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 Post subject: RAID 6 14:2
PostPosted: Thu Sep 13, 2018 8:03 am 

Joined: Tue Nov 19, 2013 4:38 pm
Posts: 115
I am using R5 (7+1) on my 8400 Allflash and want to use R6 instead for obvious reasons.
If I go for R6 14+2 I won’t lose any capacity compared to R5 7+1.
Are there any downsides to R6 14+2?
My plan is to change the CPG settings and run tunesys. Are there anything that I must be aware of.


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 Post subject: Re: RAID 6 14:2
PostPosted: Thu Sep 13, 2018 9:38 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 711
Location: Europe
RAID6 has more backend IO on writes, so the maximum frontend performance the system can deliver will decrease by some percent.


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 Post subject: Re: RAID 6 14:2
PostPosted: Thu Sep 13, 2018 9:50 am 

Joined: Wed Nov 09, 2011 12:01 pm
Posts: 282
There is a performance impact moving from R5 -> R6 due to additional reads/writes on the backend, if you have the ninjastars tool it'll show you the estimates for your config. If you aren't pushing the array hard atm you might not notice it.
As you say going from 7+1 to 14+2 uses the same amount of space for parity so shouldn't lose space once complete, it will need additional space for the conversion though so keep that in mind.
I'm not too sure of the math but seem to remember a performance hit relating to larger set sizes, in the past 3PAR was normally optimised for 3+1 which was considered the sweet spot for performance vs space. I'm not certain how that factors in on latest hardware/software versions and the use of SSD though as I've not seen any graphs/details on this since the HPE purchase (we used to get a lot more low level geeky stuff out of 3PAR when they were small and you could have dinner with the senior dev/tech people occasionally). ;)


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