Re: Delays on "first" access to a NFS mount

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Mar 07, 2007 at 07:44:18PM +0100, Simon Peter wrote:
> > > just verified that the server indeed spins up all disks before
> > > answering the request. I thus suspect it is somehow checking all
> > > exports whenever any one export is accessed. Is this correct
> > > behaviour?
> > Hm.  If you have the nfs-utils source, you can see there's a loop in
> > 	nfs-utils/utils/mountd/cache.c:nfsd_fh()
> > that stats the root of each export, in two places; the first it looks
> > like you shouldn't hit if you don't have the mountpoint export option
> > set:
> 
> Correct. This one is never hit in my case.
> 
> > The second is to figure out which filesystem the filehandle that you
> > passed in that getattr is for:
> >          if (stat(exp->m_export.e_path, &stb) != 0)
> >                  continue;
> 
> This is where the wait for the respective disk to spin up occurs.

OK, cool, so we understand the problem.

> > Could we cache the stat information in the export and then
> > double-check it if necessary when there's a match?  Or is there some
> > way we could get the kernel to keep that cached for us?
> 
> I could certainly cook up a patch for mountd to cache that information
> on its own.  I don't have too much clue about how the kernel does its
> cacheing, though. If it's useful to do that directly in mountd, I
> could get my hands on it.

There's two caches involved:

	- the filesystem caches attributes so that subsequent stat's of
	  the exported directory can be answered without having to go to
	  disk.  I guess it's not suprising that that wouldn't be cached
	  anymore if you hadn't touched the filesystem in a long time.
	  Though there's one point I'm unclear on: are the directories
	  you're exporting mountpoints?  That's the normal
	  configuration, and in that case I would've thought the inode
	  for that directory would be pinned in memory so the stat
	  wouldn't have to go to disk.  I'm probably missing something.

	- the filehandle->export mapping that this function tells the
	  kernel about is cached by nfsd for a half-hour.  That time is
	  set a little later in nfsd_fh:

	          qword_printint(f, time(0)+30*60);

	  I don't think there would be any harm to just changing that
	  time(0)+30*60 to 0x7FFFFFFF (== never expire)--nfs-utils
	  should be invalidating that cache explicitly whenever it's
	  needed.  Maybe that should be the default.

--b.

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