From: The IBM
z/VM Operating System [mailto:[EMAIL PROTECTED] On Behalf Of Wakser, David
Sent: Wednesday, September 06, 2006 2:30
PM
To:
[email protected]
Subject: Re: VM directory entry for
shared DASD
Rob:
Yes, you have
actually underscored what I have been saying: except for the volume containing
the lock file, no DASD within a single VM image needs MWV specified!
Thanks.
David
Wakser
-----Original Message-----
From: Rob van der Heij [mailto:[EMAIL PROTECTED]]
Sent: Wednesday,
September 06, 2006 2:26 PM
To: [email protected]
Subject: Re: VM directory entry for
shared DASD
On
9/6/06, Wakser, David <[EMAIL PROTECTED]>
wrote:
>
So, if the shared disk does NOT start ay Cyl 0 but at Cyl 1,
> the MWV is
worthless and I cannot share disks?
If you
want the VSE guests in this LPAR to use reserve/release, you need MWV to have
CP simulate it (that's the *virtual* reserve/release part). The cyl0 is only
when it needs to go to the real device so other LPARs can
participate.
> I
think what I need to understand is: who says CP has to issue
> any
reserve/release? Does not VSE (via the lock file or some internal
>
locking
>
mechanism) handle this? Why does CP need to be involved?
The
control unit is supposed to acknowledge R/R to provide exclusive access to the
volume for more than one channel program. If you would have guest A issue the
reserve and then get guest B from the same z/VM image try it, the control unit
would grant access because the LPAR already holds a reserve, so the control
unit thinks this is fine. What you want is CP to be arbiter and issue the real
reserve for the volume as long as one of the guests holds a
reserve.
>
BTW, there are no LPARS involved here. And the reason why not
> all shared
packs start on cyl 0 is because we wanted to retain
> uniqueness of DASD volids whenever
possible. Thus, the VSE DASD starts at REAL CYL.
If it's
just 1 z/VM LPAR then you have no need for real reserve to be issued because
there is nobody outside who can try. And afaik VSE only does the R/R for some
disks (like the lock file) and for the other disks there is logic (in the lock
file) that protects the datasets (rather than reserve the entire volume
anytime you want to write a shared dataset).
I hope I
did not make it more confusing that it has to be. Rob
--
Rob van der Heij
Velocity Software, Inc
http://velocitysoftware.com/
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