Ah, but while that was true of a ³real² 3390, is that also now true of emulated 3390¹s which are split across varying numbers of essentially SCSI disks? A single 3390 mod 27 might be split up over several 9 gig physical disks in order to implement the emulation. Is the controller smart enough to be able to start an I/O to each, even though the I/O¹s were sent to the same 3390 address?
The waters get muddier every day.... -- Robert P. Nix Mayo Foundation .~. RO-OE-5-55 200 First Street SW /V\ 507-284-0844 Rochester, MN 55905 /( )\ ----- ^^-^^ "In theory, theory and practice are the same, but in practice, theory and practice are different." On 3/25/10 1:43 PM, "Kris Buelens" <[email protected]> wrote: > There is maybe some misunderstanding: (leaving out PAV a while) a device can > handle only 1 IO at a time, guests know that, CP too. So indeed a linux will > not send a new IO if the previous one to that disk hasn't ended yet, the guets > will queue it. With several guests with minidisks on the same disk, CP will > queue the requests. Multipathing doesn't change that. > Multipathing helps in cases where the device is not busy, but the channel or > control unit is. > > PAV simplified: with PAV, you make appear a single disk as if it were many > disks, but for each PAV address the old rule is still valid: one IO at a time. > If you guest is PAV aware, it can also avoid queuing, if not, putting many > guests on the same volume and let CP exploit PAV can improve IO rates. > > 2010/3/25 RPN01 <[email protected]> >> Another point I¹ve not seen mentioned, and I¹m not sure if it¹s true or >> not... >> >> Given a dedicated volume to a Linux guest, won¹t the guest start only one I/O >> to the device at a time, and wait for it to complete? If you break up a >> larger volume into several minidisks (like a mod 27 into mod 9¹s) aren¹t you >> allowing the z/VM multipathing to do its job more efficiently, even if you >> give all the smaller volumes to the same Linux guest? >> >> Like I said, I may be totally off base, but this is the impression I¹ve >> had...
