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 Post subject: Difference Between Overall Storage and CPG Allocation
PostPosted: Wed Apr 24, 2019 11:04 am 

Joined: Wed Apr 24, 2019 10:41 am
Posts: 3
One thing I never could grasp is how to correlate the overall storage allocated with what's allocated to the CPG's, so I'm hoping someone can help me understand.

I'm using the last IMC version for this example, but I see the same on SSMC.

If I select Systems --> Storage Systems -->, then the Summary tab, I have about 70 terabytes of SSD allocated. (SSD Total - SSD Free)

If I select Provisioning --> CPGs --> then the CPGs tab, I have 5 SSD CPGs, but the sum of the allocated capacity for them is only about 45 terabytes, so I'm confused by what I'm missing.


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 Post subject: Re: Difference Between Overall Storage and CPG Allocation
PostPosted: Wed Apr 24, 2019 12:04 pm 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 884
Location: Europe
1. Overall storage is RAW, while CPG is post-RAID
2. Overall usage includes spare capacity.

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 Post subject: Re: Difference Between Overall Storage and CPG Allocation
PostPosted: Wed Apr 24, 2019 1:22 pm 

Joined: Wed Apr 24, 2019 10:41 am
Posts: 3
Thanks MammaGutt. What do you mean exactly by "spare capacity"? I'm assuming that's where my confusion comes from because the difference seems to be too much for just RAID overhead.

MammaGutt wrote:
1. Overall storage is RAW, while CPG is post-RAID
2. Overall usage includes spare capacity.


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 Post subject: Re: Difference Between Overall Storage and CPG Allocation
PostPosted: Wed Apr 24, 2019 2:21 pm 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 884
Location: Europe
The capacity equal to X amount of drives per Y amount installed is reserved as spare to handle failed chunklets and drives and you don't need to wait for a failed drive is replaced before starting to rebuild.

Look at it like hot spare, expect that it isn't dedicated drives like most arrays do, but it is distributed across all drives for performance reasons.

First of all you rebuild to all drives in the system rather than hammering one during rebuild, and all drives in the system is active during normal operation.

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 Post subject: Re: Difference Between Overall Storage and CPG Allocation
PostPosted: Mon Apr 29, 2019 9:01 am 

Joined: Wed Apr 24, 2019 10:41 am
Posts: 3
Thanks again for the response. Perhaps I've just never noticed how much space these things take up, but does it sound right that RAID overhead + reserved space would equal 30 terabytes out of a total 73 allocated (to 2 CPG's)?


MammaGutt wrote:
The capacity equal to X amount of drives per Y amount installed is reserved as spare to handle failed chunklets and drives and you don't need to wait for a failed drive is replaced before starting to rebuild.

Look at it like hot spare, expect that it isn't dedicated drives like most arrays do, but it is distributed across all drives for performance reasons.

First of all you rebuild to all drives in the system rather than hammering one during rebuild, and all drives in the system is active during normal operation.


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 Post subject: Re: Difference Between Overall Storage and CPG Allocation
PostPosted: Mon Apr 29, 2019 11:35 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 884
Location: Europe
selym wrote:
Thanks again for the response. Perhaps I've just never noticed how much space these things take up, but does it sound right that RAID overhead + reserved space would equal 30 terabytes out of a total 73 allocated (to 2 CPG's)?


It depends. Worst case scenario:
If you have 6x7.68TB(or lets say 8TB for easy math) SSDs, with default sparing and R6 (4+2) you have 48TB of space. You reserve 1 per per 24 SSDs with a minimum of 2 to spare, that is 16TB in this example. Then you have 32TB RAW left. With R6 (4+2) you have 33% RAID overhead and lose ~11TB to overhead and is left with ~21TB for data. That is 21TB of data for 48TB RAW...

Sparing can be reduced to minimum and RAID levels can be changed...

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