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Diffstat (limited to 'src/third_party/wiredtiger/src/docs/tune-capacity.dox')
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diff --git a/src/third_party/wiredtiger/src/docs/tune-capacity.dox b/src/third_party/wiredtiger/src/docs/tune-capacity.dox new file mode 100644 index 00000000000..3aad4997576 --- /dev/null +++ b/src/third_party/wiredtiger/src/docs/tune-capacity.dox @@ -0,0 +1,38 @@ +/*! @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. + + */ |