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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Sequential access</span></span>
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<p><b>Sequential access</b> is a term describing a group of elements (such as data in a memory array or a <a href="Hard_disk_drive" title="Hard disk drive">disk</a> file or on <a href="Magnetic-tape_data_storage" title="Magnetic-tape data storage">magnetic-tape data storage</a>) being accessed in a predetermined, ordered <a href="Sequence" title="Sequence">sequence</a>. It is the opposite of <a href="Random_access" title="Random access">random access</a>, the ability to access an arbitrary element of a sequence as easily and efficiently as any other at any time.
</p><p>Sequential access is sometimes the only way of accessing the data, for example if it is on a tape. It may also be the access method of choice, for example if all that is wanted is to process a sequence of data elements in order.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>
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<div class="mw-heading mw-heading2"><h2 id="Definition">Definition</h2></div>
<p>There is no consistent definition in <a href="Computer_science" title="Computer science">computer science</a> of sequential access or sequentiality.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-9" class="reference"><a href="#cite_note-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> In fact, different sequentiality definitions can lead to different sequentiality quantification results. In spatial dimension, request size, stride distance, backward accesses, re-accesses can affect sequentiality. For temporal sequentiality, characteristics such as multi-stream and inter-arrival time threshold has impact on the definition of sequentiality.<sup id="cite_ref-10" class="reference"><a href="#cite_note-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup>
</p><p>In <a href="Data_structure" title="Data structure">data structures</a>, a data structure is said to have sequential access if one can only visit the values it contains in one particular order. The canonical example is the <a href="Linked_list" title="Linked list">linked list</a>. Indexing into a list that has sequential access requires <a href="Big_O_notation" title="Big O notation">O</a>(<i>n</i>) time, where <i>n</i> is the index. As a result, many algorithms such as <a href="Quicksort" title="Quicksort">quicksort</a> and <a href="Binary_search_algorithm" class="mw-redirect" title="Binary search algorithm">binary search</a> degenerate into bad algorithms that are even less efficient than their naive alternatives; these algorithms are impractical without <a href="Random_access" title="Random access">random access</a>. On the other hand, some algorithms, typically those that do not have index, require only sequential access, such as <a href="Mergesort" class="mw-redirect" title="Mergesort">mergesort</a>, and face no penalty.
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<div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2></div>
<ul><li><a href="Direct-access_storage_device" title="Direct-access storage device">Direct-access storage device</a></li>
<li><a href="Queued_Sequential_Access_Method" title="Queued Sequential Access Method">Queued sequential access method</a></li>
<li>Sequential data</li></ul>
<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>
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<li id="cite_note-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-1">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://technet.microsoft.com/en-us/library/cc938619.aspx">Random and Sequential Data Access</a>, Microsoft TechNet</span>
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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><i>Irfan Ahmad</i>, <a rel="nofollow" class="external text" href="http://www.vmware.com/files/pdf/iiswc_2007_distribute.pdf">Easy and Efficient Disk I/O Workload Characterization in VMware ESX Server</a>, IISWC, 2007.</span>
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<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><i>Eric Anderson</i>, <a rel="nofollow" class="external text" href="https://www.usenix.org/legacy/event/fast09/tech/full_papers/anderson/anderson.pdf">Capture, Conversion, and Analysis of an Intense NFS Workload</a>, FAST, 2009.</span>
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<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><i>Yanpei Chen et al.</i> <a rel="nofollow" class="external text" href="http://dl.acm.org/citation.cfm?id=2043562">Design Implications for Enterprise Storage Systems via Multi-dimensional Trace Analysis</a>. SOSP. 2011</span>
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<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><i>Andrew Leung et al.</i> <a rel="nofollow" class="external text" href="http://www.ssrc.ucsc.edu/Papers/leung-usenix08.pdf">Measurement and Analysis of Large-scale Network File System Workloads</a>. USENIX ATC. 2008</span>
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<li id="cite_note-6"><span class="mw-cite-backlink"><b><a href="#cite_ref-6">^</a></b></span> <span class="reference-text"><i>Frank Schmuck and Roger Haskin</i>, <a rel="nofollow" class="external text" href="https://www.usenix.org/legacy/events/fast02/full_papers/schmuck/schmuck.pdf">GPFS: A Shared-Disk File System for Large Computing Clusters</a>, FAST. 2002</span>
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<li id="cite_note-7"><span class="mw-cite-backlink"><b><a href="#cite_ref-7">^</a></b></span> <span class="reference-text"><i>Alan Smith</i>. <a rel="nofollow" class="external text" href="http://www-inst.eecs.berkeley.edu/~cs266/sp10/readings/smith78.pdf">Sequentiality and Prefetching in Database Systems</a>. ACM TOS</span>
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<li id="cite_note-8"><span class="mw-cite-backlink"><b><a href="#cite_ref-8">^</a></b></span> <span class="reference-text"><i>Hyong Shim et al.</i> <a rel="nofollow" class="external text" href="http://0b4af6cdc2f0c5998459-c0245c5c937c5dedcca3f1764ecc9b2f.r43.cf2.rackcdn.com/11747-atc13-shim.pdf">Characterization of Incremental Data Changes for Efficient Data Protection</a>. USENIX ATC. 2013.</span>
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<li id="cite_note-9"><span class="mw-cite-backlink"><b><a href="#cite_ref-9">^</a></b></span> <span class="reference-text"><i>Avishay Traeger et al.</i> <a rel="nofollow" class="external text" href="http://www.fsl.cs.sunysb.edu/docs/fsbench/fsbench.pdf">A Nine Year Study of File System and Storage Benchmarking</a>. ACM TOS. 2007.</span>
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<li id="cite_note-10"><span class="mw-cite-backlink"><b><a href="#cite_ref-10">^</a></b></span> <span class="reference-text"><i>Cheng Li et al.</i> <a rel="nofollow" class="external text" href="https://www.usenix.org/node/183622">Assert(!Defined(Sequential I/O))</a>. HotStorage. 2014</span>
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