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Confusing about Space Reclamation when changing NRAID-Level

Ralf Gerresheim
Frequent Advisor

Confusing about Space Reclamation when changing NRAID-Level

Hi,

 

I hvae a problem to understand the required space on the P4000 if I change the NRaid level from 0 to 5:

The situation:

Do to space resctrictions, all volumes on a 2-node Cluster run NRAID 0.

Now, additional 5 nodes are added to that cluster and the volumes should be changed to NRAID5.

The volumes are only used for read operations there will be no changes to the data.

 

When I edit die volume propperties from NRAID 0 to NRAID 5, the consummed space is doubled!

This is, what I do not understand. As I understand, with NRAID 5 the data is striped to 2, 3, or 4 nodes with an additional node for the parity information. I would expect with 7 nodes in a Cluster, data are striped to 4 nodes with a fifth node for parity. So why does the system uses double space with NRAID 5 compared to NRAID 0 insted of only 20%  more space?

 

I read in the user guide, that with NRAID 5 (and NRAID 6) a snapshot schedule is required. If not already exist, it will be created automatically. Is that the reason for?

 

If I create a NRAID 10 volume from the NRAID 0 volume it consumes the same space than the NRAID 5 volume. Does it makes sense to change to NRAID 10 instead of NRAID5?  Which gives the higher read performance?

 

Thanks for your answers in advance.

 

2 REPLIES
oikjn
Honored Contributor

Re: Confusing about Space Reclamation when changing NRAID-Level

NR10 is definitely much better for performance than NR5.  I believe NR10 is the only NR level HP actually suggests for production. 

 

NR5 should take up less space than NR10, but you might be checking it too soon.  Did you wait for the volume to restripe completely?  During the conversion time I believe the volume ends up being both NR0 and NR5 so until it completes the NR5 transition it can't purge out the NR0 data.  I would guess this won't happen until some time AFTER the restripe completes.  try converting one of your smaller volumes and letting it complete to NR5 and then see what the space used shows a day after the restripe completes.

 

I would suggest running NR10 as long as you have the space available as performance is generally a problem for NR5, but if your load really is 90+% read, that might not actually be an issue.

Emilo
Trusted Contributor

Re: Confusing about Space Reclamation when changing NRAID-Level

Without going into a long explanation just change it to thin provisoned you will see the right voulme size then.