VDI Home Directory Sizing


To focus on IOPs or not to focus on IOPs that is the question?  As I sort out a new sizing dilemma I’ve run into a potential problem with SATA drives and IOPs contention.

The Scenario:

Storage Array: NetApp 6280, Data ONTAP 8.1.1, 3TB SATA, DS4243 Shelves, 10% Aggr Snap Reserve (Why? Because wafl iron rebuilds take a fraction of the time with Aggr snapshots)

User Home Directories: 20GB allocated to each user with 20% snapshot overhead

Disks TB / Disk IOPs / Disk
72 1.86 75
TB / Aggr GB / Aggr IOPs / Aggr
134 123,494.40 5400
GB / User Users / Aggr IOPs / User
24 5145 1.05

This leaves me with 1.05 IOPs per user.  A couple things to make note off, this assumes 100% concurrency ie. all 5,145 users accessing this volume at the same time which is highly improbable.  If 75% concurrency is plugged in the IOPs per user grows to 1.4 which seems more reasonable.

Concurrency 75% Concurrency Users IOPs / User
75% 3858.75 1.40

The assumption is that 25% of the user accounts within any organization are for temporary users who are not logged in on a consistent basis.

Possible Solutions to the potential IOPs performance shortage: Since it’s a NetApp array write performance can only be improved by either adding flash pools, upgrading to a higher performance controller, or by adding spindles/disk.  For read performance addition of flashpools, flash cache (read only),  or additional spindles would add performance.

Concurrency 75% Concurrency Users IOPs / User
75% 3750 1.44

Making the numbers more “round” yields a slight difference in IOPs / User.  This issue will be exacerbated with 4TB SATA and future large capacity drives.

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