DukeR1337
2 Iron

Question about system drives in all flash and hybrid systems

Dear all

I have read this article https://www.emc.com/collateral/white-papers/h15093-dell-emc-unity-best-practices-guide.pdf and now I have some questions.

On page 12 it says

Dell EMC recommends using only a single drive size and a single RAID width within an all-Flash pool. EXAMPLE: For an all-Flash pool, use only 1.6TB SAS Flash 3 drives, and configure them all with RAID-5 8+1.

On Page 13 it says

The first 4 drives of every Dell EMC Unity system are called the system drives.

System drives can be added to storage pools like any other drive, but offer less usable capacity due to the system partitions. To reduce the capacity difference when adding the system drives to a pool, Dell EMC recommends using a smaller RAID width for the tier which will contain the system drives. EXAMPLE: Choose RAID-5 4+1 for a tier containing the system drives, instead of 12+1.

The system drives also support a modest IOPS workload related to system management functions. For systems with Flash system drives, this workload is negligible.

DYNAMIC POOLS

At time of creation, dynamic pools will use the largest RAID width possible with the number of drives specified, up to the following maximum widths:

 RAID-5: 12+1

For RAID-5, initially create the pool with at least 14 drives

QUESTION NOW

1) What kind of disks are the system drive in a all flash array? Flash as well? Same size then the other ordered disks?

2)  It mentioned if you use the system drives in a pool configure the smallest raid width but it says also initially create the pool with at least 14 drives for a dynamic pool.

2.1) Is the number 14 with system disks or without?

2.2) Can I add the system drive to the dynamic pool or not? Any drawbacks?


I would like to buy an all flash array with approx 18 TB of usable capacity with dynamic pool and think provisioning. So do I need to buy 14 addition disks to the 4 system drives and do NOT add the system drives or do I buy 14 disks in total (with system drives).


That is confusing me quite lot 🙂


PS: In a hybrid one with classic pool how is it going there with the system drive? Using them in a small pool with 4+1 or leave them untouched?


Cheers and thanks

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tsamuel0905
2 Iron

Re: Question about system drives in all flash and hybrid systems

For Hybrid arrays, SAS drives are preferred to be used as system drive. System Drives are part of pool, RAID config serve as pool serves. Overall Pool capacity includes system drive, so actual usable capacity varies.

For AFA, it similar to Hybrid but instead of SAS drives it uses Flash drives.

More clarification, you can reach out to sales partner.

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Re: Question about system drives in all flash and hybrid systems

If you buy an AFA you should use all the drivers, including the "VAULT ones". You have to take in count that only in Hybrid configurations, because the VAULT drives supports "a modest IOPS workload". But "For systems with Flash system drives, this workload is negligible". (look page 13).

All the AFA configurations (quick configurations on "midrange sizer") came with all the disks in a single storage pool.

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