For the dsmSendObj API call from the server standpoint it is very
important for the estimated size to be accurate for a few reasons.

1 - The server supports selection of a storage pool based on the size
    of an object by setting the maximum size of an object allowed for a
    storage pool and specifying a next storage pool.  The size used will
    be the estimated size passed on this API call.

2 - The server could fail the transaction after backing up most of the
    data.  As an example say you have 50M of space available in your storage
    pool.  Suppose your estimate on the dsmSendObj call that the size of
    the object is 45M but the actual size is 55M.  The server will allocate
    the 45M, as you back up additional data the server will make additional
    allocations in the storage pool, but as soon as you go over the 50M
    that is available in the storage pool you will get an ANR9999D message
    on the server and the transaction will be aborted wasting 50M of
    network traffic because 50M of data was transmitted and then discarded
    because of the aborted transaction.

3 - You will not be able to use caching in your server storage pool because
    the cached space is not eligible to allocate additional space for an
    object being backed up over the estimated size specified in the
    dsmSendObj API call.  Cached space is eligible for the initial
    allocation as determined by your size estimate.

4 - From the performance side these allocations over the amount of data
    estimated for the size are made in small chunks in the storage pool
    so if your estimate is really low there is a lot of additional overhead
    making a determination for what space is available for allocation in
    that storage pool and allocating that space.

Hope this helps
David Bohm, ADSM Level 2