Drobo Pro with more than 16TB?

[Alex Upton]

I'm curious if anyone knows if this is expected functionality for an original Drobo Pro?

Within the past month I received an alert that I was running low on space and needed to replace one of my 3TB drives. So I replaced it with a 4TB drive and let the rebuild complete. After completion with the new 4TB drive the Drobo Dashboard still stated that I needed to replace the drive... So again I added a 6TB drive based on the last firmware release notes that stated >4TB drives were supported.

After the rebuild with the 6TB drive I'm still left with the same available storage issues... 

Here's my current drive layout. I'm using the drobo pro with support for dual drive failures.

1. 6TB
   
2. 3TB
   
3. 3TB
   
4. 3TB
   
5. 4TB
   
6. 3TB
   
7. 3TB
   
8. 3TB
   

Yet the total available is stuck at 16.32TB

Is there something I'm doing wrong or failing to understand?

Thank you in advance.

-Alex

[amo...@iknowkungfoo.com]

The total capacity is calculated using the smallest sized drive in the array.

I recently had a drive go out on my Drobo FS, I had a 1.5TB, and two 2TB drives. I replaced the faulty 2TB with a 3TB drive and the total capacity didn't change. It wasn't until I replaced the other two drives with 3TB drives that the capacity increased. This is all related to the RAID functionality. Now I have 5 x 3TB drives, with dual drive redundancy, which gives me I think ~8TB usable storage.

HTH,

Adrian

-----Original Message-----
From: "Alex Upton" <alex....@gmail.com>
Sent: Thursday, May 14, 2015 10:29am
To: drobo...@googlegroups.com
Subject: Drobo Pro with more than 16TB?

I'm curious if anyone knows if this is expected functionality for an
original Drobo Pro?

Within the past month I received an alert that I was running low on space
and needed to replace one of my 3TB drives. So I replaced it with a 4TB
drive and let the rebuild complete. After completion with the new 4TB drive
the Drobo Dashboard still stated that I needed to replace the drive... So
again I added a 6TB drive based on the last firmware release notes that
stated >4TB drives were supported.

After the rebuild with the 6TB drive I'm still left with the same available
storage issues...

Here's my current drive layout. I'm using the drobo pro with support for
dual drive failures.

1. 6TB
2. 3TB
3. 3TB
4. 3TB
5. 4TB
6. 3TB
7. 3TB
8. 3TB

Yet the total available is stuck at 16.32TB

Is there something I'm doing wrong or failing to understand?

Thank you in advance.
-Alex

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[Peter Silva]

putting in bigger disks means it needs reserve more to keep enough space to withstand failure of the two biggest ones.

To calculate capacity with disk redundancy, remove the 2 largest disks from the capacity, and subtract their capacity from the remaining ones

you started with 8*3 , so capacity was 6*3 - (3+3 reserve) = 12 TiB usable

By adding higher capacity spares,  reserve has grown from 3+3 to 6+4.

After your replacements, to have dual  3*6 = 18 - 10 -> 10 TiB of usable space.

If you keep feeding it, it should get better.  If you replace another disk of the same or smaller capacity, then the delta will be pure added capacity (the equation only changes when the new disk is one of the two largest ones.)

so add a second 6TB drive, and you should get 3 extra TB usable.  It might stop complaining at that point... 

The above assumes you want to keep dual disk redundancy..  turning that off would give you 4 TB and stop the squawking also.

[Perseus]

I know this thread is years old. Nevertheless and just in case someone does not know how DroboPro storage space and disk redundancy is calculated - there is a nice interactive tool:

 * https://www.drobo.com/storage-products/capacity-calculator/

Just select an 8 slot device, then drag and drop disk sizes from below onto the device and the calculation is done for you in an instance.

NB: I updated my DroboPro's firmware under Linux years ago from 16 to 24 TiB total storage size. Since then I can use up to to 8 x 3 TiB = 24 TiB disk space in total.

[Peter]

This all looks normal. If you have a bunch of equal sized disks and dual disk redundancy, you should really add three or four devices at a time, because the bigger disks don´t help until you get past 2.

The maximum space it had to cover for via parity when you started was 2 x 3 TiB == 6 TiB.  you have replaced them with two bigger disks, so now the biggest loss that needs to be tolerated is 4+6 = 10 TiB, so it doesn´t use anything above 3 TiB on the new drives.  You will only start getting more capacity on the third disk, and the increment will be the delta between the disk replaced and the smallest added disk.

Adding bigger disks means more space to cover for the failure.  If you now replace a third drive, either a 4TiB or a 6 TiB drive, you will  get +1 TiB (4 TiB added minus 3 TiB to replace from the disk removed.)  Since the other two drives are >= 4TiB. 

It´s better to use uniform capacity in such systems.  it works with uneven capacity which is great in a pinch, but people end up unhappy because the larger drives sort of lie fallow.