On AIX 7.2 LPAR with 800GB of memory, estimated dump device size is approx 85 GB. Having 85-90 GB of primary dumplv on rootvg is not a convenient option. Please advise if any other available option to have dump collected during reboot of LPAR from HMC.  

------------------------------
Jitendra Singam
------------------------------

Hi,

It does sound like it's possible to have a dump device outside of rootvg - https://www.ibm.com/support/pages/managing-system-dump-devices.  But that document does mention doing so on a temporary basis.

Phill.

------------------------------
Phill Rowbottom
Unix Consultant
Service Express
Bedford
------------------------------

Thank Phill, I tested it, its possible to have dump device outside rootvg in traditional type of dump. With fw-assisted type of dump primary dump device should be in rootvg.

------------------------------
Jitendra Singam
------------------------------

Original Message:
Sent: Wed June 24, 2026 04:53 AM
From: Phill Rowbottom
Subject: AIX dump device option

Hi,

It does sound like it's possible to have a dump device outside of rootvg - https://www.ibm.com/support/pages/managing-system-dump-devices.  But that document does mention doing so on a temporary basis.

Phill.

------------------------------
Phill Rowbottom
Unix Consultant
Service Express
Bedford
------------------------------

Hi,

What's exarctly inconvenient with having 90GB dump space in rootvg?

You need this space anyway, so it has to be allocated to the systems, and it cannot be a shared space. That means the total comsumption of the space remains the same no matter where the dump space comes from. 
System backup does not include contents of this space, so size of the mksysb file and the time it takes to create this file are the same no matter how large the dump space is.

------------------------------
Lech Szychowski
------------------------------

Hi Lech,

If you're using alt_rootvg, you're wasting twice the space.  For alt_rootvg setups, use a small (1PP) primary dump dev on rootvg, and a large secondary dump device on a 3rd vg.

Can't remember if this solution needs something "creative" on the inittab.

Incidentally, on alt_rootvg environments having a 3rd "offload" vg, allows you to archive audit/logs outside of the rootvg, keeping the mksysbs smaller, and the alt_disk_copys faster.

------------------------------
José Pina Coelho
IT Specialist at Kyndryl
------------------------------

Original Message:
Sent: 06/26/26 02:54 AM
From: Lech Szychowski
Subject: AIX dump device option

Hi,

What's exarctly inconvenient with having 90GB dump space in rootvg?

You need this space anyway, so it has to be allocated to the systems, and it cannot be a shared space. That means the total comsumption of the space remains the same no matter where the dump space comes from. 
System backup does not include contents of this space, so size of the mksysb file and the time it takes to create this file are the same no matter how large the dump space is.

------------------------------
Lech Szychowski
------------------------------

If the issue is disk space you can grow the LUN sizes dynamically with chvg -g. See Growing Disks in Rootvg on AIX.

On modern SAN devices the depup process reduces the real cost of a second copy to zero.

------------------------------
Alexander Pettitt
------------------------------

Given that alt_disk_copy doesn't copy the dump device contents, a sysdump will diverge the blocks.

------------------------------
José Pina Coelho
IT Specialist at Kyndryl
------------------------------

Original Message:
Sent: 06/29/26 07:07 AM
From: Alexander Pettitt
Subject: AIX dump device option

If the issue is disk space you can grow the LUN sizes dynamically with chvg -g. See Growing Disks in Rootvg on AIX.

On modern SAN devices the depup process reduces the real cost of a second copy to zero.

------------------------------
Alexander Pettitt
------------------------------

Good catch.

Honestly I am hoping to have zero dumps but maybe you could use dd to bring them into sync?

------------------------------
Alexander Pettitt
------------------------------

Original Message:
Sent: 06/29/26 07:37 AM
From: José Pina Coelho
Subject: AIX dump device option

Given that alt_disk_copy doesn't copy the dump device contents, a sysdump will diverge the blocks.

------------------------------
José Pina Coelho
IT Specialist at Kyndryl
------------------------------


Original Message:
Sent: 06/29/26 07:07 AM
From: Alexander Pettitt
Subject: AIX dump device option

If the issue is disk space you can grow the LUN sizes dynamically with chvg -g. See Growing Disks in Rootvg on AIX.

On modern SAN devices the depup process reduces the real cost of a second copy to zero.

------------------------------
Alexander Pettitt
------------------------------

Hope for the best, prepare for the worst, although there's really no point in synchronizing the dumps (a dump from a boot of rootvg makes no sense inside alt_rootvg), from a dedup perspective, you should just package it up with the snap command and zero the dump LV.

------------------------------
José Pina Coelho
IT Specialist at Kyndryl
------------------------------

Original Message:
Sent: 06/29/26 07:44 AM
From: Alexander Pettitt
Subject: AIX dump device option

Good catch.

Honestly I am hoping to have zero dumps but maybe you could use dd to bring them into sync?

------------------------------
Alexander Pettitt
------------------------------


Original Message:
Sent: 06/29/26 07:37 AM
From: José Pina Coelho
Subject: AIX dump device option

Given that alt_disk_copy doesn't copy the dump device contents, a sysdump will diverge the blocks.

------------------------------
José Pina Coelho
IT Specialist at Kyndryl
------------------------------



Original Message:
Sent: 06/29/26 07:07 AM
From: Alexander Pettitt
Subject: AIX dump device option

If the issue is disk space you can grow the LUN sizes dynamically with chvg -g. See Growing Disks in Rootvg on AIX.

On modern SAN devices the depup process reduces the real cost of a second copy to zero.

------------------------------
Alexander Pettitt
------------------------------

Let's consider what that dump device does. If we have an event that totally stops the operating system, we have a place to save the data and analyze what happened. How do we determine the estimated dump device size? We call each component of the OS and have the measure the amount of data they would and add it all together. We want to inquire at the heaviest period of workload because thing like the thread and process tables grow over time. And from my own experience, I add 50% to thee size. If things are going badly and various diagnostic routines and error handling kick in, they usually increase the amount of space you need. 

Now for you real question: Why rootvg? When AIX is booting and we are not really 'up' fully, and no other VGs have been varied on, the only place we can write the dump to is a LV in rootvg. There is a huge amount of work that happens behind the scenes to make rootvg fully functional. And if something happens during this time, where do we save the data? Rootvg. 

That is why we require the permanent dump devices to be on rootvg. 

------------------------------
Grover Davidson
------------------------------