Documentation of the block device I/O controller: description, usage,
advantages and design.
Signed-off-by: Andrea Righi <righi.andrea@gmail.com>
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+ Block device I/O bandwidth controller
+
+----------------------------------------------------------------------
+1. DESCRIPTION
+
+This controller allows to limit the I/O bandwidth of specific block devices for
+specific process containers (cgroups [1]) imposing additional delays on I/O
+requests for those processes that exceed the limits defined in the control
+group filesystem.
+
+Bandwidth limiting rules offer better control over QoS with respect to priority
+or weight-based solutions that only give information about applications'
+relative performance requirements. Nevertheless, priority based solutions are
+affected by performance bursts, when only low-priority requests are submitted
+to a general purpose resource dispatcher.
+
+The goal of the I/O bandwidth controller is to improve performance
+predictability from the applications' point of view and provide performance
+isolation of different control groups sharing the same block devices.
+
+NOTE #1: If you're looking for a way to improve the overall throughput of the
+system probably you should use a different solution.
+
+NOTE #2: The current implementation does not guarantee minimum bandwidth
+levels, the QoS is implemented only slowing down I/O "traffic" that exceeds the
+limits specified by the user; minimum I/O rate thresholds are supposed to be
+guaranteed if the user configures a proper I/O bandwidth partitioning of the
+block ...
