HPE Storage Users Group

Hi all,

Here's a scenario for you all! In one of my 7400s we have 8 x 480gb SSDs (446gb usable).

Setup in a 3+1 R5, that would give us 6 disks of usable space. 6 x 446 = 2676gb of usable space.

If I add up the "Used User Space" VVs assigned to this CPG, I get 1644gb. To me, that means that I've got roughly 1000gb of free space.

In the 3PAR Management Console, when I click on the CPG and view the "Estimated Free System Space", it estimates I have 444gb free. Somehow I'm missing ~500gb of space.

Where am I going wrong?


Thanks
Joe

Maybe you missed spare and admin space?

The spare and admin spaces are negligible (~10gb). I ran a compactcpg for good measure but no change.


Thanks

Hi,

you should add up "user reserved space", and not "user used space". If you want to know how much more data you as a user (from a server view) can put on a 3par array, the cpg's "estimated free system space" is a much better place to look imho!

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When all else fails, read the instructions.

In your scenario you have 2.12TB Usable space for raid5 3+1. Spare 0.44Tb and Raid overhed 0.71TB and sparing algorthm minimum
For Raid5 7+1 you got 2.47Tb usable space. 0.44Tb spare and overhed 0.35Tb. Sparing algorithm minimum.

Hi apol,

The CPG's "estimated free system space" is exactly what I'm referring to. That takes into consideration RAID overheads and whatnot I understand, to give me a real world value of what I can load into the array.

If I add up the reserved user space it equals 1659gb (similar to used user space).

Georgii, I'm not quite sure I understand. If I have 8 disks at 480gb (446gb actual recognised size of disk), I lose 2 disks in the RAID3+1 configuration, right? That would give me 6 x 446gb = 2676 usable.

Your numbers add up to what I'm seeing though (0.44tb free), are you able to help explain a little further how you were able to calculate this?


Thanks, Joe

Please read PM. Tnx.
In your scenario you lost 1.15 TB

You have 8 x 480 GB SSDs. That will give you a total of 446 x 8 chunklets = 3568.
Now your sparing algoritm comes into play. Minimum should be the equal of 1 SSD, default should be equal of 2 SSDs.

Minimum would leave you with 3122 chunklets (GB)
Default would leave you with 2676 chunklets (GB)

R5 3+1 (25% RAID overhead) would give you the following useable space:
on minimum : 2351.5 GB
on default : 2007 GB

From what I understand from your first post, you seem to have about 2007GB useable space. In that case I would recommend changing the sparing algoritm to minimum (SSDs fail so seldom, that it is very unlikely that you will lose two SSDs before you're able to replace the first one), and I would strongly considering upgrading to 3.2.2 MU3 to get the Express Layout feature (added in 3.2.2 MU2) which will allow you to do R5 7+1 on 8 SSDs. That should give you 2731.75GB of useable space in your current hardware configuration.

All numbers exclude admin space and OS (which usually would be on FC if this is a hybrid array)

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The views and opinions expressed are my own and do not necessarily reflect those of my current or previous employers.

Thanks for the explanation