Section 4.7. Disk Utilization


4.7. Disk Utilization

When considering disk utilization, keep in mind the following points:

  • Any level of disk utilization may degrade application performance because accessing disks is a slow activityoften measured in milliseconds.

  • Sometimes heavy disk utilization is the price of doing business; this is especially the case for database servers.

  • Whether a level of disk utilization actually affects an application greatly depends on how the application uses the disks and how the disk devices respond to requests. In particular, notice the following:

    An application may be using the disks synchronously and suffering from each delay as it occurs, or an application may be multithreaded or use asynchronous I/O to avoid stalling on each disk event.

    Many OS and disk mechanisms provide writeback caching so that although the disk may be busy, the application does not need to wait for writes to complete.

  • Utilization values are averages over time, and it is especially important to bear this in mind for disks. Often, applications and the OS access the disks in bursts: for example, when reading an entire file, when executing a new command, or when flushing writes. This can cause short bursts of heavy utilization, which may be difficult to identify if averaged over longer intervals.

  • Utilization alone doesn't convey the type of disk activityin particular, whether the activity was random or sequential.

  • An application accessing a disk sequentially may find that a heavily utilized disk often seeks heads away, causing what would have been sequential access to behave in a random manner.

  • Storage arrays may report 100% utilization when in fact they are able to accept more transactions. 100% utilization here means that Solaris believes the storage device is fully active during that interval, not that it has no further capacity to accept transactions. Solaris doesn't see what really happens on storage array disks.

  • Disk activity is complex! It involves mechanical disk properties, buses, and caching and depends on the way applications use I/O. Condensing this information to a single utilization value verges on oversimplification. The utilization value is useful as a starting point, but it's not absolute.

In summary, for simple disks and applications, utilization values are a meaningful measurement so we can understand disk behavior in a consistent way. However, as applications become more complex, the percent utilization requires careful consideration. This is also the case with complex disk devices, especially storage arrays, for which percent utilization may have little value.

While we may debate the accuracy of percent utilization, it still often serves its purpose as being a "useful starting point," which is followed by other metrics when deeper analysis is desired (especially those from DTrace).




Solaris Performance and Tools(c) Dtrace and Mdb Techniques for Solaris 10 and Opensolaris
Solaris Performance and Tools: DTrace and MDB Techniques for Solaris 10 and OpenSolaris
ISBN: 0131568191
EAN: 2147483647
Year: 2007
Pages: 180

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