Hi,
I was following this post as I am also curious about the actual meaning and calculation details for this overhead capacity.
While trying to further explore the meaning of the term for myself, I got the impression that the term "overhead capacity" is perhaps being interpreted/used in different ways and contexts. So I wanted to participate in this discussion with the goal to get to a common understanding and conclusion.
Here are my considerations and the findings from my research on this topic:
As far as I can recall from my historical knowledge, the term "overhead capacity" was already used in the context of space-efficient vdisks to express the space that is required for each grain of a vdisk to maintain e.g. the allocation info or log files of the space-efficient vdisk(-copy) in a "traditional" storage pool.
With the introduction of data-reduction pools I guess that this concept has been adapted accordingly and hence the overhead capacity is nowadays mainly being referred as the space that is required for tracking unmap and reclaim operations within in the pool, see e.g. Redbook "Implementation Guide for IBM Storage FlashSystem and IBM SAN Volume Controller Updated for IBM Storage Virtualize Version 8.6" chapter 2.12 "Storage pool configuration".
Based on the description in that Redbook (or alternatively also on knowledgecenter ) this would basically go in line with the description in the tooltip you posted.
What confuses me in that context though, is that your screenshot shows such a huge amount, as I would expect in that context also a smaller number than usable capacity. Especially now that you identified that the number is matching the number of total savings.
I checked a few FlashSystems in our lab too. Interestingly I found one FlashSystem where overhead capacity and total savings are a perfect match, while I also saw another FlashSystem where these numbers did not match, e.g.:
now look at the "Total Provisioned" numbers. While the first one only has 5.73% provisioned, the second one shows a huge overallocation of 895.02%. So my working theory is that the "Overhead Capacity" field mainly provides meaningful or relevant information when an overallocation actually exists.
For completeness (as the screenshots do not cover that):
- the first FlashSystem reports a total provisioned capacity of 4,020.00 GiB
- the second FlashSystem reports a total provisioned capacity of 196,073.13 GiB
A potential interpretation especially based on the second example could (imho) be that even though the total savings are ~187 TiB (out of 196 TiB provisioned capacity ("artifically" calculated by compression and dedup factors etc)), in fact ~93 TiB out of these are actually occupied by metadata and may have to be substracted from those savings, as they actually still occupy space.
But this is only a guess, as I must admit I am totally lost meanwhile when it comes to all those "artificial" capacities due to the combination of compressed FCMs, DRPs, Thin-provisioning last but not least backend storage virtualization behind SVC. I think many people meanwhile get nuts when they try to understand the actual figures on a SVC that uses a (data-reduction) Pool provided by FlashSystem which in turn itself also uses inline compression.
So thats just my two cents.
Is there anyone of the IBMers in this forum who has a comprehensive understanding of all those capacity figures and can shed some more light into this?
Cheers
Philipp
------------------------------
Philipp Wachtler
------------------------------
Original Message:
Sent: Thu July 17, 2025 05:56 AM
From: Nezih Boyacioglu
Subject: Storage Insights Overhead Capacity
It's really strange!
I checked a few FlashSystems and I can't see any value under "Overhead Capacity."
------------------------------
Nezih Boyacioglu
Original Message:
Sent: Thu July 17, 2025 05:11 AM
From: T Masteen
Subject: Storage Insights Overhead Capacity
Hello Nezih,
On SVC we have host_unmap off and backend_unmap on. On FS both are on
In Storage Insights I can match the number of the overhead capacity with the number of Total savings. this makes it even more confusing for me.
------------------------------
TMasteen
Original Message:
Sent: Thu July 17, 2025 02:58 AM
From: Nezih Boyacioglu
Subject: Storage Insights Overhead Capacity
Hi T.
what is the unmap settings on SVC and on 7x00?
------------------------------
Nezih Boyacioglu
Original Message:
Sent: Thu July 10, 2025 09:52 AM
From: T Masteen
Subject: Storage Insights Overhead Capacity
Hi All,
Storage Insights allows you to view the capacity figures for an FS7x00.
I can't quite place one number: "Overhead Capacity".
The description is: "The amount of usable capacity occupied by metadata in a pool or system and other data used for system operation."
I'm struck by the large number, larger than, for example, "Usable Capacity." The Flashsystem is virtualized by an SVC.
What does this number represent? And where can I find more details about it?
Thanks.
------------------------------
TMasteen
------------------------------