Yes, you will need more than T1 bandwidth for VI replication!
November 5th, 2007 | 
Author: Too many companies try to implement replication to a DR VI without upgrading the bandwidth between the primary and secondary sites. Let’s look at a simple example that can illustrate what could go wrong with inadequate bandwidth.
A company has 5 VMs that each use 20 GB virtual disks. The data is not too dynamic and data change only averages about 1o% per business day or roughly 1 GB per hr. This data change could be common activity like Active Directory replication, files saved to user home folders, application databases, and email. This is common to a small to medium sized business.
Using the Data Replication Minimum Bandwidth Requirements chart provided by NSI, makers of Double-Take, You can see that the 100 GB falls into the LAN 10Mb/s bandwidth category (in the 10% column). Click the thumbnail image to the left for a better view of the chart. We’ve already proved that this company needs better than a T1, but it’s close enough not to convince those that think their data change will be lower than 10%.
The real “gotcha” is that companies never consider how long it will take to replicate the data.
Most WAN replication solutions are scheduled to replicate once an
amount of data change has occurred or a defined amount of time has elapsed since the last replication. Obviously, the more frequent the replication schedule the less restore work will be needed at the DR site to achieve up to date fail over. Let’s assume in our example that this company is scheduled to replicate every hour, and so, on average this company will need to replicate 1 GB of data.
I used an online connection speed calculator to estimate the transfer time at T1 bandwidth. Click on the thumbnail image to the right to see the cropped results. 1000 MB transferred across T1 1.544 MB will take over 1 hour and 25 minutes.
There goes the hourly replication schedule. Sure, you could back down the schedule to 2 hours or greater. You could even have alternate schedules for different VMs. There is a lot you could do to make it work better with a T1, but do you really want to monitor and administer that complexity? What if you also want to leverage your DR site for development and testing? Now you’ve just added more data flow between your sites.
Upgrade the link between the sites. Using a T3 link the 1000 MB transfer takes 3 minutes.
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Posted in dr, esx, replication, SAN, vizioncore, vmware | 
Tags: calculator, disaster recovery, fail over, networking, replication
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