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DCS UMN Initial Version

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Reviewed-by: Hasko, Vladimir <vladimir.hasko@t-systems.com>
Co-authored-by: Chen, Junjie <chenjunjie@huawei.com>
Co-committed-by: Chen, Junjie <chenjunjie@huawei.com>
This commit is contained in:
Chen, Junjie 2022-12-12 14:07:56 +00:00 committed by zuul
parent 50686af1a0
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178 changed files with 1132 additions and 378 deletions
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</tbody>
</table>
</div>
<div class="fignone" id="CacheCluster__fig10333132616620"><span class="figcap"><b>Figure 1 </b>Proxy Cluster DCS Redis instance architecture</span><br><span><img id="CacheCluster__image3165104135518" src="en-us_image_0296786822.png"></span></div>
<div class="fignone" id="CacheCluster__fig10333132616620"><span class="figcap"><b>Figure 1 </b>Proxy Cluster DCS Redis instance architecture</span><br><span><img id="CacheCluster__image203718541976" src="en-us_image_0000001383102132.png"></span></div>
<p id="CacheCluster__p153106583326">Architecture description:</p>
<ul id="CacheCluster__ul2171627184718"><li id="CacheCluster__li20179270479"><strong id="CacheCluster__b264814218559">VPC</strong><p id="CacheCluster__p89401530124718">All server nodes of the instance run in the same VPC.</p>
<div class="note" id="CacheCluster__note04501339113"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="CacheCluster__p124516392019">For intra-VPC access, the client and the instance must be in the same VPC with specified security group rule configurations.</p>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-pd-200312001.html">DCS Instance Types</a></div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-pd-200312001.html">DCS Instance Types</a></div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-pd-200312001.html">DCS Instance Types</a></div>
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<div class="fignone" id="CacheSingleNode__fig15457185394718"><a name="CacheSingleNode__fig15457185394718"></a><a name="fig15457185394718"></a><span class="figcap"><b>Figure 1 </b>Single-node DCS Redis instance architecture</span><br><span><img id="CacheSingleNode__image9134201365216" src="en-us_image_0296784660.png"></span></div>
<p id="CacheSingleNode__p153106583326">Architecture description:</p>
<ul id="CacheSingleNode__ul1631417191120"><li id="CacheSingleNode__li20179270479"><strong id="CacheSingleNode__b264814218559">VPC</strong><p id="CacheSingleNode__p89401530124718">All server nodes of the instance run in the same VPC.</p>
<div class="note" id="CacheSingleNode__note04501339113"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="CacheSingleNode__p124516392019">For intra-VPC access, the client and the instance must be in the same VPC with specified security group rule configurations.</p>
<div class="note" id="CacheSingleNode__note04501339113"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="CacheSingleNode__p124516392019">For intra-VPC access, the client and the instance must be in the same VPC with specified security group rule configurations with specified security group rule configurations.</p>
<p id="CacheSingleNode__p14803202620398">For details, see <a href="en-us_topic_0090662012.html">Security Group Configurations</a>.</p>
</div></div>
</li><li id="CacheSingleNode__li017927194718"><strong id="CacheSingleNode__b386793812488">Application</strong><p id="CacheSingleNode__p36064574817">The client of the instance, which is the application running on an Elastic Cloud Server (ECS).</p>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-pd-200312001.html">DCS Instance Types</a></div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-pd-0326001.html">Service Overview</a></div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-ug-0312003.html">Creating an Instance</a></div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-pd-0326001.html">Service Overview</a></div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-ug-0312039.html">Managing Passwords</a></div>
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<h1 class="topictitle1">Will Cached Data Be Retained After an Instance Is Restarted?</h1>
<div id="body8662426"><p id="dcs-faq-0220330__en-us_topic_0000001226633976_p2938536145015">After a single-node DCS instance is restarted, data in the instance is deleted.</p>
<p id="dcs-faq-0220330__en-us_topic_0000001226633976_p157943519577">Master/standby and cluster instances (except single-replica clusters) support AOF persistence by default. Data is retained after these instances are restarted.</p>
<p id="dcs-faq-0220330__en-us_topic_0000001226633976_p2938236195019">If AOF persistence is disabled (<strong id="dcs-faq-0220330__en-us_topic_0000001226633976_b1775833072617">appendonly</strong> is set to <strong id="dcs-faq-0220330__en-us_topic_0000001226633976_b1880493211267">no</strong>), data is deleted after the instances are restarted.</p>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326021.html">Redis Usage</a></div>
</div>
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<h1 class="topictitle1">Why Is Flow Control Triggered? How Do I Handle It?</h1>
<div id="body8662426"><p id="dcs-faq-0220721__en-us_topic_0000001357973609_p11171144915210">Flow control is triggered when the traffic used by a Redis instance in a period exceeds the maximum bandwidth.</p>
<div class="note" id="dcs-faq-0220721__en-us_topic_0000001357973609_note19860103015720"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="dcs-faq-0220721__en-us_topic_0000001357973609_p662045412561">For details about the maximum allowed bandwidth, see the "Assured/Maximum Bandwidth" column of different instance types listed in <a href="en-us_topic_0054235835.html">DCS Instance Specifications</a>.</p>
