This is a rather interesting "feature" of ADSM.  Before a client transmits
data to the server, it asks the server if there is enough room, i.e., the
client specifies exactly how much space is needed to hold the object to be
transmitted.  The server then attempts to make enough room for that object.
 This step is usually successfully completed by disposing of cached data in
a disk pool.  The server only makes as much room as requested.  The key is
it can't make more once the transmission of the object starts.  So, if the
client lies about the size, or the object grows during transmission, you
will get the dreaded "server out of space" message.  How can a file grow?
 If compression is attempted on an already compressed object, it can
actually grow in size.  Files that will display this tendency include zip
files, jpg files, etc.

V3 clients fixed this problem.  If the file actually grew during
compression, then it sent it uncompressed.

I'm an advocate of turning compression off at the client in most cases.
 You only get to compress data once and most tape devices provide
compression.  This is usually hardware compression and more efficient than
most software compression.

Use client compression on clients that are very fast connected to networks
that are very slow or over-utilized.  In many cases turning off compression
will actually improve backup times.

I hope some of this helps...


Kelly Lipp
Storage Solutions Specialists, Inc.
lipp AT storsol DOT com
www.storsol.com
(719) 531-5926