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Raid 5+0
https://3parug.net/viewtopic.php?f=18&t=2852		 Page 1 of 1
Author:  Jim Mc [ Wed Apr 11, 2018 5:38 am ]
Post subject:  Raid 5+0
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?
Author:  MammaGutt [ Wed Apr 11, 2018 6:41 am ]
Post subject:  Re: Raid 5+0
RAID 0 is stripe, not mirror :)

Not sure why you are expecting 2x the capacity usage :)
Author:  Jim Mc [ Thu Apr 12, 2018 2:44 am ]
Post subject:  Re: Raid 5+0
: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?
Author:  MammaGutt [ Thu Apr 12, 2018 7:31 am ]
Post subject:  Re: Raid 5+0
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 .
Author:  Richard Siemers [ Thu Apr 12, 2018 9:49 pm ]
Post subject:  Re: Raid 5+0
What documentation are you referring to with "5+0"?
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