FAS 2020 disks per controller and no. shelves to get usable capacity

[MAHMOUD_MAGDY]

Hello,

been reading a lot about how to calculate usable capacity with HA pairs and single controllers but would like to have some feedback if I am on the correct track or not

for HA pairs in FAS 2020, each controller can have a root volume with 2 disks RAID4, so if my system got 1 shelf 12 Disk, then I will lose 4, leaving me with 8.

those 8 if placed in single aggregate assuming that they are 2 TB and with 1 spare and 2 Parity then I will run away with 5 Disks (arount 10 TB).

am I correct so far?

for the second shelf I won't have to assign more disks to the controllers so I run away with single aggregate of (1 spare, 2 parity) of 18 TB, since fas 2020 with 7.3.1 has max. of 16 TB then I will have to assign 1 disk to the aggregate of the first shelf and I will enjoy my 16 TB aggregate on the second shelf and 12 TB aggregate (1st shelf + second shelf).

am I on the correct track.

THanks.

[aborzenkov · Accepted solution]

It is not mandatory to have separate root aggregate. It is perfectly acceptable for (relatively) small system to have single aggregate which hosts all volumes.

You do need separate root volume though.

[MAHMOUD_MAGDY]

WOW, Thanks for the very quick reply.

so for small deployment I can run away with 1 aggregate with all volumes, looks nice.

any recommendations for the root volume size.

also can you double check if my understanding are right for the second shelf in case I got a larger system configuration, I mean are my understanding and calculation right?

[aborzenkov]

The minimal required root volume size depends on model and is documented in Storage Management Guide. I never had pressing needs to go beyond this minimum. The only reason I can think of is to keep longer performance archive history.

It does not matter whether disks are in base or extension shelf; you can assign them any way you need to each controller in HA pair. So there is no single answer. For two shelf configuration I’d probably use all disks in first shelf for one head and all disks in extension shelf for another head. Purely out of aesthetic reasons ☺ But at the end it is performance requirements and sizing that decide.

[MAHMOUD_MAGDY]

Thanks a lot for your awesome detailed replies.

[MAHMOUD_MAGDY]

also wondering if I can make controllers disks 1 TB instead to lower the waste of hards?