MSA 2040 Add vitual disk-groups of same RAID, but different size, in existing Pool

[markos_stefanou]

Hello,

excuse me, but I am new to HP MSA technologies and I have some questions:
We have the following existing setup:

MSA 2040
Pool A, 8 virtual disk-groups of RAID1 (2 x 400GB each, SSD)
Pool B  (not-important)

We want to add some more space to Pool A, so we bought two 800GB SSD drives, that will build a RAID 1 virtual diskgroup, with 800GB size.
So, can we add this diskgroup to the existing Pool A, or we should only add 400GB RAID1 diskgroups?
Will there be any performance issues?

Thank you in advance

[SUBHAJIT KHANBARMAN_1]

@markos_stefanou 

If the capacity of existing VDG different from new VDG then yes number of pages stored on VDG will eventually differ compared to other Virtual disk group in that Tier, and would therefore also lead to unpredictable performance.

That's why it's always suggested to match the capacity, RAID and disk type with existing VDG if you like to get optimum performance for the same tier.

Hope this helps!
Regards
Subhajit

I am an HPE employee

If you feel this was helpful please click the KUDOS! thumb below!

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[markos_stefanou]

Hello Subjajit,

thank you very much for your answer.

Since there aren't any 400GB SSD sold anymore, we had to buy 800GB SSD. We don't mind using half the space in order to have optimum and predictable performance.

How can I create a VDG (400GB, RAID1) using the two 800GB hard disks, in order to go with the suggested HPE practice? 

When I try to create the VDG with the two disks and add it the pool, it doesn't allow me to set the desired size. It just auto-sets it at maximum available space. Also I couldn't find any command for that in the CLI reference.

Thank you

[SUBHAJIT KHANBARMAN_1 · Accepted solution]

@markos_stefanou 

There is no way that we can limit drive size. So you can create new VDG with 800GB drives and use it and you have to accept unpredictable performance.

Otherwise you have to take data backup and replace all 400GB drives with 800GB drives and use them.

You can use 400GB drives in different Pool also and use them for other volumes.

Hope this helps!
Regards
Subhajit

I am an HPE employee

If you feel this was helpful please click the KUDOS! thumb below!

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[Shawn_K]

Hello,

Once you create a vdisk there is metadata associated with the vdisk that determines the space per drive. In your case you started with 400GB drives. So the metadata of the vdisk requires all newly added drives to use 400GB in order to be added ot the vdisk. You can add the 800GB drives to the existing vdisk but it will only use 400GB of those drives. You cannot define how much space each drive will utilize.

If you are creating a vdisk with different capacity drives, the metadata of the vdisk will be limited to the smallest drive. For example, if you have a 400GB, 600GB, 800GB, and a 1.2TB drive, your vdisk drive size will be 400GB. It is recommended to use drives of the same capacity so you get the most use of each drive. However, as drive vendors change their drive models we sometimes see the issue you are facing.

Your performance will be impacted in every vdisk by the slowest drive member. This is often difficult to determine because the performance difference is so minimal. However, I have also seen one drive impact the vdisk. When this happens, the suggestion is to replace the drive. Unless your drives are hugely different speeds, you should not see a noticable performance impact.

As long as you are ok with the 800GB drives being used as 400GB drives you should be fine adding those two drives into your exisiting vdisk. Unfortunately, if you want the larger capacity, you will need to replace all the 400GB drives. Be aware, if you plan to replace the 400GB drives you cannot add the 800GB drives in as replacement drives one at a time. This is due to the metadata of the vdisk being set at 400GB. You would need to backup the data, replace all the 400GB drives with the new 800GB drives, create a new vdisk, and restore the data.

I work for Hewlett Packard Enterprise. The comments in this post are my own and do not represent an official reply from HPE. No warranty or guarantees of any kind are expressed in my reply.

Cheers,
Shawn

[ArunKKR]

Hi Markos,

MSA 2040 supports both linear vdisks and virtual pool.

I believe that you are using virtual pools.

Shawn's comments would be based on linear vdisk configuration.

In this scenario you may create RAID 1 DG using new 2 X 800GB SSD disks under Pool A and it should work with out any major impact on performance.

[ArunKKR]

Hi Markos,

Please let us know whether the information shared helped.