</div></div>
<p id="dcs-faq-0220721__en-us_topic_0000001357973609_p1273182501517">Even if the bandwidth usage is low, flow control may still be triggered. The real-time bandwidth usage is reported once in each reporting period. Flow controls are checked every second. The traffic may surge within seconds and then fall back between reporting periods. By the time the bandwidth usage is reported, it may have already restored to the normal level.</p>
<p id="dcs-faq-0220721__en-us_topic_0000001357973609_p206341115114814">For master/standby instances:</p>
<ul id="dcs-faq-0220721__en-us_topic_0000001357973609_ul117685904813"><li id="dcs-faq-0220721__en-us_topic_0000001357973609_li517635914813">If flow control is always triggered when the bandwidth usage is low, there may be service microbursts or big or hot keys. In this case, check for big or hot keys.</li><li id="dcs-faq-0220721__en-us_topic_0000001357973609_li21761559164816">If the bandwidth usage remains high, the bandwidth limit may be exceeded. In this case, expand the capacity. Larger capacity supports higher bandwidth.</li></ul>
<p id="dcs-faq-0220721__en-us_topic_0000001357973609_p737634574812">For cluster instances:</p>
<ul id="dcs-faq-0220721__en-us_topic_0000001357973609_ul20813103175118"><li id="dcs-faq-0220721__en-us_topic_0000001357973609_li188139365110">If flow control is triggered only on one or a few shards, the shards may have big or hot keys.</li><li id="dcs-faq-0220721__en-us_topic_0000001357973609_li178131439513">If flow control or high bandwidth usage occurs on all or most shards at the same time, bandwidth usage of the instance has reached the limit. In this case, expand the instance capacity.</li></ul>
<div class="note" id="dcs-faq-0220721__en-us_topic_0000001357973609_note13959936343"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="dcs-faq-0220721__en-us_topic_0000001357973609_p16190307316">You can analyze big keys and hot keys on the DCS console. For details, see <a href="dcs-ug-190808001.html">Cache Analysis</a>.</p>
</div></div>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326023.html">Monitoring and Alarm</a></div>
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</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0326024.html">Data Backup, Export, and Migration</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0805001.html">Analysis of Big Keys and Hot Keys</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-191204001.html">Master/Standby Switchover</a></strong><br>
</li>
</ul>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326019.html">FAQs</a></div>
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</li>
<li class="ulchildlink"><strong><a href="dcs-faq-210223001.html">Why Is "Error in execution" Returned When I Access Redis?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-210409001.html">Why Does a Key Disappear in Redis?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0220330.html">Will Cached Data Be Retained After an Instance Is Restarted?</a></strong><br>
</li>
</ul>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326019.html">FAQs</a></div>
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<div>
<ul class="ullinks">
<li class="ulchildlink"><strong><a href="dcs-faq-0427070.html">How Do I View Current Concurrent Connections and Maximum Connections of a DCS Redis Instance?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0730049.html">Does Redis Support Command Audits?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0730050.html">What Should I Do If the Monitoring Data of a DCS Redis Instance Is Abnormal?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0730052.html">Why Is Used Memory Greater Than Available Memory?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0513001.html">Why Does Bandwidth Usage Exceed 100%?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0603001.html">Why Is the Rejected Connections Metric Displayed?</a></strong><br>
</li>
<li class="ulchildlink"><strong><a href="dcs-faq-0220721.html">Why Is Flow Control Triggered? How Do I Handle It?</a></strong><br>
</li>
</ul>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326019.html">FAQs</a></div>
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<h1 class="topictitle1">How Do I View Current Concurrent Connections and Maximum Connections of a DCS Redis Instance?</h1>
<div id="body8662426"><div class="section" id="dcs-faq-0427070__en-us_topic_0000001152024136_section1930172420179"><h4 class="sectiontitle">Viewing Concurrent Connections of a DCS Redis Instance</h4><p id="dcs-faq-0427070__en-us_topic_0000001152024136_p10599203110321">The number of connections received by a DCS instance is a metric that is monitored by Cloud Eye. For details on how to view metrics, see <a href="dcs-ug-0312045.html">Viewing DCS Monitoring Metrics</a>.</p>
<p id="dcs-faq-0427070__en-us_topic_0000001152024136_p6675918164416">On the Cloud Eye console, find the <strong id="dcs-faq-0427070__en-us_topic_0000001152024136_b244683293117">Connected Clients</strong> metric. Click <span><img id="dcs-faq-0427070__en-us_topic_0000001152024136_image13301111610473" src="en-us_image_0000001383077054.png"></span> to view monitoring details on an enlarged graph.</p>
<p id="dcs-faq-0427070__en-us_topic_0000001152024136_p1087210448476">Specify a time range to view the metric in a specific monitoring period. For example, you can select a 10-minute period to view the number of connections received during the period. On the graph, you can view the trend and the total number of connections received during the period.</p>
