I plan to expand an array size by adding two FCMs on a production system.
The current DRAID6 array consists of 13 19.2TB FCMs.

I have no plan to stop the attached servers for this expansion work.

I have two questions:

Q1:
The expandarray command usage says the following:
"Expansion can have an impact on host performance and latency when performed on an array that has over 50% load."

Would you please tell me what does "50% load" mean?
I need to know the exact metrics so that I can check the "50% load" on the FS7300
https://www.ibm.com/docs/en/flashsystem-7x00/8.5.x?topic=ac-expandarray

Q2:
How long does it take to complete the array expansion process after executing expandarray?  Are there any good estimation method?

Best regards, Eiichiro

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EIICHIROH NAKAGAWA
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Hello Nakagawa-san,

Q1 is a valid question, which I cannot offer an answer for as of now.
To know 50%, we'd need to know what the 100% load would be, I will consult on that with the community.
Perhaps, someone else can contribute some useful insight on that in the meantime.

For Q2: it is hard to estimate, if not even impossible. Since the array expansion process is a background task with low priority, the duration may vary during the process depending on the array's workload.
Once kicked off, lsarrayexpansionprogress provides an ETA.

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Best regards, 

Christian Schroeder
IBM Storage Virtualize Support with Passion
------------------------------

By digging further, I came across these essential development statements with regards the factors impacting the array expansion process

• amount of data on the array, as all data will be moved during the expansion due to change in the RAID geometry
• number of drives in the array; the more drives, the faster
• DRAID array expansion generally is designed to cause minimal impact to host I/O; expansion activity is being throttled therefore to ensure little performance impact. The heavier the host I/O , the more the expansion activity will be throttled 

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Best regards, 

Christian Schroeder
IBM Storage Virtualize Support with Passion
------------------------------

Original Message:
Sent: Tue June 11, 2024 02:59 AM
From: Christian Schroeder
Subject: FlashSystem 7300 expandarray command

Hello Nakagawa-san,

Q1 is a valid question, which I cannot offer an answer for as of now.
To know 50%, we'd need to know what the 100% load would be, I will consult on that with the community.
Perhaps, someone else can contribute some useful insight on that in the meantime.

For Q2: it is hard to estimate, if not even impossible. Since the array expansion process is a background task with low priority, the duration may vary during the process depending on the array's workload.
Once kicked off, lsarrayexpansionprogress provides an ETA.

------------------------------
Best regards, 

Christian Schroeder
IBM Storage Virtualize Support with Passion
------------------------------

Original Message:
Sent: 6/11/2024 2:59:00 AM
From: Christian Schroeder
Subject: RE: FlashSystem 7300 expandarray command

Hello Nakagawa-san,

Q1 is a valid question, which I cannot offer an answer for as of now.
To know 50%, we'd need to know what the 100% load would be, I will consult on that with the community.
Perhaps, someone else can contribute some useful insight on that in the meantime.

For Q2: it is hard to estimate, if not even impossible. Since the array expansion process is a background task with low priority, the duration may vary during the process depending on the array's workload.
Once kicked off, lsarrayexpansionprogress provides an ETA.

------------------------------
Best regards, 

Christian Schroeder
IBM Storage Virtualize Support with Passion
------------------------------

Hola NAKAGAWA san!

I didn't found info specifically about the 50%. I'd say the 100% is the max workload that the array can satisfy. From the Flashsystems redbook, in the section "General Planning", it  talks about performance requirements, then this note:

From my tests in lab, the workload also affects the rebuild time of the arrays, so probably also affecting the total time to complete the expansion. Up to 4x faster when no IO. So you might also want to try it once the storage system has the less work.

Sorry I don't expectations on how it should take your array to expand to the new size.

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Luis Lopez
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Hi,

What is referred to here as 100% is actually the highest utilization of resources in your system. If you are expecting a maximum latency of 2 milliseconds from the 7300, the time when it is working with an average latency of 2 milliseconds is not the right time to expand the array. in such a situation, it would be a better option to prefer time periods with lower utilization for you rather than percentage rates. 

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Nezih Boyacioglu
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