I/O Latency And Performance Tuning

Andy Galbraith is starting a new toolbox series.  His first post is an introduction and a look at drive latency:

You look at the numbers again, and now you find that disk latency, which had previously been fine, is now completely in the tank during the business day, showing that I/O delays are through the roof.
What happened?
This demonstrates the concept of shifting bottleneck – while CPU use was through the roof, the engine so bogged down that it couldn’t generate that much I/O, but once the CPU issue was resolved queries started moving through more quickly until the next choke point was met at the I/O limit.  Odds are once you resolve the I/O situation, you would find a new bottleneck.
How do you ever defeat a bad guy that constantly moves around and frequently changes form?

Click through for some pointers on disk latency and trying to figure out when it becomes a problem.

Related Posts

I/O Read-Ahead In SQL Server

Kendra Little shows how read-ahead works, using the example of index seeks: Looking at an actual execution plan, I dig into the index seek operator and it shows me information about the physical IO. Almost all of the requests were read-ahead reads. Read-ahead is a mechanism that SQL Server can use when it’s pulling a lot […]

Read More

Non-Cost-Based Optimizations In Relational Databases

Lukas Eder has a big article on ten query optimizations that don’t involve looking at statistics or query costs: This optimisation is really silly, but hey, why not. If users write impossible predicates, then why even execute them? Here are some examples: -- "Obvious" SELECT * FROM actor WHERE 1 = 0 -- "Subtle" SELECT […]

Read More

Categories

September 2017
MTWTFSS
« Aug Oct »
 123
45678910
11121314151617
18192021222324
252627282930