<div class="p" id="dcs-faq-0427070__en-us_topic_0000001152024136_p363521310447">On the Cloud Eye console, you can also view other monitoring metrics of your DCS instances, for example:<ul id="dcs-faq-0427070__en-us_topic_0000001152024136_ul10189131214415"><li id="dcs-faq-0427070__en-us_topic_0000001152024136_li111881312104413">CPU Usage</li><li id="dcs-faq-0427070__en-us_topic_0000001152024136_li1618920126442">Memory Usage</li><li id="dcs-faq-0427070__en-us_topic_0000001152024136_li3189131218449">Used Memory</li><li id="dcs-faq-0427070__en-us_topic_0000001152024136_li19189171217445">Ops per Second</li></ul>
</div>
</div>
<div class="section" id="dcs-faq-0427070__en-us_topic_0000001152024136_section108331358191915"><h4 class="sectiontitle">Viewing or Modifying the Maximum Connections of an Instance</h4><p id="dcs-faq-0427070__en-us_topic_0000001152024136_p468215206202">When creating an instance on the console, you can view the default maximum number of connections and the limit that can be configured.</p>
<p id="dcs-faq-0427070__en-us_topic_0000001152024136_p624072363315">After an instance is created, you can view or change the value of <strong id="dcs-faq-0427070__en-us_topic_0000001152024136_b11239923183317">maxclients</strong> (the maximum number of connections) on the <strong id="dcs-faq-0427070__b2086918457373">Parameters</strong> page of the DCS console. (This parameter cannot be modified for Proxy Cluster instances.)</p>
</div>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326023.html">Monitoring and Alarm</a></div>
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<h1 class="topictitle1">Why Does Bandwidth Usage Exceed 100%?</h1>
<div id="body8662426"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p8060118">The basic information about the bandwidth usage metric is as follows.</p>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" id="dcs-faq-0513001__en-us_topic_0000001152183968_table111117367367" frame="border" border="1" rules="all"><thead align="left"><tr id="dcs-faq-0513001__en-us_topic_0000001152183968_row611114364367"><th align="left" class="cellrowborder" valign="top" width="18.72187218721872%" id="mcps1.3.2.1.7.1.1"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p011115366360">Metric ID</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="13.301330133013304%" id="mcps1.3.2.1.7.1.2"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p611143643620">Metric Name</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="18.101810181018102%" id="mcps1.3.2.1.7.1.3"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p1911214361369">Description</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="13.16131613161316%" id="mcps1.3.2.1.7.1.4"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p8112123614366">Value Range</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="21.782178217821784%" id="mcps1.3.2.1.7.1.5"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p11112153616365">Monitored Object and Dimension</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="14.931493149314932%" id="mcps1.3.2.1.7.1.6"><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p1627205117465">Monitoring Period (Raw Data)</p>
</th>
</tr>
</thead>
<tbody><tr id="dcs-faq-0513001__en-us_topic_0000001152183968_row9112183612365"><td class="cellrowborder" valign="top" width="18.72187218721872%" headers="mcps1.3.2.1.7.1.1 "><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p191120363366">bandwidth_usage</p>
</td>
<td class="cellrowborder" valign="top" width="13.301330133013304%" headers="mcps1.3.2.1.7.1.2 "><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p1511243693615">Bandwidth Usage</p>
</td>
<td class="cellrowborder" valign="top" width="18.101810181018102%" headers="mcps1.3.2.1.7.1.3 "><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p611203623618">Percentage of the used bandwidth to the maximum bandwidth limit</p>
</td>
<td class="cellrowborder" valign="top" width="13.16131613161316%" headers="mcps1.3.2.1.7.1.4 "><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p911211362367">0–200%</p>
</td>
<td class="cellrowborder" valign="top" width="21.782178217821784%" headers="mcps1.3.2.1.7.1.5 "><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p32179217504">Monitored object:</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p1721716225014">Master/standby DCS Redis 4.0 or 5.0 instances</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p112176213507">Redis Server of Redis Cluster DCS Redis 4.0 or 5.0 instance</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p1321782175018">Dimension:</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p1521772185016">dcs_cluster_node</p>
</td>
<td class="cellrowborder" valign="top" width="14.931493149314932%" headers="mcps1.3.2.1.7.1.6 "><p id="dcs-faq-0513001__en-us_topic_0000001152183968_p727210510464">1 minute</p>
</td>
</tr>
</tbody>
</table>
</div>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p81021619517">Bandwidth usage = (Input flow + Output flow)/(2 x Maximum bandwidth) x 100%</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p9331103545411">According to the formula, the bandwidth usage counts in the input flow and output flow, which include the traffic for replication between the master and replicas. Therefore, the total traffic is greater than the normal service traffic.</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p17209224155218">If the value of the <strong id="dcs-faq-0513001__en-us_topic_0000001152183968_b4139183117258">Flow Control Times</strong> metric is larger than 0, the maximum bandwidth has been reached and flow control has been performed.</p>
