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 Post subject: Raid 5+0
PostPosted: Wed Apr 11, 2018 5:38 am 

Joined: Thu May 28, 2015 3:45 am
Posts: 18
Hi there,

I'm trying to wrap my head around space consumption on a 7400.

If I provision a thick volume of 5 TiB in size, it only results in a space consumption overhead of 27%.

Which is about what I would expect for Raid 5, 3 Data + 1 Parity. But it should be double this quantity if it's including a mirror as well? Can anyone explain how come the mirror space doesn't appear to show?

Here's how I worked it out:

5TiB thick provision in FC_r5 CPG
FC space prior to provision: 28108.08 GiB
FC space post provision: 21599.76 GiB

Difference: 6508.32

6508.32 (actually used) / 5120 (provisioned) = 127% but I would expect this to be 252% due to raid 0 mirror?


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 Post subject: Re: Raid 5+0
PostPosted: Wed Apr 11, 2018 6:41 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 1570
Location: Europe
RAID 0 is stripe, not mirror :)

Not sure why you are expecting 2x the capacity usage :)

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 Post subject: Re: Raid 5+0
PostPosted: Thu Apr 12, 2018 2:44 am 

Joined: Thu May 28, 2015 3:45 am
Posts: 18
:oops: Not the first time I've gotten stripe and mirror mixed up.

Now I'm even more confused though, because my understanding is raid 5 is a stripe anyway? Why does the documentation specify it as 5+0?


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 Post subject: Re: Raid 5+0
PostPosted: Thu Apr 12, 2018 7:31 am 

Joined: Mon Sep 21, 2015 2:11 pm
Posts: 1570
Location: Europe
RAID 0 = stripe
RAID 1 = mirror
RAID 5 = single parity
RAID 6 = dual parity

A normal CPG could be Raid 5 (3+1) which means 3 data for each parity. As 3PAR is using wide-striping it is generally considered to be RAID 50 (or 5+0) when multiple 3+1 is used to generate a data set.

Going a bit deeper, each disk in a 3PAR is divided into chunklets (1GB for Gen4 and Gen5 systems). Without the "+0" you wouldn't be able to have a volume (or CPG) larger than 3GB with RAID 5 (3+1).

I would recommend reading pages 42-44 or something in the concepts guide https://support.hpe.com/hpsc/doc/public ... =c04204225 .

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 Post subject: Re: Raid 5+0
PostPosted: Thu Apr 12, 2018 9:49 pm 
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Joined: Tue Aug 18, 2009 10:35 pm
Posts: 1328
Location: Dallas, Texas
What documentation are you referring to with "5+0"?

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Richard Siemers
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