Configuring a new Pegasus2 R8 32TB

  • Last Post 01 March 2017
Barry Sharp posted this 28 February 2017


I have a new Pegasus2 R8 32 TB (8x 4TB Hitachi disks) arriving later this week. It will arrive as a full RAID-5 with capacity of 7x 4TB or some 28 TB.

I want to configure this R8 as follows

5x 4TB RAID-5 Volumes named PEG-1, PEG-2, PEG-3, PEG-4 and PEG-5 spanning 7 disks. I assume each of these Volumes will use 4/7 TB or some 0.571 TB on each of the 7 disks, right ? If this is correct, then the 5 Volumes will be using 2.857 TB on each of the 7 disks, and 2.857 TB on the 8th parity disk, right ?

The above will leave me with 8x1.143 TB or some 9.143 TB, right. Can I therefore use the 9.143 TB for a RAID-0 spanning the 8 disks and name this Volume as PEG-Scratch.

If my arithmetic is correct then...

The 5x RAID-5 Volumes will consume 22.856 TB across the 8 disks.

The PEG-Scratch RAID-0 will consume 9.143 TB

Thus 22.856 + 9.143 = 31.999 TB (this seems to be correct as the raw capicity for the 8x 4TB disks is 32TB)

Please let me know if I can accomplish the above or if not, what are my other options ?

BTW... the reason for configuring 5x 4TB RAID-5 Volumes PEG-1/2/3/4/5 is because we have an eSATA 5-bay enclosure with 5x 4TB disks. Each of these disks will be used to backup PEG-1/2/3/4/5 each day.

Thank you.

Order By: Standard | Latest | Votes
Dinesh Kannusamy posted this 28 February 2017

Hi Barry,

No, it is not possible to share the individual disk space to create a new disk array instead you can have 5 seperate(Pass Thru) drives so you will get 5 (4TB) volumes mounted on your desktop and you can name them as PEG 1/2/3/4/5(No RAID)  OR you can create a Disk array with 6 drives and then Create 5 Logical volumes(RAID5)  and use the other 2 drives for RAID 0/RAID1. 

Please contact Promise Tech support for more information.


Barry Sharp posted this 28 February 2017


Thank you for your response. Very helpful. If I understand you, one option for me is to configure as follows...

1) Using 6 4TB disks to form an array which then can be setup as 5x 4TB RAID-5 logical volumes named PEG-1/2/3/4/5. Performance should be what 5 disks can provide in a RAID-5 configuration.

2) Using the remaining two 4TB disks to form an array which then can be setup as 2x 4TB RAID-0 logical volume named PEG-Scratch. Performance should be what 2 disks can privide as RAID-0 configuration.

Do I have this correct ?

Thank you.

Barry Sharp posted this 28 February 2017

One other option I can think of but it's not as optimal for restricting PEG-1/2/3/4/5 from exceeding 4TB would be to configre all 8 disks as an array and then configure them as a RAID-5 logical volume. This would provide performance for RAID-5 using 7 disks.

The trick here would be to define six folder/directories at the top level in the file system as PEG-1, PEG-2, PEG-3, PEG-4, PEG-5 and PEG-Scratch.

During production the size of each PEG-1/2/3/4/5 will need to be monitored for not exceeding 4TB so they can be backed up to our 5-bay eSATA enclosure having 5 seperate 4TB disks.

The PEG-Scratch can grow to no more than 8TB or can be greater only if all of the PEG-1/2/3/4/5 logical volumes have not exceeded their 4TB limits.

I believe this will provide best performance for all the PEG-1/2/3/4/5 volumes as well as the PEG-Scratch volume as RAID-5 with protection against a single disk failure.

Is the above a configuable option for me using the Promise Utility ?


Thank you.

Richard Oettinger posted this 28 February 2017

Hi Barry,

the Promise RAID controller in the Pegasus will allow you to create one Disk Array using all 8 disks, and then create several Logical Drives in it.

A Logical Drive is the "RAID" volume, so you could create one RAID0 LD of a certain capacity, and then some RAID5 LDs of other capacities.

The thing to remember is that you would get great performance from all the LDs as they are striped over all eight drives - but if you access more than one at a time there will be a drop in that performance...

~ Richard

Barry Sharp posted this 28 February 2017

Richard: Thanks.

What you've advised is, I think, what I was trying to convey in my OP.

So here is what I think I need to do based on me understanding what you've posted...

1. Creat a Disk Array using all 8 disks.

2. Create a RAID-0 Logical Drive of size 8TB and call it PEG-Scratch

3. Create a RAID-5 Logival Drive of size 4TB and call it PEG-1

4. Repeat step 3 naming additional Logical Drives as PEG-2, PEG-3, PEG-4 and PEG-5, each with a size of 4TB

I do understand that doing i/o to two or more of these LDs at same time will cause a decrease in performance for each of the i/o streams. Our typical workflow usually only works on a single PEG-1/2/3/4 or 5 at any time so only a single i/o stream would be used. The PEG-Scratch really is there to facilate moving data around at times. Over the years I've always found having a nice large scratch-pad is usefull when rearranging storage space.

Have I understood you correctly with respect to the 4 steps above ?

Richard Oettinger posted this 28 February 2017

That is exactly what you do, Barry.

Remember, all of the LDs are striped across the set of disks, so for example, IO to a LD that is using the inner cylinders of the disks and one that is using the outer cylinders would mean that the disk heads would have to move in and out for each IO to the two LDs (the technical term is "shoe shining" 8^).


Barry Sharp posted this 01 March 2017



I practiced this configuration on my Pegasus2 R4 that had 4x 3TB disks.

1) I setup a Disk Array; 12TB

2) I setup a Logical Drive; 4TB RAID5 Alias PEG41

3) I setup a Logical Drive; 4TB RAID5 Alias PEG42

4) I setup a Logical Drive; 1.33TB RAID0 Alias PEG43

PEG41 started to be Synchronized

In Finder's sidebar under Devices "PROMISE RAID" displayed 3 times

I figured out which was which and renamed them using Finder to be PEG4-1-RAID5, PEG4-2-RAID5 and PEG4-Stripe.

It all seems AOK. Thanks very much for your help and prompting. I'm now ready for my Pegasus2 R8 32TB.