HP OpenVMS Systems

Ask the Wizard Questions

I/O size

The Question is:

I am the system manager for data archive for the Hubble
Space Telescope.  We are currently porting the system to
a 2100 Alpha server.  Our problem is that the incoming
data rate will go up to at about 7-10 Gigabytes starting
in early 1997.  The outgoing data rate will be about the
same.  We need all the I/O performance we can get.

Are there any plans to raise the maximum I/O size above
64K?

The Sony optical drives we are using have a 4Mb buffer.
We think that if we could throw more data at the system
per I/O we could increase our throughput.  At least that
is what the software developers are telling me.

The Answer is:

    The size of I/O depends on your hardware. In the SCSI world, it
    requires that the port AND class driver support larger single
    I/O requests. The basic $QIO system service on Alpha accepts
    transfer sizes of up to 32 bits' worth of bytes (I'd limit it
    to 31 for safety though...).

    There are a number of SCSI port drivers...most of the current
    generation in fact...that for one reason or another can handle at most
    64K at a time. The drivers break this up for you. We are looking at
    supporting longer I/O, but be aware that this may well be handled as
    a site override of some sort, since huge I/O could tie up the bus
    between commands for longer than we like to see it in general
    purpose situations.

    Another point to be made: the measurements of SCSI driver performance
    on several adapters indicate we are driving them at pretty close to
    their maximum I/O rates, and the internal splits into 64K units may
    not be the bottleneck here. We are working also on getting faster SCSI
    controllers and wide support, and in fact wide SCSI drives are working
    in house on several controllers now. Wide drives have twice the data
    rate limits and are appearing on the market.

    Your I/O drain devices (tapes? disks?) may want to be on different
    busses than the I/O source ones.

    Some other performance boost techniques are also in the works in the
    SCSI space.