It's IO. A SSCH (start channel) instruction is issued with the modify bit on. Then the program can issue a RSCH (resume subchannel) which continues the channel program with the initial i/o ending, allowing for an almost continuous data feed. You can throw new CCWs (channel commands) at it for read and write. While most shops no longer are concerned with physical device geometry, what with everything stored electronically, obviating true seeks, it is still expensive and unnecessary to end the channel program and fire it up again. That's why it is always recommended to keep paging "cylinders" isolated from all others.
All CP io uses the seldom ending channel program (paging, spool, directory, warm, ckpt). Consider if you mix page and drct. Drct access will occur. If a page RSCH in in process it will end (only one i/o at a time san PAV!) so the drct i/o can start. Then a page is needed for paging; the drct RSCH is ended then the page SSCH/RSCH starts. Not good. David ________________________________ From: The IBM z/VM Operating System on behalf of Edward M. Martin Sent: Tue 4/10/2007 3:55 PM To: [email protected] Subject: Re: [IBMVM] Time to move Hello Everyone, I have seen, read, and been told about having a separate page volume. It just seems like a true waste of space. Yes, I understand that disk is getting cheaper all the time, but budgets are kept here. I do have room, so the question is with all the VSE systems V=V but with enough real to cover them, and z/VM paging at 0 to 1 per sec, how much of an impact would it be to have paging on other volumes? q alloc page EXTENT EXTENT TOTAL PAGES HIGH % VOLID RDEV START END PAGES IN USE PAGE USED ------ ---- ------ ------ ------ ------ ------ ---- 430RES 0752 257 390 24120 533 563 2% 430W04 0715 1 2000 360000 0 0 0% ------ ------ ---- SUMMARY 384120 533 1% USABLE 384120 533 1% And could you explain (again) the seldom ending channel program for paging? Thanks, Ed Martin Aultman Health Foundation 330-588-4723 [EMAIL PROTECTED] ext. 40441 > -----Original Message----- > From: The IBM z/VM Operating System [mailto:[EMAIL PROTECTED] On > Behalf Of David Kreuter > Sent: Tuesday, April 10, 2007 3:20 PM > To: [email protected] > Subject: Re: Time to move > > On the page volume with drct space you will defeat the seldom ending > channel program for paging. Directory access uses the same type of channel > program but will initiate its own io. And the directory disk pages will be > accessed from time to time. Must they be on page volumes? > David > > ________________________________
