MD3800i pool expansion

Question

Hi,

I found a fun new rule I didn't know about before:

You can only add 12 physical disks to an existing disk pool. Ever! That's all you get! Why? It doesn't say. But wait, you actually CAN add more than 12 physical disks, it just only utilizes the capacity of the first 12, the rest is marked as "unusable". Why? It also doesn't say. My best guess, because I can find nothing else on the subject, is because rebalancing would take too long / affect performance too adversely if the number of spindles grows past a certain point (after the initial creation, which appears to have no hard limits other than the max # of spindles allowed by the entire array). Minimum of 11 disks in a pool to start, up to the max allowed by the array, but after creation, you can only add 12. not 12 at a time, 12 ever. I don't understand.

If anyone can enlighten me further, I'd sure appreciate it. All I wanted to do was add 17 disks to a 17 disk pool. Found that seemingly-arbitrary rule in the manual today and frustration ensued.

DELL-Chris H · Accepted Answer

Adding drives to a DDP is capped at a max of 12. If you need to add more than 12 you will need to kill & fill. DDP is different than disk groups, as disk groups you can add as many drives as you want as long as they are added in pairs.

DiegoLopez · Answer

Hello @Mtexter,

Are you sure about this? The information I have for Disk Pool expansion, is that we can add max 12 drives each time.

Are you using compatible disks? Do you have any errors on the array? When you look at the MD3800i from the front of it is there any amber lights on the 7 physical disk that are showing as offline in MDSM? Also have you tried to reboot the MD3800i as well?

Regards.

Mtexter · Answer

If it's true that you can add 12 at a time, and not 12 ever, that needs to be elucidated in the documentation. I attempted to add 17 disks all at once, but rather than having any sort of warning message from the MD array, it just let me do it and left 5 disks' worth of capacity as "unusable". We only noticed this because the final usable capacity was significantly less than expected. We did not attempt to re-add storage to the existing pool after we read about the 12-disk rule, we just created a new one with all 17 new disks.

To answer your prior questions, all the disks are certified Dell drives, there are no amber lights, no offline disks, etc. We were not able to reboot the MD3800 because it's a production system.

DELL-Chris H · Answer

Mtexter,

There is not enough info to answer completely. Would you confirm a couple things?

What is the FW version?

Are your using DDP or DG?

How many enclousers are attached?

If you're doing a fresh deployment then rules are different

Let me know

Mtexter · Answer

FW version is 8.20A mixture of disk pools and disk groups, although the attempt here was to grow a pool.2 MD1220 enclosures attached, in addition to the MD3820i head unit.not a fresh deployment.

Mtexter · Answer

Thanks for that clarification.  Not sure if possible on such an old product but it'd be nice to have that level of clarity in the manual somewhere.  Appreciate the help!

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MD3800i pool expansion

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