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   <div class="section" id="leveled-compaction-strategy">
 <span id="lcs"></span><h1>Leveled Compaction Strategy<a class="headerlink" href="#leveled-compaction-strategy" title="Permalink to this headline">¶</a></h1>
 <p>The idea of <code class="docutils literal notranslate"><span class="pre">LeveledCompactionStrategy</span></code> (LCS) is that all sstables are put into different levels where we guarantee
 that no overlapping sstables are in the same level. By overlapping we mean that the first/last token of a single sstable
 are never overlapping with other sstables. This means that for a SELECT we will only have to look for the partition key
 in a single sstable per level. Each level is 10x the size of the previous one and each sstable is 160MB by default. L0
 is where sstables are streamed/flushed - no overlap guarantees are given here.</p>
 <p>When picking compaction candidates we have to make sure that the compaction does not create overlap in the target level.
 This is done by always including all overlapping sstables in the next level. For example if we select an sstable in L3,
 we need to guarantee that we pick all overlapping sstables in L4 and make sure that no currently ongoing compactions
 will create overlap if we start that compaction. We can start many parallel compactions in a level if we guarantee that
 we wont create overlap. For L0 -&gt; L1 compactions we almost always need to include all L1 sstables since most L0 sstables
 cover the full range. We also can’t compact all L0 sstables with all L1 sstables in a single compaction since that can
 use too much memory.</p>
 <p>When deciding which level to compact LCS checks the higher levels first (with LCS, a “higher” level is one with a higher
 number: L0 is the lowest one, L8 is the highest one) and if the level is behind a compaction will be started
 in that level.</p>
 <div class="section" id="major-compaction">
 <h2>Major compaction<a class="headerlink" href="#major-compaction" title="Permalink to this headline">¶</a></h2>
 <p>It is possible to do a major compaction with LCS - it will currently start by filling out L1 and then once L1 is full,
 it continues with L2 etc. This is sub optimal and will change to create all the sstables in a high level instead,
 CASSANDRA-11817.</p>
 </div>
 <div class="section" id="bootstrapping">
 <h2>Bootstrapping<a class="headerlink" href="#bootstrapping" title="Permalink to this headline">¶</a></h2>
 <p>During bootstrap sstables are streamed from other nodes. The level of the remote sstable is kept to avoid many
 compactions after the bootstrap is done. During bootstrap the new node also takes writes while it is streaming the data
 from a remote node - these writes are flushed to L0 like all other writes and to avoid those sstables blocking the
 remote sstables from going to the correct level, we only do STCS in L0 until the bootstrap is done.</p>
 </div>
 <div class="section" id="stcs-in-l0">
 <h2>STCS in L0<a class="headerlink" href="#stcs-in-l0" title="Permalink to this headline">¶</a></h2>
 <p>If LCS gets very many L0 sstables reads are going to hit all (or most) of the L0 sstables since they are likely to be
 overlapping. To more quickly remedy this LCS does STCS compactions in L0 if there are more than 32 sstables there. This
 should improve read performance more quickly compared to letting LCS do its L0 -&gt; L1 compactions. If you keep getting
 too many sstables in L0 it is likely that LCS is not the best fit for your workload and STCS could work out better.</p>
 </div>
 <div class="section" id="starved-sstables">
 <h2>Starved sstables<a class="headerlink" href="#starved-sstables" title="Permalink to this headline">¶</a></h2>
 <p>If a node ends up with a leveling where there are a few very high level sstables that are not getting compacted they
 might make it impossible for lower levels to drop tombstones etc. For example, if there are sstables in L6 but there is
 only enough data to actually get a L4 on the node the left over sstables in L6 will get starved and not compacted.  This
 can happen if a user changes sstable_size_in_mb from 5MB to 160MB for example. To avoid this LCS tries to include
 those starved high level sstables in other compactions if there has been 25 compaction rounds where the highest level
 has not been involved.</p>
 </div>
 <div class="section" id="lcs-options">
 <span id="id1"></span><h2>LCS options<a class="headerlink" href="#lcs-options" title="Permalink to this headline">¶</a></h2>
 <dl class="simple">
 <dt><code class="docutils literal notranslate"><span class="pre">sstable_size_in_mb</span></code> (default: 160MB)</dt><dd><p>The target compressed (if using compression) sstable size - the sstables can end up being larger if there are very
 large partitions on the node.</p>
 </dd>
 <dt><code class="docutils literal notranslate"><span class="pre">fanout_size</span></code> (default: 10)</dt><dd><p>The target size of levels increases by this fanout_size multiplier. You can reduce the space amplification by tuning
 this option.</p>
 </dd>
 </dl>
 <p>LCS also support the <code class="docutils literal notranslate"><span class="pre">cassandra.disable_stcs_in_l0</span></code> startup option (<code class="docutils literal notranslate"><span class="pre">-Dcassandra.disable_stcs_in_l0=true</span></code>) to avoid
 doing STCS in L0.</p>
 </div>
 </div>


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	<div class="section" id="leveled-compaction-strategy">
	<span id="lcs"></span><h1>Leveled Compaction Strategy<a class="headerlink" href="#leveled-compaction-strategy" title="Permalink to this headline">¶</a></h1>
	<p>The idea of <code class="docutils literal notranslate"><span class="pre">LeveledCompactionStrategy</span></code> (LCS) is that all sstables are put into different levels where we guarantee
	that no overlapping sstables are in the same level. By overlapping we mean that the first/last token of a single sstable
	are never overlapping with other sstables. This means that for a SELECT we will only have to look for the partition key
	in a single sstable per level. Each level is 10x the size of the previous one and each sstable is 160MB by default. L0
	is where sstables are streamed/flushed - no overlap guarantees are given here.</p>
	<p>When picking compaction candidates we have to make sure that the compaction does not create overlap in the target level.
