SAN Replication: Bandwidth Math

A common question I get a lot is, “how much bandwidth will I need to replicate X amount of data”. The answer is simple, it depends. It depends on your change rate on amount of data to be replicated.

You would need to calculate how much data will change for each of the LUNs over any given time period. After the initial sync only the changed blocks will be copied over. I would work out how much data changes over the busiest hour on the LUNs. Lets figure out how much bandwidth in Mb/s is needed to move our changed data that is in GBs.

Find the size of all the changed data:
Lun 1 changes 2GB of data an hour
Lun 2 changes 1GB of data during the same hour
Lun 3 changes 2GB of data during the same hour
Then the total data changed is 5 GB for that hour

Convert from GB to MB
5GB * 1024 = 5120MB

Convert the MB to Mb
5120*8 = 40960Mb (8 bytes = 1 bit)

40960Mb = total amount of changed data for our LUNs during our highest hour peak.

Now convert this to the average per second.
40960 / (60*60) = 40960 / 3600 = 11.38 Mb/s

So you would need 11.38 Mb/s to move 5GB of data in an hour. Now obviously this doesn’t take into account the latency latency between sites or different times of change rate but it does give you a good over view on how the math works for calculating bandwidth needs you may have.

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