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  109.     <h1 class="post-title" itemprop="name headline">lvcache: a tool for managing LVM caches</h1>
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  111.     <p class="post-date post-line">
    112. 

  112.         <span>Published <time datetime="2014-08-16" itemprop="datePublished">Sat, Aug 16, 2014</time></span>
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  113.         <span>by</span>
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    116. 

  116.                 <a href="/about/" itemprop="url" rel="author">Lars Kellogg-Stedman</a>
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    124. 

  124.     
    125. 

  125. 

  126.     
    127. 

  127. 

  128.     <p>Until recently I had a <a href="http://bcache.evilpiepirate.org/">bcache</a> based setup on my laptop, but when
    129. 

  129. forced by circumstance to reinstall everything I spent some time
    130. 

  130. looking for alternatives that were less disruptive to configure on an
    131. 

  131. existing system.</p>
    132. 

  132. <p>I came across <a href="http://rwmj.wordpress.com/2014/05/22/using-lvms-new-cache-feature/">Richard Jones' article</a> discussing the recent work to
    133. 

  133. integrate <a href="https://en.wikipedia.org/wiki/Dm-cache">dm-cache</a> into <a href="http://en.wikipedia.org/wiki/Logical_Volume_Manager_(Linux)">LVM</a>.  Unlike <em>bcache</em> and unlike using
    134. 

  134. <em>dm-cache</em> directly, the integration with LVM makes it easy to
    135. 

  135. associate devices with an existing logical volume.</p>
    136. 

  136. <p>I have put together a small tool called <a href="https://github.com/larsks/lvcache">lvcache</a> that simplies the
    137. 

  137. process of:</p>
    138. 

  138. <ul>
    139. 

  139. <li>Creating and attaching cache volumes</li>
    140. 

  140. <li>Detaching and removing cache volumes</li>
    141. 

  141. <li>Getting cache statistics for logical volumes</li>
    142. 

  142. <li>Listing the cache status of all logical volumes</li>
    143. 

  143. </ul>
    144. 

  144. <p>With <code>lvcache</code> installed, you can run (as root) the following command
    145. 

  145. to create a new cache volume that is 20% the size of your origin
    146. 

  146. volume and attach it to the specified origin volume:</p>
    147. 

  147. <pre><code># lvcache create myvg/home
    148. 

  148. </code></pre>
    149. 

  149. <p>You can control the size of the cache LV relative to the origin
    150. 

  150. volume.  To create a cache LV that is 40% the size of the origin
    151. 

  151. volume:</p>
    152. 

  152. <pre><code># lvcache create -% 40 myvg/home
    153. 

  153. </code></pre>
    154. 

  154. <p>You can query <code>dm-setup</code> for cache statistics with the <code>status</code>
    155. 

  155. command (the <code>-H</code> translates raw bytes counts into human readable
    156. 

  156. numbers with SI suffixes):</p>
    157. 

  157. <pre><code># lvcache status -H myvg/home
    158. 

  158. +-----------------------+------------------+
    159. 

  159. | Field                 | Value            |
    160. 

  160. +-----------------------+------------------+
    161. 

  161. | cached                | True             |
    162. 

  162. | size                  | 32G              |
    163. 

  163. | cache_lv              | home_cache       |
    164. 

  164. | cache_lv_size         | 6G               |
    165. 

  165. | metadata_lv           | home_cache_cmeta |
    166. 

  166. | metadata_lv_size      | 8M               |
    167. 

  167. | cache_block_size      | 128              |
    168. 

  168. | cache_utilization     | 0/98304          |
    169. 

  169. | cache_utilization_pct | 0.0              |
    170. 

  170. | demotions             | 0                |
    171. 

  171. | dirty                 | 0                |
    172. 

  172. | end                   | 62914560         |
    173. 

  173. | features              | 1                |
    174. 

  174. | md_block_size         | 8                |
    175. 

  175. | md_utilization        | 200/2048         |
    176. 

  176. | md_utilization_pct    | 9.765625         |
    177. 

  177. | promotions            | 0                |
    178. 

  178. | read_hits             | 0                |
    179. 

  179. | read_misses           | 0                |
    180. 

  180. | segment_type          | cache            |
    181. 

  181. | start                 | 0                |
    182. 

  182. | write_hits            | 0                |
    183. 

  183. | write_misses          | 0                |
    184. 

  184. +-----------------------+------------------+
    185. 

  185. </code></pre>
    186. 

  186. <p>Because <code>lvcache</code> is using the <a href="http://cliff.readthedocs.org/en/latest/">cliff</a> framework, it is very easy to
    187. 

  187. extract individual values from this list for graphing or monitoring
    188. 

  188. purposes:</p>
    189. 

  189. <pre><code># lvcache status tank.home -f value -c md_utilization_pct
    190. 

  190. 9.765625
    191. 

  191. </code></pre>
    192. 

  192. <p>Or:</p>
    193. 

  193. <pre><code># lvcache status tank.home -f shell
    194. 

  194. cached=&quot;True&quot;
    195. 

  195. size=&quot;32G&quot;
    196. 

  196. cache_lv=&quot;nova_cache&quot;
    197. 

  197. cache_lv_size=&quot;6G&quot;
    198. 

  198. metadata_lv=&quot;nova_cache_cmeta&quot;
    199. 

  199. metadata_lv_size=&quot;8M&quot;
    200. 

  200. cache_block_size=&quot;128&quot;
    201. 

  201. cache_utilization=&quot;0/98304&quot;
    202. 

  202. cache_utilization_pct=&quot;0.0&quot;
    203. 

  203. demotions=&quot;0&quot;
    204. 

  204. dirty=&quot;0&quot;
    205. 

  205. end=&quot;62914560&quot;
    206. 

  206. features=&quot;1&quot;
    207. 

  207. md_block_size=&quot;8&quot;
    208. 

  208. md_utilization=&quot;200/2048&quot;
    209. 

  209. md_utilization_pct=&quot;9.765625&quot;
    210. 

  210. promotions=&quot;0&quot;
    211. 

  211. read_hits=&quot;0&quot;
    212. 

  212. read_misses=&quot;0&quot;
    213. 

  213. segment_type=&quot;cache&quot;
    214. 

  214. start=&quot;0&quot;
    215. 

  215. write_hits=&quot;0&quot;
    216. 

  216. write_misses=&quot;0&quot;
    217. 

  217. </code></pre>
    218. 

  218. <p>This is a very rough tool right now, but it seems to get the job done
    219. 

  219. on my system.  If you do find this useful, let me know!</p>
    220. 

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