<p id="dcs-faq-0513001__en-us_topic_0000001152183968_p62091247528">However, flow control decisions are made without considering the traffic for replication between the master and replicas. Therefore, sometimes the bandwidth usage exceeds 100% but the number of flow control times is 0.</p>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326023.html">Monitoring and Alarm</a></div>
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<h1 class="topictitle1">What Are Big Keys and Hot Keys?</h1>
<div id="body0000001430506037">
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" id="dcs-faq-0521003__table124661749173110" frame="border" border="1" rules="all"><thead align="left"><tr id="dcs-faq-0521003__en-us_topic_0000001233623890_row135345162174"><th align="left" class="cellrowborder" valign="top" width="20.49%" id="mcps1.3.1.1.3.1.1"><p id="dcs-faq-0521003__en-us_topic_0000001233623890_p35351416161711">Term</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="79.51%" id="mcps1.3.1.1.3.1.2"><p id="dcs-faq-0521003__en-us_topic_0000001233623890_p1853561641714">Definition</p>
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<tbody><tr id="dcs-faq-0521003__en-us_topic_0000001233623890_row9535101618175"><td class="cellrowborder" valign="top" width="20.49%" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521003__en-us_topic_0000001233623890_p115352016141716">Big key</p>
</td>
<td class="cellrowborder" valign="top" width="79.51%" headers="mcps1.3.1.1.3.1.2 "><p id="dcs-faq-0521003__en-us_topic_0000001233623890_p1346816261178">There are two types of big keys:</p>
<ul id="dcs-faq-0521003__en-us_topic_0000001233623890_ul11469122611716"><li id="dcs-faq-0521003__en-us_topic_0000001233623890_li64692264172">Keys that have a large value, for example, a 10 MB String key, or a 100 MB Hash, List, or Set key (all elements combined). If the size of a single String key exceeds 10 KB, or if the size of all elements of a key combined exceeds 50 MB, the key is defined as a big key.</li><li id="dcs-faq-0521003__en-us_topic_0000001233623890_li846917264173">Keys that have a large number of elements, for example, a Hash key that has 10,000 elements. If the number of elements in a key exceeds 5000, the key is defined as a big key.</li></ul>
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<tr id="dcs-faq-0521003__en-us_topic_0000001233623890_row19535181616171"><td class="cellrowborder" valign="top" width="20.49%" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521003__en-us_topic_0000001233623890_p4535016181713">Hot key</p>
</td>
<td class="cellrowborder" valign="top" width="79.51%" headers="mcps1.3.1.1.3.1.2 "><p id="dcs-faq-0521003__en-us_topic_0000001233623890_p18689338178">A key is defined as a hot key if it is frequently requested or if it occupies a large number of resources. For example:</p>
<ul id="dcs-faq-0521003__en-us_topic_0000001233623890_ul196833311171"><li id="dcs-faq-0521003__en-us_topic_0000001233623890_li126833312175">In a cluster instance, a shard processes 10,000 requests per second, among which 3000 are performed on the same key.</li><li id="dcs-faq-0521003__en-us_topic_0000001233623890_li106813336173">In a cluster instance, a shard uses a total of 100 Mbits/s inbound and outbound bandwidth, among which 80 Mbits/s is used by the <strong id="dcs-faq-0521003__en-us_topic_0000001233623890_b55302156146">HGETALL</strong> operation on a Hash key.</li></ul>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0805001.html">Analysis of Big Keys and Hot Keys</a></div>
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<h1 class="topictitle1">What Is the Impact of Big Keys or Hot Keys?</h1>
<div id="body0000001379866278">
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" id="dcs-faq-0521004__table1344311351293" frame="border" border="1" rules="all"><thead align="left"><tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row227713376217"><th align="left" class="cellrowborder" valign="top" width="20.49%" id="mcps1.3.1.1.3.1.1"><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p72771337152115">Category</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="79.51%" id="mcps1.3.1.1.3.1.2"><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p727763702117">Impact</p>
</th>
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</thead>
<tbody><tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row143611959162317"><td class="cellrowborder" rowspan="6" valign="top" width="20.49%" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p117218274247">Big key</p>
</td>
<td class="cellrowborder" valign="top" width="79.51%" headers="mcps1.3.1.1.3.1.2 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p1073475212246"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b159111656162117">Instance specifications fail to be modified.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p185156484240">Specification modification of a Redis Cluster instance involves rebalancing (data migration between nodes). Redis has a limit on key migration. If the instance has any single key bigger than 512 MB, the modification will fail when big key migration between nodes times out. The bigger the key, the more likely the migration will fail.</p>
</td>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row8597175962314"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p1043575512248"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b5941185119254">Data migration fails.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p1051584852414">During data migration, if a key has many elements, other keys will be blocked and will be stored in the memory buffer of the migration ECS. If they are blocked for a long time, the migration will fail.</p>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row1383818599231"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p7516204814247"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b668243662219">Cluster shards are unbalanced.</strong></p>
