Databases Reference
In-Depth Information
What DoYouWant toMeasure?
Now that you have established that you want to measure performance, what do you actually want to
measure? This section covers a number of different metrics relevant to storage performance.
IOPS
Input/Output operations per second (IOPS) is a measure of how many input or output requests you
serviced per second. This is a good starting point, but the number you get is a function of the subsystem's
actual throughput and the size of the I/O request.
Here is an example that helps illustrate this is a simple single disk test. The disk is capable of delivering
5MB/Sec for Random read activity. Test that throughput by issuing 4 KB I/Os. You get an IOPS of
5MB/4KB
1250. That sounds great, but now test with a 64KB block size, and you get only 78 IOPS,
That doesn't sound so great when you compare it with 1250, yet you are still getting the same 5MB/Sec
throughput.
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So although IOPS is an interesting metric, you really need to know more about what kind of I/O was
being used for the test.
Best Practice
Consider what kind of I/O was being issued when reading IOPS.
BlockSize
Block size is very important to your test results, but it's a parameter you want to drive into the tests,
rather than being something you need to measure. When attempting to measure I/O performance of a
real-life storage subsystem, block size is something you will want to measure. In this case block size gives
you an indication of what size I/Os SQL is issuing.
MB/Sec
This is one of the key metrics for any storage subsystems performance. This tells you how many MB of
data you got on or of off the storage subsystem in one second. Watch carefully for the differences between
MB (Mega Bytes) and Mb (Mega bits) as one MB is 8 times larger than one Mb, or 8Mb
1MB,andthe
disk manufacturers and other storage vendors love those big numbers (Mb) to make their particular
product look better than the rest.
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Another example of the difference between MB and Mb is relevant to I/O throughput. A disk capable of
delivering a throughput of 5MB/Sec is the same as a disk delivering a throughput of 40Mb/Sec.
When looking at MB/Sec numbers, you must also consider the latency at which that throughput was
achieved. MB/Sec and Latency are both a function of the overall disk throughput. Be careful of very high
throughput (MB/Sec) figures that are achieved at the cost of excessively high latencies.
Latency
This is another key metric, and is tells you how long you had to wait for each I/O to complete. It needs
to be considered along with the throughput in MB/Sec. Latencies of less than 5 milliseconds (mSec)
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