Article ID: 238145 - View products that this article applies to.
This article was previously published under Q238145
When you enable bandwidth throttling, Internet Information Services (IIS) activates a set of counters to monitor it in System Monitor (SysMon). These counters can be identified by "Async I/O" in the counter name. These counters are active only when bandwidth throttling is enabled. (If bandwidth throttling is not enabled, the counters appear in SysMon, but they always have a value of zero.)
The Async I/O counters are part of the Internet Information Services Global performance object. They represent totals for all of the IIS services. Bandwidth is not measured for each service. The following table lists and describes the Async I/O counters.
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The bandwidth setting determines whether IIS accepts or rejects a request for a static HTML page, based on periodic samples of the rate at which bytes are sent on the server.
If the bandwidth used (as indicated by the sample) approaches the maximum set by the user, bandwidth throttling blocks read requests, but allows write requests and transmission requests. Read requests are blocked first, because they are likely to result in further requests.
If the bandwidth used exceeds the maximum set by the user, bandwidth throttling rejects read requests, blocks large write requests and transmission requests, and allows small write requests and transmission requests.
To determine how many requests are being blocked and rejected, monitor the Async I/O counters. These counters display cumulative totals, so it's best to use the SysMon log to log the counter values. A spreadsheet can also be used to calculate the rate over time. SysMon can be used to notify administrators when the number of blocked or rejected requests exceeds a threshold.