<ul id="dcs-faq-0521004__en-us_topic_0000001233623890_ul165161448142414"><li id="dcs-faq-0521004__en-us_topic_0000001233623890_li1251624820245">The memory usage of shards is unbalanced. For example, if a shard uses a large memory or even uses up the memory, keys on this shard are evicted, and resources of other shards are wasted.</li><li id="dcs-faq-0521004__en-us_topic_0000001233623890_li17516164852410">The bandwidth usage of shards is unbalanced. For example, flow control is repeatedly triggered on a shard.</li></ul>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row15507012413"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p2524720202510"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b788134511264">Latency of client command execution increases.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p2516448122416">Slow operations on a big key block other commands, resulting in a large number of slow queries.</p>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row1925717015243"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p176051181254"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b7696252172616">Flow control is triggered on the instance.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p1351664832415">Frequently reading data from big keys exhausts the outbound bandwidth of the instance, triggering flow control. As a result, a large number of commands time out or slow queries occur, affecting services.</p>
</td>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row12465110202417"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p1338401612516"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b1096285512619">Master/standby switchover is triggered.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p13516144813243">If the high-risk <strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b727174317117">DEL</strong> operation is performed on a big key, the master node may be blocked for a long time, causing a master/standby switchover.</p>
</td>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row1269712013248"><td class="cellrowborder" rowspan="3" valign="top" width="20.49%" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p182132311242">Hot key</p>
</td>
<td class="cellrowborder" valign="top" width="79.51%" headers="mcps1.3.1.1.3.1.2 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p135813396253"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b1549517213918">Cluster shards are unbalanced.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p175841737102519">If only the shard where the hot key is located is busy processing service queries, there may be performance bottlenecks on a single shard, and the compute resources of other shards may be wasted.</p>
</td>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row7228111132418"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p182361241112512"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b7871310271">CPU usage surges.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p7584173718257">A large number of operations on hot keys may cause high CPU usage. If the operations are on a single cluster shard, the CPU usage of the shard where the hot key is located will surge. This will slow down other requests and the overall performance. If the service volume increases sharply, a master/standby switchover may be triggered.</p>
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<tr id="dcs-faq-0521004__en-us_topic_0000001233623890_row92771737192113"><td class="cellrowborder" valign="top" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0521004__en-us_topic_0000001233623890_p621394416251"><strong id="dcs-faq-0521004__en-us_topic_0000001233623890_b101411964273">Cache breakdown may occur.</strong></p>
<p id="dcs-faq-0521004__en-us_topic_0000001233623890_p3584937202518">If Redis cannot handle the pressure on hot keys, requests will hit the database. The database may break down as its load increases dramatically, affecting other services.</p>
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</tbody>
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<div>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0805001.html">Analysis of Big Keys and Hot Keys</a></div>
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<h1 class="topictitle1">How Do I Avoid Big Keys and Hot Keys?</h1>
<div id="body0000001380026226"><ul id="dcs-faq-0521005__ul1627391114415"><li id="dcs-faq-0521005__li132743113411"><strong id="dcs-faq-0521005__b197041440153910">Keep the size of Strings within 10 KB</strong> and <strong id="dcs-faq-0521005__b113402035453910">the quantity</strong> of Hashes, Lists, Sets, or Zsets <strong id="dcs-faq-0521005__b159670428253910">within 5000</strong>.</li><li id="dcs-faq-0521005__li1427414112414">When naming keys, use the service name abbreviation as the prefix and do not use special characters such as spaces, line brakes, single or double quotation marks, and other escape characters.</li><li id="dcs-faq-0521005__li9274511114120">Do not rely too much on Redis transactions.</li><li id="dcs-faq-0521005__li0274161164112">The performance of short connections ("connect" in Redis terminology) is poor. Use clients with connection pools.</li><li id="dcs-faq-0521005__li127491184119">Do not enable data persistence if you use Redis just for caching and can tolerate data loss.</li><li id="dcs-faq-0521005__li13952142316363">For details about how to optimize big keys and hot keys, see the following table.</li></ul>
