Re: Can two Hercules instances share a base DASD file in a shadow configuration?

Thanks for the confirmation.  I wasn't imagining a branched network, only simple linear "versions" for each instance with the base kept unmodified.
 

Peter

---In [email protected], <ivan@...> wrote :
 
 Le 6/13/2019 à 7:58 AM, pjfarley3@... mailto:pjfarley3@... [hercules-390] a écrit :
 > This is a question out of curiosity, not out of any actual present 
 > need to use it.
 >
 > I see nothing in the docs that would prevent this scenario from 
 > working, but I'm asking here in case there is anything that would 
 > cause corruption in the base DASD file.
 >
 > The idea is that the base DASD is to be strictly read-only, never 
 > updated.  All DASD updates are to be stored in the shadow file only.  
 > Both Hercules instances could and would be active at the same time.  
 > Both Hercules instances could and would be both reading and writing to 
 > the DASD via their own separate shadow file at the same time.

 Peter,
 
 I see absolutely no reason why this wouldn't work. When a shadow file is 
 created, the base file (or any shadow file that is hierachicaly 
 underneath) are only accessed read-only. This would be equivalent to a 
 single branched VMWare Workstation 'linked clone'
 
 The caveats are :
 
 - You have to be carefull not to integrate any shadow file back into the 
 base file or any other instance will be out of sync (and unusable) - 
 unless it is intentional. There is no actual protection against that, 
 contrary to VMWare which prevent you from removing a snapshot if a 
 linked clone exists that uses that snapshot. On linux you'd probably 
 want to force the file read only or under windows make it read only 
 using ACLs to prevent accidental damage.
 
 - You can't make multi branched generations of clones since CCKD/CCKD64 
 only support a linear generation of shadow files.
 
 --Ivan