Because DataKeeper can replicate data across any available network, special consideration must be given to the question, “Is there sufficient bandwidth to successfully replicate the volume and keep the mirror in the mirroring state as the source volume is updated throughout the day?”
Keeping the mirror in the mirroring state is critical, because a switchover of the volume is not allowed unless the mirror is in the mirroring state.
Determine Network Bandwidth Requirements
Prior to installing SIOS DataKeeper, you should determine the network bandwidth requirements for replicating your data. Use the method below to measure the rate of change for the data that you plan to replicate. This value indicates the amount of network bandwidth that will be required to replicate that data.
After determining the network bandwidth requirements, ensure that your network is configured to perform optimally. If your network bandwidth requirements are above your current available network capacity, you must consider one or more of the following options:
- Enable compression in DataKeeper, or in the network hardware, if possible
- Create a local, non-replicated storage repository for temporary data and swap files if you are replicating Hyper-V virtual machines
- Reduce the amount of data being replicated
- Increase your network capacity
If the network capacity is not sufficient to keep up with the rate of change that occurs on your disks, DataKeeper mirrors will remain in a resynchronizing state for considerable periods of time. During resynchronization, data on the target volume is not guaranteed to be consistent.
Measuring Rate of Change
Use Performance Monitor (perfmon) to measure the rate of change that occurs on your volumes that are to be replicated. The best way to do this is to create a log of disk write activity for some period of time (one day, for instance) to determine what the peak disk write periods are.
To track disk write activity,
- use perfmon to create a user-defined data collector set on Windows 2008 or Windows 2012.
- add the counter “Disk Write Bytes/sec” for each volume – the volume counters can be found in the logical disks group.
- start the log and let it run for the predetermined amount of time, then stop and open the log.
An alternative to creating a log of disk writes is to use perfmon to track disk write bytes/sec interactively, in the Performance Monitor tool, and to observe the maximum and average values there.
SIOS DataKeeper handles short bursts of write activity by adding that data to its async queue. However, make sure that over any extended period of time, the disk write activity for all replicated volumes combined remains, on average, below the amount of change that DataKeeper and your network can transmit.
Tabled below are the approximate replication times in accordance to Network Bandwidth and its Rate of Change with a data change of 100GB:
|Network Bandwidth||Rate of Change (Approximately)||Replication Time (Approximately)|
|1.5 Mbps (T1)||180,000 Bytes/sec||154 hours|
|10 Mbps||1,200,000 Bytes/sec||23 hours|
|45 Mbps (T3)||5,400,000 Bytes/sec||5 hours|
|100 Mbps||12,000,000 Bytes/sec||2 hours|
|1000 Mbps (Gigabit)||120,000,000 Bytes/sec||14 minutes|
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