Some name server operations can be quite resource intensive, and in order to tune your system properly it is sometimes necessary to change BIND's internal quotas. This is accomplished via
limit <name> <value>
directives in the bootfile. Limits, and their default values, are as follows:
limit transfers-in 10
This is the number of simultaneous named-xfer processes BIND is willing to start. Higher numbers yield faster convergence to primary servers if your secondary server has hundreds or thousands of zones to maintain, but setting this number too high can cause thrashing due to starvation of resources such as network bandwidth or swap space. NOTE: this limit can also be expressed via the deprecated directive max-fetch NN.
limit transfers-per-ns 2
This is the number of simultaneous named-xfer processes BIND is willing to initiate to any given name server. In most cases, you should not need to change it. If your secondary server is pulling hundreds or thousands of zones from a single primary server, increasing transfers-per-ns may speed convergence. It should be kept as small as possible, to avoid causing thrashing and resource starvation on the primary server.
limit datasize <system-dependent>
Most systems have a quota that limits the size of the so-called ``data segment,'' which is where BIND keeps all of its authority and cache data. BIND will behave suboptimally (perhaps even exiting) if it runs up against this quota. If your system supports a system call to change this quota for a given process, you can ask BIND to use that system call via the limit datasize NN directive. The value given here may be scaled by postfixing k for 1024X, m for (1024^2)X, and g for (1024^3)X. In 1995, the root servers all use limit datasize 64m.