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BP1018 Sizing and Best Practices for Citrix XenDesktop with Dell EqualLogic Storage 35
In this formula, we use 15% headroom for the array capacity and 25% headroom for VM RAM and the
write cache allocated on the virtual desktop.
The following table describes the formula components and provides a sample calculation.
Table 8 Formula components
Variable
Description
Value in our test
configuration
NumVM Number of virtual desktops
to be deployed.
VMMem
Amount of RAM allocated
on each virtual desktop.
1 GB
vDiskSize Size of the master OS disk
image.
25 GB
NumvDisk Number of vDisks 6
WCacheSize
Size of the Write cache.
1.5 GB
ArrayCapacity
Total capacity of the
PS6010XVS array in GB.
2539.5 GB
The size of the write cache and RAM on the virtual desktop varies depending on the applications used
in different customer environments. The actual usage needs to be monitored and the formula should
be used as guideline for sizing storage capacity in deploying virtual desktops.
Sample capacity calculation from our test configuration:
NumVM = (2539 – (0.15*2539 + 25*6) ) / (1.25*1 + 1.25*1.5)
This calculation shows that we can deploy around 640 desktops using a single PS6010XVS array to
meet our test capacity requirements. In our test configuration, we deployed 630 virtual desktops and,
from a performance standpoint, the storage disk latency was well within 10 ms and there was no
bottleneck with storage array performance. The observed IOPS for our workload is described above in
Table 6.
If the number of virtual desktops needs to be scaled, then we recommend adding additional
EqualLogic arrays. Based on our test results, linear scaling can be achieved by adding additional
storage arrays as long as there is no bottleneck on other system resources. It is important to note that
when sizing for desktop count, both the capacity and IOPS needs should be considered. Either
capacity or IOPS requirement may warrant more storage arrays than required by the other. It is a best
practice to determine the number of arrays needed for capacity and IOPS separately. The maximum of
these two values can be considered as the final requirement. This approach ensures that the
underlying storage can handle both performance and capacity requirements needed by the VDI
environment.