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" id="dcs-faq-0521005__table51721132203610" frame="border" border="1" rules="all"><thead align="left"><tr id="dcs-faq-0521005__row8173532133619"><th align="left" class="cellrowborder" valign="top" width="22.89%" id="mcps1.3.2.1.3.1.1"><p id="dcs-faq-0521005__p35279463368">Category</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="77.11%" id="mcps1.3.2.1.3.1.2"><p id="dcs-faq-0521005__p1652710469363">Method</p>
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</thead>
<tbody><tr id="dcs-faq-0521005__row817319327363"><td class="cellrowborder" rowspan="2" valign="top" width="22.89%" headers="mcps1.3.2.1.3.1.1 "><p id="dcs-faq-0521005__p20263527163812">Big key</p>
</td>
<td class="cellrowborder" valign="top" width="77.11%" headers="mcps1.3.2.1.3.1.2 "><p id="dcs-faq-0521005__p834719309371"><strong id="dcs-faq-0521005__b834703033715">Split big keys.</strong></p>
<p id="dcs-faq-0521005__p9347153013373">Scenarios:</p>
<ul id="dcs-faq-0521005__ul12347130153716"><li id="dcs-faq-0521005__li73471030183712"><strong id="dcs-faq-0521005__b57304051153910">If the big key is a String</strong>, you can split it into several key-value pairs and use <strong id="dcs-faq-0521005__b162589608453910">MGET</strong> or a pipeline consisting of multiple <strong id="dcs-faq-0521005__b164200870853910">GET</strong> operations to obtain the values. In this way, the pressure of a single operation can be split. For a cluster instance, the operation pressure can be evenly distributed to multiple shards, reducing the impact on a single shard.</li><li id="dcs-faq-0521005__li43471330143717"><strong id="dcs-faq-0521005__b7713182391514">If the big key contains multiple elements, and the elements must be operated together</strong>, the big key cannot be split. You can remove the big key from Redis and store it on other storage media instead. This scenario should be avoided by design.</li><li id="dcs-faq-0521005__li234703023715"><strong id="dcs-faq-0521005__b1534743016372">If the big key contains multiple elements, and only some elements need to be operated each time</strong>, separate the elements. Take a Hash key as an example. Each time you run the <strong id="dcs-faq-0521005__b52932189953910">HGET</strong> or <strong id="dcs-faq-0521005__b110362695053910">HSET</strong> command, the result of the hash value modulo <em id="dcs-faq-0521005__i196395322153910">N</em> (customized on the client) determines which key the field falls on. This algorithm is similar to that used for calculating slots in Redis Cluster.</li></ul>
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<tr id="dcs-faq-0521005__row1517316326368"><td class="cellrowborder" valign="top" headers="mcps1.3.2.1.3.1.1 "><p id="dcs-faq-0521005__p6347163013717"><strong id="dcs-faq-0521005__b5347130123717">Store big keys on other storage media.</strong></p>
<p id="dcs-faq-0521005__p19347113020373">If a big key cannot be split, it is not suitable to be stored in Redis. You can store it on other storage media, and delete the big key from Redis.</p>
<div class="caution" id="dcs-faq-0521005__note034833019375"><span class="cautiontitle"> CAUTION: </span><div class="cautionbody"><p id="dcs-faq-0521005__p17348130193710">Do not use the <strong id="dcs-faq-0521005__b66232654753910">DEL</strong> command to delete big keys. Otherwise, Redis may be blocked or even a master/standby switchover may occur.</p>
</div></div>
</td>
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<tr id="dcs-faq-0521005__row15174193215363"><td class="cellrowborder" rowspan="2" valign="top" width="22.89%" headers="mcps1.3.2.1.3.1.1 "><p id="dcs-faq-0521005__p166981631143815">Hot key</p>
</td>
<td class="cellrowborder" valign="top" width="77.11%" headers="mcps1.3.2.1.3.1.2 "><p id="dcs-faq-0521005__p11227567386"><strong id="dcs-faq-0521005__b9227263381">Use the client cache or local cache.</strong></p>
<p id="dcs-faq-0521005__p1227562383">If you know what keys are frequently used, you can design a two-level cache architecture (client/local cache and remote Redis). Frequently used data is obtained from the local cache first. The local cache and remote cache are updated with data writes at the same time. In this way, the read pressure on frequently accessed data can be separated. This method is costly because it requires changes to the client architecture and code.</p>
</td>
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<tr id="dcs-faq-0521005__row1617483216361"><td class="cellrowborder" valign="top" headers="mcps1.3.2.1.3.1.1 "><p id="dcs-faq-0521005__p1222717623814"><strong id="dcs-faq-0521005__b11227186103816">Design a circuit breaker or degradation mechanism.</strong></p>
<p id="dcs-faq-0521005__p222756143820">Hot keys can easily result in cache breakdown. During peak hours, requests are passed through to the backend database, causing service avalanche. To ensure availability, the system must have a circuit breaker or degradation mechanism to limit the traffic and degrade services if breakdown occurs.</p>
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</table>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0805001.html">Analysis of Big Keys and Hot Keys</a></div>
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<h1 class="topictitle1">How Do I Analyze the Hot Keys of a DCS Redis 3.0 Instance?</h1>
<div id="body0000001430066485"><p id="dcs-faq-0521006__p193920413448">DCS for Redis 3.0 does not support hot key analysis on the console. Alternatively, you can use the following methods to analyze hot keys:</p>
