1. Sizing for IOPS - Manual Method 1
How do you get this?
And this?
Citrix documentation says:
And provides this:
What’s missing? - Number of users
- Read/Write Ratio
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2. Sizing for IOPS - Manual Method 2
• Another common IOPS sizing formula is:
– Usable IOPS = ((raw IOPS x write%)/write penalty)+ (raw IOPS x read%)
- Number of disks to get raw IOPS
• What’s missing here? - Read/Write Ratio
• Read/Write Ratio is needed in both cases
– Without actual numbers, best practice for R/W:
• 30/70 R/W = average
• 0/100 R/W = worst case
• Still required: Number of disks OR Number of users
• If disk count is known, use above formula
• If sizing for storage requirements, determine number of users

3. Determine Usable IOPS
PowerEdge R720
16 x 146GB 15k SAS – RAID10
Disk IOPS = 175 Write Penalty = 2
30/70 R/W = average and 0/100 R/W = worst case
Usable IOPS = ((raw IOPS x write %)/write penalty)+ (raw IOPS x read%)
IOPS Disk Write Write Write Read Usable
Scenario IOPS Qty Raw IOPS Write % subtotal Penalty IOPS Read % IOPS IOPS
Average 175 16 2800 .7 1960 2 980 .3 840 1820
Worst
175 16 2800 1 2800 2 1400 0 0 1400
Case
The heavy write VDI workload reduces IOPS up to 50% !
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4. Determine Users per Usable IOPS
• How many users can I support?
– Available IOPS Range = 1400-1820
– Required variable – IOPS/User
• Citrix XenDesktop Workload Standards
• Average: 1820 / 10 = 182 users
• Worst Case: 1400 /10 = 140 users

5. Brining it Together
• From real XenDesktop test results:
Lifecycle IOPS/desktop Read : Write
Steady State 7-8 1% : 99%
Boot Storm 16 - 18 13% : 87%
Disk Raw Write Write Write Read
IOPS Qty IOPS Write % subtotal Penalty IOPS Read % IOPS Usable IOPS
175 16 2800 .99 2722 2 1386 .01 28 1414
• Results: 1414 / 8 = 176 users (theoretical at 100%)
• Results at 70% IOPS consumption = 124 users
• Let’s look at the Boot Storm scenario, requiring recalculation
Disk Raw Write Write Write Read
IOPS Qty IOPS Write % subtotal Penalty IOPS Read % IOPS Usable IOPS
175 16 2800 .87 2436 2 1218 .13 364 1582
• 124 users x 18 = 2232 IOPS required in Boot Storm
650 IOPS Short = high latency and
slow reboot/user access times
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6. •
Know your Workloads
As I opened with, sizing storage for VDI workloads is complex
• Using Best Practice numbers provide a guideline only
• Where on this chart do your users fall in?
We walked
through this
But what
about that?
• Using our scenario of 1414 IOPS and Heavy Windows 7 users
– At 70% utilization – 1414 x .7 = 990 / 50 = 20 users / server
• Conversely, if you have 500 users, to determine IOPS
– 500 x 50 = 25,000 IOPS + 30% headroom = 32,500 IOPS required

7. Shared Storage Sizing
• Sizing Shared Storage for Capacity
– Straight forward and easy, no tricks or gotchas
– Only caveat is knowing the RAID implications OR
– Use published Usable Capacity numbers
• Sizing Shared Storage for IOPS – different story
– Everything discussed applies
– What we cannot account for is vendor implementations
• Controller features and capabilities
• “Secret Sauce”
• Hybrid disk configurations / algorithms
• We conduct an incredible amount of testing and validation to provide the
information to make informed decisions.
– Login VSI to generate workload
– Liquidware Labs Stratusphere to measure user experience

8. Recommendations
• Use a performance tool to define IOPS for each Use Case
– Perfmon works for Windows
– Liquidware Labs Stratusphere
– Lakeside Software Systrack
• Once IOPS requirement established multiply by Users
• Add at least 30% headroom for storms, growth, etc.
• Determine functional requirements
– Are the virtual desktops stateless? (persistent vs. non-persistent)
– Do you require live migration, high-availability…
– Ultimately determine whether local host storage is appropriate
• If host local is specified, use the enclosed methods
• If shared storage is specified, require your vendor of choice or partner to
provide storage specifications