	This is done by always including all overlapping sstables in the next level. For example if we select an sstable in L3,
	we need to guarantee that we pick all overlapping sstables in L4 and make sure that no currently ongoing compactions
	will create overlap if we start that compaction. We can start many parallel compactions in a level if we guarantee that
	we wont create overlap. For L0 -> L1 compactions we almost always need to include all L1 sstables since most L0 sstables
	cover the full range. We also can’t compact all L0 sstables with all L1 sstables in a single compaction since that can
	use too much memory.</p>
	<p>When deciding which level to compact LCS checks the higher levels first (with LCS, a “higher” level is one with a higher
	number: L0 is the lowest one, L8 is the highest one) and if the level is behind a compaction will be started
	in that level.</p>
	<div class="section" id="major-compaction">
	<h2>Major compaction<a class="headerlink" href="#major-compaction" title="Permalink to this headline">¶</a></h2>
	<p>It is possible to do a major compaction with LCS - it will currently start by filling out L1 and then once L1 is full,
	it continues with L2 etc. This is sub optimal and will change to create all the sstables in a high level instead,
	CASSANDRA-11817.</p>
	</div>
	<div class="section" id="bootstrapping">
	<h2>Bootstrapping<a class="headerlink" href="#bootstrapping" title="Permalink to this headline">¶</a></h2>
	<p>During bootstrap sstables are streamed from other nodes. The level of the remote sstable is kept to avoid many
	compactions after the bootstrap is done. During bootstrap the new node also takes writes while it is streaming the data
	from a remote node - these writes are flushed to L0 like all other writes and to avoid those sstables blocking the
	remote sstables from going to the correct level, we only do STCS in L0 until the bootstrap is done.</p>
	</div>
	<div class="section" id="stcs-in-l0">
	<h2>STCS in L0<a class="headerlink" href="#stcs-in-l0" title="Permalink to this headline">¶</a></h2>
	<p>If LCS gets very many L0 sstables reads are going to hit all (or most) of the L0 sstables since they are likely to be
	overlapping. To more quickly remedy this LCS does STCS compactions in L0 if there are more than 32 sstables there. This
	should improve read performance more quickly compared to letting LCS do its L0 -> L1 compactions. If you keep getting
	too many sstables in L0 it is likely that LCS is not the best fit for your workload and STCS could work out better.</p>
	</div>
	<div class="section" id="starved-sstables">
	<h2>Starved sstables<a class="headerlink" href="#starved-sstables" title="Permalink to this headline">¶</a></h2>
	<p>If a node ends up with a leveling where there are a few very high level sstables that are not getting compacted they
	might make it impossible for lower levels to drop tombstones etc. For example, if there are sstables in L6 but there is
	only enough data to actually get a L4 on the node the left over sstables in L6 will get starved and not compacted. This
	can happen if a user changes sstable_size_in_mb from 5MB to 160MB for example. To avoid this LCS tries to include
	those starved high level sstables in other compactions if there has been 25 compaction rounds where the highest level
	has not been involved.</p>
	</div>
	<div class="section" id="lcs-options">
	<span id="id1"></span><h2>LCS options<a class="headerlink" href="#lcs-options" title="Permalink to this headline">¶</a></h2>
	<dl class="simple">
	<dt><code class="docutils literal notranslate"><span class="pre">sstable_size_in_mb</span></code> (default: 160MB)</dt><dd><p>The target compressed (if using compression) sstable size - the sstables can end up being larger if there are very
	large partitions on the node.</p>
	</dd>
	<dt><code class="docutils literal notranslate"><span class="pre">fanout_size</span></code> (default: 10)</dt><dd><p>The target size of levels increases by this fanout_size multiplier. You can reduce the space amplification by tuning
	this option.</p>
	</dd>
	</dl>
	<p>LCS also support the <code class="docutils literal notranslate"><span class="pre">cassandra.disable_stcs_in_l0</span></code> startup option (<code class="docutils literal notranslate"><span class="pre">-Dcassandra.disable_stcs_in_l0=true</span></code>) to avoid
	doing STCS in L0.</p>
	</div>
	</div>


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