<ul id="dcs-faq-0521006__ul130321203"><li id="dcs-faq-0521006__li113031219018">Method 1: Analyze the service structure and service implementation to discover possible hot keys.<p id="dcs-faq-0521006__p1359010423112"><a name="dcs-faq-0521006__li113031219018"></a><a name="li113031219018"></a>For example, hot keys can be easily found in the service code during flash sales or user logins.</p>
<p id="dcs-faq-0521006__p512912548016">Advantage: Simple and easy to implement.</p>
<p id="dcs-faq-0521006__p329410281312">Disadvantage: Requires familiarity with the service code. In addition, the analysis become more difficult as the service scenarios become more complex.</p>
</li><li id="dcs-faq-0521006__li141471715212">Method 2: Collect key access statistics in the client code to discover hot keys.<p id="dcs-faq-0521006__p15792112616215"><a name="dcs-faq-0521006__li141471715212"></a><a name="li141471715212"></a>Disadvantage: Requires intrusive code modification.</p>
</li><li id="dcs-faq-0521006__li204874411217">Method 3: Capture and analyze packets.<p id="dcs-faq-0521006__p1037613506214"><a name="dcs-faq-0521006__li204874411217"></a><a name="li204874411217"></a>Advantage: Simple and easy to implement.</p>
</li></ul>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0805001.html">Analysis of Big Keys and Hot Keys</a></div>
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<h1 class="topictitle1">Why Is the Rejected Connections Metric Displayed?</h1>
<div id="body8662426"><p id="dcs-faq-0603001__en-us_topic_0000001198023693_p73321412506">If the <strong id="dcs-faq-0603001__en-us_topic_0000001198023693_b10926205117259">Rejected Connections</strong> metric is displayed, check if the number of connected clients exceeds the maximum allowed number of connections of the instances. To check the maximum allowed number of connections, go to the <strong id="dcs-faq-0603001__en-us_topic_0000001198023693_b143881647112614">Parameters</strong> tab page of the instance and check the value of the <strong id="dcs-faq-0603001__en-us_topic_0000001198023693_b19671140122712">maxclients</strong> parameter.</p>
</div>
<div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="dcs-faq-0326023.html">Monitoring and Alarm</a></div>
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<h1 class="topictitle1">How Do I Detect Big Keys and Hot Keys in Advance?</h1>
<div id="body0000001379547102">
<div class="tablenoborder"><table cellpadding="4" cellspacing="0" summary="" id="dcs-faq-0606002__table180183710324" frame="border" border="1" rules="all"><thead align="left"><tr id="dcs-faq-0606002__row178021637143215"><th align="left" class="cellrowborder" valign="top" width="34.96%" id="mcps1.3.1.1.3.1.1"><p id="dcs-faq-0606002__p12579129338">Method</p>
</th>
<th align="left" class="cellrowborder" valign="top" width="65.03999999999999%" id="mcps1.3.1.1.3.1.2"><p id="dcs-faq-0606002__p625751214333">Description</p>
</th>
</tr>
</thead>
<tbody><tr id="dcs-faq-0606002__row1980212370326"><td class="cellrowborder" valign="top" width="34.96%" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0606002__p1980293733210">Through <strong id="dcs-faq-0606002__b79321032553939">Big Key Analysis</strong> and <strong id="dcs-faq-0606002__b103292901953939">Hot Key Analysis</strong> on the DCS console</p>
</td>
<td class="cellrowborder" valign="top" width="65.03999999999999%" headers="mcps1.3.1.1.3.1.2 "><p id="dcs-faq-0606002__p1582215495119">See <a href="dcs-ug-190808001.html">Cache Analysis</a>.</p>
</td>
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<tr id="dcs-faq-0606002__row38022037183218"><td class="cellrowborder" valign="top" width="34.96%" headers="mcps1.3.1.1.3.1.1 "><p id="dcs-faq-0606002__p12802137113218">By using the <strong id="dcs-faq-0606002__b1859015253209">bigkeys</strong> and <strong id="dcs-faq-0606002__b1659014257208">hotkeys</strong> options on redis-cli</p>
</td>
<td class="cellrowborder" valign="top" width="65.03999999999999%" headers="mcps1.3.1.1.3.1.2 "><ul id="dcs-faq-0606002__ul8127320134911"><li id="dcs-faq-0606002__li181277206497">redis-cli uses the <strong id="dcs-faq-0606002__b101075574853939">bigkeys</strong> option to traverse all keys in a Redis instance and returns the overall key statistics and the biggest key of six data types: Strings, Lists, Hashes, Sets, Zsets, and Streams. The command is <strong id="dcs-faq-0606002__b121893929053939">redis-cli -h </strong><em id="dcs-faq-0606002__i190866199153939">&lt;Instance connection address&gt;</em><strong id="dcs-faq-0606002__b211779146253939"> -p </strong><em id="dcs-faq-0606002__i89825748553939">&lt;Port number&gt;</em><strong id="dcs-faq-0606002__b110034694353939"> -a </strong><em id="dcs-faq-0606002__i77687502053939">&lt;Password&gt;</em><strong id="dcs-faq-0606002__b12862540553939"> --bigkeys</strong>.</li><li id="dcs-faq-0606002__li10127172084912">In Redis 4.0 and later, you can use the <strong id="dcs-faq-0606002__b116132887053939">hotkeys</strong> option to quickly find hot keys in redis-cli. Run this command during service running to find hot keys: <strong id="dcs-faq-0606002__b139556488653939">redis-cli -h </strong><em id="dcs-faq-0606002__i91175973053939">&lt;Instance connection address&gt;</em><strong id="dcs-faq-0606002__b14872896153939"> -p </strong><em id="dcs-faq-0606002__i113591880953939">&lt;Port number&gt;</em><strong id="dcs-faq-0606002__b170646013653939"> -a </strong><em id="dcs-faq-0606002__i29057584553939">&lt;Password&gt;</em><strong id="dcs-faq-0606002__b192171473053939"> --hotkeys</strong>. The hot key details can be obtained from the summary part in the returned result.</li></ul>
