[gpfsug-discuss] data replicas and metadata space used

[Russell Nordquist]

I was reading this slide deck on GPFS metadata sizing and I ran across something
http://files.gpfsug.org/presentations/2016/south-bank/D2_P2_A_spectrum_scale_metadata_dark_V2a.pdf

On slide 51 it says

"Max replicas for Data, multiplies the MD capacity used

• Reserves space in MD for the replicas even if no files replicated!”

This is something I did not realize - setting data replicas to 2 or even 3 consumes metadata space even if you are not using the data replicas. For metadata replicas it says unused replica’s have little impact - great. I like to set data and metadata replica’s to 3 when I make a filesystem even when the initial replicas used are 1 since you never know what will change down the road. However this makes me wonder about that idea for the data replica’s - it’s really expensive metadata spacewise.

This information was written prior to GPFSv5 when subblocks changed from only 32. Does it still hold true that unused data replica’s use metadata space with v5? 

thanks
Russell


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