The simple MW approach is surely wrong, it will not create a PAV environment: Linux will think it has 3 different devices, accidents will happen.
MDC will avoid the IO; Control Unit cache hit is still IO as concerned for the z Series, but here PAV would help. AFAIK, PAV will not help if the concurrent IOs are not satisfied mostly from the control unit cache: the real disk can only handle one IO anyhow. I never implemented PAV (my former customer didn't have PAV enabled for the VM disks: it wouldn't help with DB2 nor SFS, and that's what they used heavily). You need to use the MDISK's MINIOPT directory record to tell CP to create a PAV group; keyword PAVALIAS. This way Linux will recognize all addresses as a PAV group. My guess: MDISK 391 3390 1500 500 VOL001 M (I removed the W) MINIOPT PAVALIAS 1391 2391 would create 391 as base and 1391 plus 2391 as PAV alias addresses. 2009/7/2 RPN01 <[email protected]> > Your response verifies what I’d thought was happening, but doesn’t > address the whole “multiple writable minidisk” quandary. > > I’m considering something like the following: > > USER LINUXGUEST > MDISK 391 3390 1500 500 VOL001 MW > LINK * 391 1391 MW > LINK * 391 2391 MW > > Which would give me three virtual devices all pointing to the same minidisk > within the Linux guest. > > First big question: Have I shot myself in the foot? Common z/VM wisdom says > that multiple write enabled links to the same minidisk lead down a slippery > slope to disaster. But would that be the case here? > > Second big question: Would PAV see the various I/O requests and assign them > to separate PAV aliases, allowing for better thorughput to the device? I’m > thinking that, if I can get past the first question, then the second would > be “yes”. > > One thing you didn’t mention in your response is that, hopefully, many of > the requests can be satisfied from cache, either via MDC or control unit > caching, avoiding the actual I/O. The thing that PAV and the multiple > minidisks would give you is the ability to get those I/Os started sooner > than with a single path in Linux. > > -- > 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 7/2/09 12:15 PM, "Kris Buelens" <[email protected]> wrote: > > I don't have a full answer to your question. But I want to avoid a > misconception: > > - mutipathing in z Architecture means a device can be reached by more > than one path, most often this means more than one CHPID leads to the > device, and each CHPID is connected to a different controlunit. > - Without PAV: when a device in handling an IO, other IOs will be > queued, for example by CP (reported by Pefkit). But also Linux, SFS, > DB2VM, > xxx know that classically a device can handle one one IO, and will queue > other IOs (not reported by Perfkit). > - PAV at the other hand makes it possible to have more than 1 I/O > active on a single device. PAV is kind of a ly: a given device address can > still have only one IO active; with PAV one assigns alternate device > addresses to a single device. > - With PAV: when CP gets IO requests from different users for the same > device, it will look for a free PAV address and may be able to launch it > instead of queueing it. Linux -as far as I know- is also PAV aware, so it > can launch more than one IO on condition that one gives it PAV addresses, > otherwise it won't be able to exploit it. > > > 2009/6/30 RPN01 <[email protected]> > > Before I put something huge together to test this, I thought I’d pass it by > all the experts. > > Linux has the ability to multipath, and z/VM supports multipathing via PAV. > There’s lots of documentation and studies showing that you can attach / > dedicate the PAV addresses to a Linux LPAR or guest, and implement > multipathing to DASD devices. This seems to be fairly clearly researched and > understood. > > What I’m wondering about would be multiple links to the same minidisk > (partial 3390, as opposed to a full volume) backed by a PAV multi-address > environment sustained by z/VM. Would it help I/O throughput to have multiple > MW minidisks set up in Linux as multipathing, if they were on a DASD with > PAV enabled, and having several physical addresses? Are there any got’chas > to this configuration? Any reason why it wouldn’t work? > > > -- Kris Buelens, IBM Belgium, VM customer support
