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

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. If there are unfixable problems, consider something
more complicated (like turning on i_version automatically when
someone asks for it).
- We need to check that i_version is also doing something
sensible on directory as well as on file inodes.
- We also need to think about what it does after reboots. (E.g.
what is an nfs server to do if clients see the i_version go
backwards (and hence possible repeat old values) after a
- Double-check the order that data updates and i_version updates
are done in. (Ideal would be if they were atomic, but for
nfsd's purposes at least it should be adequate if the
i_version comes after, and no later than the next commit.)

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