15.2. Throughput overhead expectations
When estimating the throughput overhead associated with DRBD, it is
important to consider the following natural limitations:
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DRBD throughput is limited by that of the raw I/O subsystem.
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DRBD throughput is limited by the available network bandwidth.
The minimum between the two establishes the theoretical throughput
maximum available to DRBD. DRBD then reduces that throughput maximum
by its additional throughput overhead, which can be expected to be
less than 3 percent.
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Consider the example of two cluster nodes containing I/O subsystems
capable of 200 MB/s throughput, with a Gigabit Ethernet link
available between them. Gigabit Ethernet can be expected to produce
110 MB/s throughput for TCP connections, thus the network connection
would be the bottleneck in this configuration and one would
expect about 107 MB/s maximum DRBD throughput.
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By contrast, if the I/O subsystem is capable of only 100 MB/s for
sustained writes, then it constitutes the bottleneck, and you would
be able to expect only 97 MB/s maximum DRBD throughput.