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<p id="dcs-faq-0606002__p520894814584">Hot key analysis is not supported by DCS Redis 3.0 instances. You can <a href="dcs-ug-190905001.html#dcs-ug-190905001__en-us_topic_0190235954_section1118571110427">configure alarms</a> to detect hot keys.</p>
<ul id="dcs-faq-0606002__ul13208164815815"><li id="dcs-faq-0606002__li0208114825812">Configure alarm rules for the <strong id="dcs-faq-0606002__b122081048195811">Memory Usage</strong> metric of the instance nodes.<p id="dcs-faq-0606002__p6208194845818">If a node has a big key, the memory usage of the node is much higher than that of other nodes. In this case, an alarm is triggered to help you find the potentially problematic key.</p>
</li></ul>
<ul id="dcs-faq-0606002__ul19208124805818"><li id="dcs-faq-0606002__li220864855812">Configure alarm rules for the <strong id="dcs-faq-0606002__b42324711217">Maximum Inbound Bandwidth</strong>, <strong id="dcs-faq-0606002__b15242047182116">Maximum Outbound Bandwidth</strong>, and <strong id="dcs-faq-0606002__b42484716213">CPU Usage</strong> metrics of the instance nodes.<p id="dcs-faq-0606002__p1120884855810">If a node has a hot key, the bandwidth and CPU usage of the node is much higher than that of other nodes. In this case, an alarm is triggered to help you find the potentially problematic key.</p>
</li></ul>
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<a name="dcs-faq-0730022"></a><a name="dcs-faq-0730022"></a>
<h1 class="topictitle1">What Is the Default Data Eviction Policy?</h1>
<div id="body1596093268950"><p id="dcs-faq-0730022__en-us_topic_0152099097_p891453012313">Data is evicted from cache based on a user-defined space limit in order to make space for new data. In the current versions of DCS, you can select an eviction policy.</p>
<p id="dcs-faq-0730022__en-us_topic_0152099097_p84517544616"><strong id="dcs-faq-0730022__b9531817917">noeviction</strong> is the <span class="keyword" id="dcs-faq-0730022__keyword13587172119">default eviction policy</span> for single-node and master/standby DCS Redis instances. You can change the eviction policy by configuring the instance parameters on the DCS console.</p>
<p id="dcs-faq-0730022__en-us_topic_0152099097_p4839017143534"><strong id="dcs-faq-0730022__b131721719919">volatile-lru</strong> is the default eviction policy for cluster DCS Redis instances. To change the eviction policy for cluster instances, contact technical support.</p>
<div id="body1596093268950"><p id="dcs-faq-0730022__en-us_topic_0152099097_p891453012313">Data is evicted from cache based on a user-defined space limit in order to make space for new data.</p>
<p id="dcs-faq-0730022__p28092072506">You can change the eviction policy by configuring the <strong id="dcs-faq-0730022__b1461933614396">maxmemory-policy</strong> parameter.</p>
<p id="dcs-faq-0730022__en-us_topic_0152099097_p24119933153714">When <strong id="dcs-faq-0730022__b9411301292">maxmemory</strong> is reached, you can select one of the following eight eviction policies:</p>
<ul id="dcs-faq-0730022__ul63511053153341"><li id="dcs-faq-0730022__li46713159155023"><strong id="dcs-faq-0730022__b148525218919">noeviction</strong>: When the memory limit is reached, DCS instances return errors to clients and no longer process write requests and other requests that could result in more memory to be used. However, <strong id="dcs-faq-0730022__b23887235114">DEL</strong> and a few more exception requests can continue to be processed.</li><li id="dcs-faq-0730022__li33146435153341"><strong id="dcs-faq-0730022__b12386224217">allkeys-lru</strong>: DCS instances try to evict the least recently used keys first, in order to make space for new data.</li><li id="dcs-faq-0730022__li62982494153346"><strong id="dcs-faq-0730022__b1523412252113">volatile-lru</strong>: DCS instances try to evict the least recently used keys with an expire set first, in order to make space for new data.</li><li id="dcs-faq-0730022__li61473187153348"><strong id="dcs-faq-0730022__b9967425718">allkeys-random</strong>: DCS instances recycle random keys so that new data can be stored.</li><li id="dcs-faq-0730022__li9870151153350"><strong id="dcs-faq-0730022__b15787152616119">volatile-random</strong>: DCS instances evict random keys with an expire set, in order to make space for new data.</li><li id="dcs-faq-0730022__li3073190415343"><strong id="dcs-faq-0730022__b989313618914">volatile-ttl</strong>: DCS instances evict keys with an expire set, and try to evict keys with a shorter time to live (TTL) first, in order to make space for new data.</li><li id="dcs-faq-0730022__li1918134611344"><strong id="dcs-faq-0730022__b188119815917">allkeys-lfu</strong>: DCS instances evict the least frequently used keys from all keys.</li><li id="dcs-faq-0730022__li188701956123617"><strong id="dcs-faq-0730022__b12439131113918">volatile-lfu</strong>: DCS instances evict the least frequently used keys with an <strong id="dcs-faq-0730022__b1944015111698">expire</strong> field from all keys.</li></ul>
<div class="note" id="dcs-faq-0730022__en-us_topic_0152099097_note33192730155638"><img src="public_sys-resources/note_3.0-en-us.png"><span class="notetitle"> </span><div class="notebody"><p id="dcs-faq-0730022__en-us_topic_0152099097_p30299119155638">If no key can be recycled, <strong id="dcs-faq-0730022__b122631228518">volatile-lru</strong>, <strong id="dcs-faq-0730022__b626410282110">volatile-random</strong>, and <strong id="dcs-faq-0730022__b62641528915">volatile-ttl</strong> are the same as <strong id="dcs-faq-0730022__b1526417284111">noeviction</strong>. For details, see the description of <strong id="dcs-faq-0730022__b42649283119">noeviction</strong>.</p>
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