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/*! @page tune_capacity Capacity tuning
In some cases, it can be helpful to constrain the overall I/O bandwidth
generated by the database. This can be beneficial when resources are shared,
for example, in cloud or virtual environments.
The total bandwidth capacity is configured by setting the
\c io_capacity configuration string when calling the ::wiredtiger_open
function. The capacity can be adjusted with WT_CONNECTION::reconfigure.
An example of setting a capacity limit to 40MB per second:
@snippet ex_all.c Configure capacity
When a total capacity is set the volume of system reads and writes totalled
will not exceed the given I/O capacity.
If a read or write is scheduled and would overflow the capacity, the issuing
thread will sleep to guarantee the capacity ceiling. The policy used is
fair to all threads, and gives some weight to both readers and writers to
try to ensure that each session can make progress when bandwidth
resources are limited.
System reads and writes do not directly translate to disk I/O
operations. These operations go through the operating system cache. To ensure
the steady flow of data to the disk, setting a capacity also enables an
additional thread that monitors the writes performed for each file. For each
file that has sufficient data written to it, a call to an
asynchronous \c fsync will be made. This call normally queues the flush
in the operating system, though there is no guarantee about when it will
actually occur. On Windows, there is no equivalent support for asynchronously
scheduling writes to disk, so this extra "sync" thread is not active.
When a total capacity is not set, or equivalently, when it is set to 0,
there are no capacity constraints on the database, and pauses will never
be inserted before I/O is done, nor are extra asynchronous \c fsync calls
performed.
*/
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