Re: Multiple Channels to DASD
On Thu, May 20, 2010 at 3:39 PM, Billy Bingham billy.bingham...@suddenlink.net wrote: Hello all, Risking the old saying that it's better to have people think you're stupid than to open your mouth and remove all doubt :) If VM is doing I/O to one DASD volume only, say a DDR DUMP or RESTORE, and everything else is equal, will VM use multiple channels to get to the one volume, or will it do all it's I/O on whatever channel it gets first and only that one channel? DDR will wait for an I/O to complete before it starts the next one. And with all I/O to the same real volume, it would also be serialized by z/VM. So if nothing else is going on z/VM can do with just one channel. I don't think CP will try to balance over the channels, but picks another one when the first choice is busy. Some DASD configurations encourage CP to prefer one channel over the other. In your scenario, you would see it all go via one channel. Something like this (with apologies for those who read mail in proportional font): ESACHAN Velocity Software-Test VSIVM4 ESAMON 3.7 Channel Performance AnalysisCHANNEL 00 Pct Channel --Data Channel Utilization ---Reads/Second-- CHP Shr Class Typ LPAR Total LPAR TOTAL Pct Max --- --- - --- - - - --- 00 Yes FICON FC0.1 0.14 6 0 195K 01 Yes FICON FC5.8 5.8 6405 6416 3 195K I can't really think of real life situations where that would be a concern. Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/
Re: Multiple Channels to DASD
Rob, are you sure CP selects the channel to use? That was the case in the 370 era. AFAIK, in XA, CP will create a mask telling which channels *can* be used, but it is the IO subsystem that will select the channel and maybe even queue the IO if all channels would be busy. In XA mode CP only queues an IO if it knows the device is busy (ie it launched an IO itself), device busy due to IO's from other LPARs or CECs will be queued by the IO subsystem. 2010/5/21 Rob van der Heij rvdh...@gmail.com On Thu, May 20, 2010 at 3:39 PM, Billy Bingham billy.bingham...@suddenlink.net wrote: Hello all, Risking the old saying that it's better to have people think you're stupid than to open your mouth and remove all doubt :) If VM is doing I/O to one DASD volume only, say a DDR DUMP or RESTORE, and everything else is equal, will VM use multiple channels to get to the one volume, or will it do all it's I/O on whatever channel it gets first and only that one channel? DDR will wait for an I/O to complete before it starts the next one. And with all I/O to the same real volume, it would also be serialized by z/VM. So if nothing else is going on z/VM can do with just one channel. I don't think CP will try to balance over the channels, but picks another one when the first choice is busy. Some DASD configurations encourage CP to prefer one channel over the other. In your scenario, you would see it all go via one channel. Something like this (with apologies for those who read mail in proportional font): ESACHAN Velocity Software-Test VSIVM4 ESAMON 3.7 Channel Performance AnalysisCHANNEL 00 Pct Channel --Data Channel Utilization ---Reads/Second-- CHP Shr Class Typ LPAR Total LPAR TOTAL Pct Max --- --- - --- - - - --- 00 Yes FICON FC0.1 0.14 6 0 195K 01 Yes FICON FC5.8 5.8 6405 6416 3 195K I can't really think of real life situations where that would be a concern. Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/ -- Kris Buelens, IBM Belgium, VM customer support
Re: Multiple Channels to DASD
On Fri, May 21, 2010 at 11:45 AM, Kris Buelens kris.buel...@gmail.com wrote: Rob, are you sure CP selects the channel to use? That was the case in the 370 era. AFAIK, in XA, CP will create a mask telling which channels *can* be used, but it is the IO subsystem that will select the channel and maybe even queue the IO if all channels would be busy. In XA mode CP only queues an IO if it knows the device is busy (ie it launched an IO itself), device busy due to IO's from other LPARs or CECs will be queued by the IO subsystem. Thanks Kris. I see my sentence could be read in different ways. I think we agree that CP does not attempt to make the I/O subsystem balance the paths. But I just looked it up and stand corrected about the intentions of the channel subsystem. I was mislead because our DASD subsystem does have a preferred path. You can specify a preferred channel path to an I/O device. When you do not specify a preferred path, the channel subsystem uses a rotation order for the initiation of I/O requests to a device. Rob -- Rob van der Heij Velocity Software http://www.velocitysoftware.com/
Re: Multiple Channels to DASD
Good question Billy, I'm waiting to see what the responses are ! In my environment, the SAD screen will show all 4 FICON channels to the DASD with activity. What I don't know is it VM balancing the load or the SAP engine doing the I/O. Inquiring minds want to know ! LOL Bubba ... snip If VM is doing I/O to one DASD volume only, say a DDR DUMP or RESTORE, and everything else is equal, will VM use multiple channels to get to the one volume, or will it do all it's I/O on whatever channel it gets first and only that one channel? Thanks, Billy
