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 Post subject: Physical Disk Unbalanced
PostPosted: Fri May 24, 2019 1:00 am 

Joined: Sun Feb 23, 2014 9:29 am
Posts: 8
Hello,

I have a 3Par 7400 4node with 2 shelfs (4cages total). My configuration has 80x450g 10k FC and 12x400g SSD.
I have 20 10k FC per cage and 3 SSD per cage.
The system complain about unbalanced physical drives for SSD. Also i tried to create a CPG with RAID 5 3+1 for SSD drives and it gives 1.85tb of space. Is it normal? What is the optimal SSD physical disk placement for my setup?
Thank you.


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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Fri May 24, 2019 3:15 pm 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 854
Location: Europe
No, you should always install drives in pair in each cage.

So 3 SSDs in a cage isn't just not according to best practice or optimal setup. It is actually unsupported.

12x400GB with default sparing will lose the capacity of two drives to sparing. 400GB drives give ~370 chunklets (GiB), so 3700 chunklets to build LDs. 25% RAID overhead should give about 2.8TiB of capacity as long as it is installed and configured correctly... Did you configure HA Cage? And what system (hardware and OS version)?

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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Fri May 24, 2019 5:47 pm 

Joined: Sun Feb 23, 2014 9:29 am
Posts: 8
Thank you for the answer. I created the SSD CPG with magazine availability. The software version of the system is 3.2.1MU5. So with 12xSSD what is the supported configuration? Can i install 4 SSD in two cages and two in the other two cages?


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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Fri May 24, 2019 8:14 pm 
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Joined: Tue Aug 18, 2009 10:35 pm
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Location: Dallas, Texas
Correct. 4 in the in the head unit shelves, 2 in the expansion shelves.

Optimal configuration depends on your workload requirements. Is this just a lab box, or do you have a specific goal in mind for it and know how much TB you need, and how much iops or throughput you need? You can configure for speed, for capacity, or somewhere in between.

Don't forget you can also use the SSDs for Adaptive flash cache.

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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Sat May 25, 2019 4:14 am 

Joined: Sun Feb 23, 2014 9:29 am
Posts: 8
OK. I will try to place the physical drives in the cages with this way and i will let you know about the results.
About the configuration, it will be a production system.
Can you tell me the way i should follow in order to move the SSD disks to cages? Should i use the relocatepd command and then the tunesys or is it better to initialize (OOTB) the system again? In the first case should I delete the SSD CPG that already exists?

Finally, how the space is calculated in the 3PAR. For example in the raid 3+1 CPG, i should calculate 1 disk for parity data, for each 3 disks of data?


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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Sat May 25, 2019 8:29 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 854
Location: Europe
agree wrote:

Finally, how the space is calculated in the 3PAR. For example in the raid 3+1 CPG, i should calculate 1 disk for parity data, for each 3 disks of data?


Disk or to be more correct, chunklet.

If you are able I would try to get the system upgraded to 3.2.2MU6. It will give you express layout for SSD, a bunch of bug fixes and a currently supported version.

If the system is not in use and have no data, the easiest way is doing another ootb. If there is data there, relocatepd or similar would be your option but it could become time consuming.

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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Mon May 27, 2019 3:04 am 

Joined: Sun Feb 23, 2014 9:29 am
Posts: 8
Hello again,

I tried to move the disks (4 in each head unit and 2 in expansion shelfs) and the warning about unbalanced disks is gone. Now the space in CPG is 2.4tb. It only allows to create Raid 1 and Raid 5 2+1. I preferred the second option in order to get more space. Why with current configuration cannot create eg raid 3+1? Do you think that the raid 5 2+1 is safe option for production system?


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 Post subject: Re: Physical Disk Unbalanced
PostPosted: Mon May 27, 2019 3:30 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 854
Location: Europe
agree wrote:
Why with current configuration cannot create eg raid 3+1?

1. You do not have a version supporting Express Layout
2. You have 12 SSDs equally distributed across 4 nodes

Without Express Layout, one SSD is only seen and accessed from one node. Each node has only 3 drives. With 3 drives you can only do normal R1, 3-way R1 or R5 2+1.

With Express Layout (from 3.2.2 MU2 or something) each SSD is seen and accessed from both nodes in a nodepair. That will give you a max stripesize of 6 allowing all options of R1, R5 2+1,3+1,4+1 and 5+1 and R6 4+2.

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