Thank you Frank.
Here I prepared a complete step by step of what is happening to that GDG.
MYJCLA is the IEBGENER I stated before, quite simple stuff:
//STEP01 EXEC PGM=IEBGENER
//STEPLIB DD DSN=MYLIBRARY,DISP=SHR
//*
//SYSIN DD DUMMY
//*
//SYSUT1 DD DSN=MYGDG(0),DISP=OLD
//SYSUT2 DD DSN=MYGDG(+1),
// DISP=(,CATLG,DELETE),
// UNIT=MYUNIT,
// SPACE=(CYL,(40,20),RLSE),
// DCB=(GDG,RECFM=FB,LRECL=80,BLKSIZE=6160)
//SYSPRINT DD SYSOUT=R
MYJCLB is only this DD, I'm sure this is all you need to understand my point:
//TESTFL DD DSN=MYGDG(+1),
// DISP=(MOD,KEEP,DELETE)
This is what happens to the GDG step by step:
MYJCLA At 11:15 no problems, as it usually runs:
IGD103I SMS ALLOCATED TO DDNAME SYSUT1
----------------------------------------------------------------
IGD101I SMS ALLOCATED TO DDNAME (SYSUT2 )
DSN (MYGDG.G6317V00)
STORCLAS (SCSTD) MGMTCLAS (MCNOABK) DATACLAS (DCSTD)
VOL SER NOS= XA9345
----------------------------------------------------------------
IGD104I MYGDG.G6316V00 RETAINED, DDNAME=SYSUT1
----------------------------------------------------------------
IGD107I MYGDG.G6317V00 ROLLED IN, DDNAME=SYSUT2
----------------------------------------------------------------
IGD104I MYGDG.G6317V00 RETAINED, DDNAME=MYDD
----------------------------------------------------------------
MYJCLB At 11:18 doesn't provide allocation parameters for the GDG:
IGD101I SMS ALLOCATED TO DDNAME (TESTFL )
DSN (MYGDG.G6318V00)
STORCLAS (SCSTD) MGMTCLAS (MCNOABK) DATACLAS (DCSTD)
VOL SER NOS= XA9334
----------------------------------------------------------------
IGD104I MYGDG.G6318V00 RETAINED, DDNAME=TESTFL
----------------------------------------------------------------
MYJCLA At 11:25 doesn't abend but doesn't seem as normal either:
IGD103I SMS ALLOCATED TO DDNAME SYSUT1
----------------------------------------------------------------
IGD17356I GDG RECLAIM REQUEST WAS SUCCESSFULLY PROCESSED FOR DATA SET MYGDG.G6318V00
----------------------------------------------------------------
IGD101I SMS ALLOCATED TO DDNAME (SYSUT2 )
DSN (MYGDG.G6318V00)
STORCLAS (SCSTD) MGMTCLAS (MCNOABK) DATACLAS (DCSTD)
VOL SER NOS= XA9334
----------------------------------------------------------------
IGD104I MYGDG.G6317V00 RETAINED, DDNAME=SYSUT1
----------------------------------------------------------------
IGD107I MYGDG.G6318V00 ROLLED IN, DDNAME=SYSUT2
----------------------------------------------------------------
IGD104I MYGDG.G6318V00 RETAINED, DDNAME=MYDD
----------------------------------------------------------------
MYJCLA At 11:35 abend:
IGD103I SMS ALLOCATED TO DDNAME SYSUT1
----------------------------------------------------------------
IGD101I SMS ALLOCATED TO DDNAME (SYSUT2 )
DSN (MYGDG.G6319V00)
STORCLAS (SCSTD) MGMTCLAS (MCNOABK) DATACLAS (DCSTD)
VOL SER NOS= XA9338
----------------------------------------------------------------
ABEND: CONFLICTING DCB PARAMETERS
----------------------------------------------------------------
IGD104I MYGDG.G6318V00 RETAINED, DDNAME=SYSUT1
----------------------------------------------------------------
IGD107I MYGDG.G6319V00 ROLLED IN, DDNAME=SYSUT2
----------------------------------------------------------------
MYJCLA At 11:45 last run, all fine again:
IGD103I SMS ALLOCATED TO DDNAME SYSUT1
----------------------------------------------------------------
IGD101I SMS ALLOCATED TO DDNAME (SYSUT2 )
DSN (MYGDG.G6320V00)
STORCLAS (SCSTD) MGMTCLAS (MCNOABK) DATACLAS (DCSTD)
VOL SER NOS= XA9325
----------------------------------------------------------------
IGD104I MYGDG.G6319V00 RETAINED, DDNAME=SYSUT1
----------------------------------------------------------------
IGD107I MYGDG.G6320V00 ROLLED IN, DDNAME=SYSUT2
----------------------------------------------------------------
IGD104I MYGDG.G6320V00 RETAINED, DDNAME=MYDD
----------------------------------------------------------------
Nothing else happens to MYGDG other than these reports.
My guess is, the execution of MYJCLB at 11:18 got the GDG messed up by not providing its allocation parameters. For that reason MYJCLA that followed on couldn't see g6318 was already created by MYJCLB. The system noticed something was wrong (WHAT?) and RECLAIMED g6318, but apparently ignoring its definition as SYSUT2 stated explicitly a full and nice DCB not coincidental at all with the one provided by the already allocated g6318. Then the problem went on to the next execution to finally abend.