Hi Alan, Not to belabor the subject I just want to make sure I understand. Here is what my issue was.
I processed many SQL queries of DB2 tables via DB2 connect running on a z/OS LPAR down to a Linux server running on the same machine but a different z/VM LPAR without any issues. Until, we ran across some queries that were larger than all of the others. These queries failed (timed out). We saw in the DB2 connect trace that the size of the queries was about 2.3K. We knew it was a size issue because when we did a 'SELECT *' on the tables, basically reducing the size it worked. After a lot of effort we found that there was an extra route in the Linux routing table that sent the data bound for one of the z/OS LPARS over to our z/VM where it then was routed by the TCP/IP stack on z/VM to the z/OS LPAR over the HiperSockets LINK. This LINK happened to have a MTU size of 1500. When we removed the bogus route from the Linux routing table causing the SQL queries in question to travel over a HiperSockets LINK with a MTU size of 16348 they were successful. It appears that the difference in the MTU sizes was the problem. My question is does the MTU size mismatch cause the problem? I thought that even if the packets were fragmented they would still get over in full. Sorry for being so wordy but I thought I would present to try to help you help me and to also help others that may be just getting into this as I am. Thanks again for all of your help!! Terry -----Original Message----- From: The IBM z/VM Operating System [mailto:[EMAIL PROTECTED] On Behalf Of Alan Altmark Sent: Wednesday, August 20, 2008 4:39 PM To: IBMVM@LISTSERV.UARK.EDU Subject: Re: HiperSockets Question On Wednesday, 08/20/2008 at 03:43 EDT, "Martin, Terry R. (CMS/CTR) (CTR)" <[EMAIL PROTECTED]> wrote: > If I am using HiperSockets and my MTU size is 1500 and a pass a packet size of > 2000 will the data in the packet be truncated since it is bigger than 1500? I > thought I read that since HiperSockets is memory transfers that the MTU can not > be split like it would over a normal transmission pipe, meaning that for a 3K > packet size over a MTU 1500 link would not be split into 2 transmissions. Am I > correct in the assumption? Yes and no. :-) It won't be truncated, but it will be fragmented. The MTU is the Guardian of the Gate. It prevents the IP layer from sending a packet too large for the underlying interface to handle. So a 3K packet over an MTU 1500 link will result in two fragments, each 1500 bytes. Alan Altmark z/VM Development IBM Endicott
