Diagnosis and Performance Monitoring - Deploying and Managing a Cloud Infrastructure - page 124

IOPS measurements vary greatly depending on the system configuration as well as other
variables that the tester inputs into the testing software, including the ratio of read and write
operations, the mixture of random or sequential access patterns, the size of data blocks, and
the number of threads and the queue depth. The storage and device drivers and the applica-
tions running in the background as well as the overall health of the running operating system
affect the results of IOPS measurement.
Taking all performance variables into consideration, a real-world test can
best be done when the system is working at near full capacity. This will
show the real working measurement of IOPS for most users in the same
situation. A lot of experts will disagree, saying, that this will not be the
actual performance of the hardware, but it will be similar to the perfor-
mance users will experience.
IOPS measurement is still broken down into different performance characteristics, that
is, different aspects relating to the different ways that storage devices store and retrieve
data: random and sequential read/write operations.
Sequential Operations Sequential read or write operations access storage locations from a
device in a contiguous manner and usually occur when large transfer sizes are involved. There
are two measurements for sequential operations: sequential read IOPS and sequential write
IOPS, which are the average number of sequential read and write I/O operations per second,
respectively. On a hard disk drive and solid-state drive, these indicate the maximum sustained
bandwidth capability of the storage device and are reported as megabytes per second (MBps)
and calculated using the formula IOPS × transfer size = (mega)bytes per second .
Random Operations Random read or write operations access storage locations from
a device randomly or in a noncontiguous manner and are associated with operations on
small data sizes. The two measurements are random read IOPS and random write IOPS,
which indicate the average random read and write I/O operations per second, respectively.
On hard drives and other electromechanical storage devices, this is usually dependent on
the device's random seek time, and in an HDD, this depends primarily on the speed of its
actuator arm and read/write head. But on a solid-state drive, these numbers depend on
the device's controller and memory interface speeds, which are all electronic so in theory
should be much faster compared to that of HDDs.
Read vs. Write Files
The file system performance, as we will discuss in the next section, has its dependencies on
the hardware being used as well as the programming of the file system itself. Although the
capabilities of file systems may vary, we use file systems that offer the best performance for
our specific application set. For example, for a server cluster configuration, Lustre comes
to mind. In this case, the IOPS performance of a given set of storage hardware will be the