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

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

On Wed, Mar 07, 2007 at 04:23:23PM -0500, Talpey, Thomas wrote:
> At 04:17 PM 3/7/2007, J. Bruce Fields wrote:
> >There's an expiry time that's passed down with each cache entry.  In
> >this particular case it's 30 minutes.  There's also a "flush" file you
> >can write to to ask that the whole cache be flushed.  I don't remember
> >how this works in detail, though.
> 
> Aha - so the time comes from mountd. There's some sort of refresh
> timer that the kernel triggers though. So it's not a deadline of this
> time (I think). Or is it.

The kernel sweeps through the cache every now and then and cleans out
expired entries.  I think it also takes note of the earliest future
expiry it runs across in the process, and uses that to decide when to
check next.  This is all in linux/net/sunrpc/cache.c:cache_clean().

> The "flush" file lives in /proc/net/rpc/nfsd.export, and you write an
> integer value to it. I *think* it then flushes any entries which are
> more than that many seconds old.

Right.

> The whole thing makes my head hurt and I try not to look at it.

Well, non-head-hurty ideas always welcomed.  I've got two export-related
problems to fix:

	- Our current NFSv4 pseudofs fsid=0 hack is a pain to administer
	  and results in inconsistent paths across different NFS
	  versions.

	- The trick of using the pseudoflavor as a client name (so doing

		/export	 gss/krb5(rw)
	
	  instead of

		/export *(sec=krb5,rw)

	  ), is inconsistent with what other os's do, and makes it
	  impossible to specify restrictions based both on flavor and on
	  ip network/dns name/netgroup.

While I'm at it Trond and Christoph and others seem to be asking whether
we can't make some more fundamental changes, such as:

	- Maintaining a static in-kernel exports table instead of
	  loading it on demand from mountd, and

	- divorcing the exports namespace completely from any local
	  process namespace, to the extent that you could even just say
	  "I want to export /dev/sda7 as /usr/local/bin" without first 
	  mounting /dev/sda7 someplace.

But I really need a better idea of the requirements on the exports
system.  And some other examples to look at wouldn't hurt either.  (Take
pity on me, Linux is all I know....)

--b.

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