With the approaching end of life date for XenApp 5, one of my customers is currently designing their XenApp 6.5 farm and asked for some assistance calculating the amount of data that would be replicated between each zone data collector.
Let’s detail the existing environment:
- XenApp 5 Feature Pack 2
- Hotfix Rollup Pack 7
- 350 servers
- 14 zones in 10 countries globally.
- Core farm infrastructure (SQL publisher datastore, licence server) located in the same datacentre as the “UK 5″ zone.
Here are the zone names and the average number of user logons per day.
|Zones||Logons per day|
The customer has had 4 years of good service from their XenApp 5 farm, and plans to replace the farm with a XenApp 6.5 environment following broadly the same design principles. One key change that the customer will benefit from is the removal of the many SQL replicas that they maintained to provide datastore services for their more remote datacentres. The XenApp 6.5 design is intended to use a single SQL server (SQL cluster to be precise) to host the datastore, and make use of the new Controller/Worker functionality within XenApp to dramatically reduce the amount of data that needs to be read across the WAN for the IMA startup process.
The customer had a specific question about the amount of data that would be replicated between the zone data collectors during normal farm operation. I soon found that while we document the communication process flows and the quantities of data that are replicated, I could not find a complete worked example with real numbers. So here we go.
IMA uses event-driven data replication. By this I mean “when something happens in one part of the farm the rest of the farm needs to know about it”. In this blog post I am not going to discuss data that is written into the datastore, for example when an application is published or a server is added to the farm. Instead I will focus on those farm changes that cause Zone Data Collectors to push data to each other. The key session events are:
|Event||Data transmitted (approximate)|
So let’s look at event-driven data replication a bit more closely. What are we saying here?
When a session is started, information is pushed from the ZDC for that zone to all other ZDCs. So each ZDC will send 0.19KB of data to all other ZDCs in the farm.
For my customer’s environment this means that the “pushing” ZDC will send 0.19 x 13 = 2.47 KB of data, but each “receiving” ZDC only receives 0.19KB of data.
When calculating the amount of data to be sent and received over each WAN link, we must consider how often the users start sessions and how often they log off per day. Each user is likely to log on and off at least once per day (note that if they turn their workstation off they are disconnecting from their session which will still send some data over the WAN). Lets assume that each user logs on in the morning, disconnects and reconnects once during the day (lunchtime?) and then logs off at the end of the day. How will these numbers look?
|Zones||Logons per day||Data sent over WAN link for session startData in KB = number of logons x 0.19 x 13||Data sent over WAN link for disconnectData in KB = number of disconnects x 0.51 x 13||Data sent over WAN link for reconnectData in KB = number of reconnects x 0.29 x 13||Data sent over WAN link for logoffData in KB = number of logoffs x 0.31 x 13||Total quantity of data sent by Zone Data Collector in KB|
|Zones||Logons per day||Data Received over WAN link for session startData in KB = number of logons outside of this zone x 0.19||Data Received over WAN link for disconnectData in KB = number of disconnects outside zone x 0.51||Data Received over WAN link for reconnectData in KB = number of reconnects x 0.29||Data Received over WAN link for logoffData in KB = number of Logoffs x 0.31||Total quantity of data Received by Zone Data Collector in KB|
Some cautionary notes:
These calculations show the total amount of data sent and received per ZDC. For my customer each Zone is in a separate data centre with a WAN link connecting that data centre to the rest of the world, hence I have considered ZDC data to be equal to WAN link data.
These calaculations show the total quantity of data for our fictitious scenario where users only logon, logoff, disconnect and reconnect once per day.
These calculations show the total data transmitted per day. We would expect each zone to have peaks and troughs in terms of logon and logoff activity, for exmaple during shift changes or Start-Of-Day and End-Of-Day.
A note about Inter-Site Replication Links/Replication Topologies etc.
When planning a XenApp farm, or trying to visualise the communication, it is tempting to try to map the functionality to Active Directory. To do this however is false. XenApp does not have an equivalent to the inter-site topology generator, analysing the farm structure and implementing the most efficient data replication structure. With XenApp every ZDC needs to communication with every other ZDC.
If you want a mathematical understanding of XenApp replication, consider reading the wikipedia articles about Mesh networks, and fully-connected networks. Remember, every ZDC needs to know what is happening in all the other zones in the farm.
XenApp 6.5 Enterprise Scalable XenApp Deployments – http://support.citrix.com/article/CTX131102
Communication Bandwidth Requirements and Application of IMA Bandwidth Formulas on XenApp for Windows Server 2003 http://support.citrix.com/article/ctx114843
Improving Farm Performance and Resiliency with Hotfix Rollup Pack 3 – http://support.citrix.com/article/CTX119922