fix docs

docs/FAQ.rst

@@ -52,7 +52,7 @@ The `Apache 2.0 License`_.
 
 Does Graphite use RRDtool?
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-No, not since Graphite was first written in 2006 at least. Graphite has its own specialized database library called :doc:`whisper </whisper>`, which is very similar in design to RRD, but has a subtle yet fundamentally important difference that Graphite requires. There are two reasons whisper was created. The first reason is that RRD is designed under the assumption that data will always be inserted into the database on a regular basis, and this assumption causes RRD behave undesirably when given irregularly occurring data. Graphite was built to facilitate visualization of various application metrics that do not always occur regularly, like when an uncommon request is handled and the latency is measured and sent to Graphite. Using RRD, the data gets put into a temporary area inside the database where it is not accessible until the current time interval has passed *and* another value is inserted into the database for the following interval. If that does not happen within an allotted period of time, the original data point will get overwritten and is lost. Now for some metrics, the lack of a value can be correctly interpreted as a value of zero, however this is not the case for metrics like latency because a zero indicates that work was done in zero time, which is different than saying no work was done. Assuming a zero value for latency also screws up analysis like calculating the average latency, etc.
+No, not since Graphite was first written in 2006 at least. Graphite has its own specialized database library called :doc:`whisper </Whisper>`, which is very similar in design to RRD, but has a subtle yet fundamentally important difference that Graphite requires. There are two reasons whisper was created. The first reason is that RRD is designed under the assumption that data will always be inserted into the database on a regular basis, and this assumption causes RRD behave undesirably when given irregularly occurring data. Graphite was built to facilitate visualization of various application metrics that do not always occur regularly, like when an uncommon request is handled and the latency is measured and sent to Graphite. Using RRD, the data gets put into a temporary area inside the database where it is not accessible until the current time interval has passed *and* another value is inserted into the database for the following interval. If that does not happen within an allotted period of time, the original data point will get overwritten and is lost. Now for some metrics, the lack of a value can be correctly interpreted as a value of zero, however this is not the case for metrics like latency because a zero indicates that work was done in zero time, which is different than saying no work was done. Assuming a zero value for latency also screws up analysis like calculating the average latency, etc.
 
 The second reason whisper was written is performance. RRDtool is very fast; in fact it is much faster than whisper. But the problem with RRD (at the time whisper was written) was that RRD only allowed you to insert a single value into a database at a time, while whisper was written to allow the insertion of multiple data points at once, compacting them into a single write operation. This improves performance drastically under high load because Graphite operates on many many files, and with such small operations being done (write a few bytes here, a few over there, etc) the bottleneck is caused by the *number of I/O operations*. Consider the scenario where Graphite is receiving 100,000 distinct metric values each minute; in order to sustain that load Graphite must be able to write that many data points to disk each minute. But assume that your underlying storage can only handle 20,000 I/O operations per minute. With RRD (at the time whisper was written), there was no chance of keeping up. But with whisper, we can keep caching the incoming data until we accumulate say 10 minutes worth of data for a given metric, then instead of doing 10 I/O operations to write those 10 data points, whisper can do it in one operation. The reason I have kept mentioning "at the time whisper was written" is that RRD now supports this behavior. However Graphite will continue to use whisper as long as the first issue still exists.