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Disk I/Os are expensive and random disk I/Os are very expensive. Accessing a piece of data on a disk drive is much slower than accessing the same piece of data in memory. How much slower A conventional disk drive is an electromechanical device with a spinning spindle and one or more magnetic platters. No matter how fast the platters may spin, moving data through mechanical parts is inherently slower than moving data through electronic circuitry. In fact,
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Disk performance counters
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disk access speed is measured in milliseconds (for example, 5 ms), whereas memory access speed is measured in nanoseconds (for example 100 ns). What is important to recognize is that not all disk I/Os are equal in their performance. More specifically, random I/Os are far slower, or more expensive, than sequential I/Os. Whether I/Os are random or sequential can be defined by the relative data locations of two consecutive I/O requests. If the next I/O request is for data at a random location, the I/O requests are random, whereas if the next I/O request is for data residing next to the currently requested data, the I/O requests are sequential. On a conventional disk, the time it takes to complete the operation of a random I/O typically includes moving the disk drive head to the right track on the platter and then waiting for the disk sector to rotate to the disk drive head. The time it takes to complete the operation of a sequential I/O typically involves moving the disk head between adjacent tracks and waiting for the right sector to rotate to the disk head. Relatively speaking, the performance of a sequential I/O depends on the track-totrack seek time, whereas the performance of a random I/O depends on the average seek time. Why does this matter Take a typical 15,000 rpm disk drive as an example. Its track-to-track seek time is 0.2 ms, whereas its average seek time is 2 ms. The performance difference has an order of magnitude! SQL Server recognizes the performance difference between sequential I/Os and random I/Os, and its database engine employs many techniques to optimize for sequential I/Os. For example, in addition to being a technique for crash recovery, transaction logging can be viewed as an optimization that converts random writes to sequential writes. Read-ahead is another optimization that attempts to take advantage of sequential reads.
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Windows exposes a large number of disk performance counters. You can use the following counters under the LogicalDisk object as the key performance indicators to evaluate disk I/O performance:
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Avg. Disk sec/Read The number of seconds to complete a read operation on
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the disk drive, averaged over the polling interval
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Avg. Disk sec/Write The number of seconds to complete a write operation
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on the disk drive, averaged over the polling interval Avg. Disk Bytes/Read The number of bytes transferred from the disk drive per read operation, averaged over the polling interval Avg. Disk Bytes/Write The number of bytes transferred to the disk drive per write operation, averaged over the polling interval Disk Reads/sec The number of read operations on the disk drive per second Disk Writes/sec The number of write operations on the disk drive per second Disk Read Bytes/sec The number of bytes transferred from the disk drive per second
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Query performance and disk I/O counters
Disk Write Bytes/sec The number of bytes transferred to the disk drive per second Current Disk Queue Length The number of requests outstanding on the disk drive
These counters measure five key I/O metrics: Disk I/O latency For example, Avg. Disk sec/Read Disk I/O size For example, Avg. Disk Bytes/Read I/O operations per second For example, Disk Reads/sec I/O throughput For example, Disk Read Bytes/sec I/O queue length For example, Current Disk Queue Length
I/O latency
The latency of an I/O request is also known as I/O response time. It is measured by taking the difference between the time the I/O request is submitted and the time the completion acknowledgement is received. I/O latency can be measured at different levels of the I/O stack. The Avg. Disk sec/Read counter is a measure taken in the Windows logical disk driver.
It is critical, but often forgotten, that all the preceding disk I/O performance counters be collected and evaluated as a whole in order to see the complete picture of the disk I/O activities. In particular, if you focus on the disk latency counters without also checking the I/O size counters, you may end up drawing a wrong conclusion. For instance, a commonly accepted threshold for judging whether a disk I/O is taking too long is 10 ms. In practice, you need to make sure that this threshold applies only to smaller I/Os. When a system is doing large I/Os, the threshold of 10 ms may be too low, and can lead to false alarms.
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