[Beowulf] automount on high ports

Tim Cutts tjrc at sanger.ac.uk
Tue Jul 1 08:40:27 PDT 2008

On 1 Jul 2008, at 3:53 pm, Perry E. Metzger wrote:

> Henning Fehrmann <henning.fehrmann at aei.mpg.de> writes:
>> we need to automount NFS directories on high ports to increase the
>> number of possible mounts. Currently, we are limited up to ca 360  
>> mounts.
> A TCP socket is a 4-tuple of localhost:localport:remotehost:remoteport
> A given localhost:localport pair can speak to an unlimted array of
> remotehost:remoteport sets. For example, in theory, your SMTP port can
> get connections from up to 2^32 different hosts on each of 2^16
> different sockets from each, for a total space of 2^48 connections to
> a single local socket number. This in no way restricts how many
> connections can come in to another port, either, because a given
> socket is again the full 4-tuple -- if you have an SSH port, it too
> can get 2^48 connections.
> Now, there is this (odd) convention that only root can open a socket
> below 1024, so hosts "trust" (what a bad idea) sockets under that
> number. You can still, however, get up to 1023 connections from any
> given remote host to a given local host's port.
> Thus, your problem sounds rather odd. There is no obvious reason you
> should be limited to 360 connections.
> Perhaps your problem is not what you think it is at all. Could you
> explain it in more detail?

Certainly on my systems where I use the am-utils automounter, I find  
the limit on the number of simultaneously mounted filesystems is more  
in the region of 1500.  I've been desperately trying to reduce the  
number of NFS filesystems we have though.  Currently our automount map  
has about 600 entries, I think.


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