Hub/Switch Installation Guide

Chapter 2 HPSS Planning
60 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
a stateless protocol. This allows use of a connectionless networking transport protocol (UDP) that
requires much less overhead than the more robust TCP. As a result, client systems must time out
requeststoserversand retryrequeststhathavetimed outbeforearesponseis received.Clienttime-
out values and retransmission limits are specified when a remote file system is mounted on the
client system.
The twomainadvantagesof usingNFS insteadof a utility likeFTPare (1)filescanbe accessedand
managed through standard system mechanisms without calling a special program or library to
translate commands, and (2) problems associated with producing multiple copies of files can be
eliminated because files can remain on the NFS server. The primary disadvantages of NFS are the
2GBfile sizelimitationsof theVersion 2protocol,thefact thatUDPdoesnot providedata integrity
capabilities, and the data transfer performance due to the limitation of sending data via the RPC
mechanism.Ingeneral,NFSshouldnotbetheinterfaceofchoiceforlargeHPSSdatatransfers.NFS
is recommended for enabling functionality not provided through other interfaces available to the
client system.
The HPSS NFS interface does not support Access Control Lists (ACLs), so don’t attempt to use
them with NFS-exported portions of the HPSS name space.
Because of the distributed nature of HPSS and the potential for data being stored on tertiary
storage, the time required to complete an NFS request may be greater than the time required for
non-HPSS NFS servers. The HPSS NFS server implements caching mechanisms to minimize these
delays, but time-out values (timeo option) and retransmission limits (retrans option) should be
adjusted accordingly. A time-out value ofno less than 10 and a transmission limit of no lessthan 3
(the default) are recommended. The values of timeo and retrans should be coordinated carefully
with the daemon’s disk and memory cache configuration parameters, in particular, the thread
interval andtouch interval.The larger these valuesare,the larger the timeo and retrans times will
need to be to avoid timeouts.
Refer to the HPSS User’s Guide for details of the NFS interface.
2.5.6 MPI-IO API
The HPSS MPI-IO API provides access to the HPSS file system through the interfaces defined by
the MPI-2 standard (MPI-2: Extensions to the Message-Passing Interface, July, 1997).
The MPI-IO API is layered on top of a host MPI library. The characteristics of a specific host MPI
aredesignatedthroughtheinclude/mpio_MPI_config.h,whichisgeneratedatHPSS creationtime
fromthe MPIO_MPI settingin the Makefile.macros.The configuration forMPI-IO is describedin
Section 7.5: MPI-IO API Configuration on page 434
The host MPI library must support multithreading. Specifically, it must permit multiple threads
within a process to issue MPI calls concurrently, subject to the limitations described in the MPI-2
standard.
The threads used by MPI-IO must be compatible with the HPSS CLAPI or NDAPI threads use.
Threaded applications must be loaded with the appropriate threads libraries.
This raises some thread-safety issues with the Sun and MPICH hosts. Neither of these host MPIs
supportmultithreading,perse.Theyareinconformancewith theMPI-1standardwhichprescribes