| (window.webpackJsonp=window.webpackJsonp||[]).push([[471],{866:function(s,n,a){"use strict";a.r(n);var e=a(29),t=Object(e.a)({},(function(){var s=this,n=s._self._c;return n("ContentSlotsDistributor",{attrs:{"slot-key":s.$parent.slotKey}},[n("h1",{attrs:{id:"压缩方式"}},[n("a",{staticClass:"header-anchor",attrs:{href:"#压缩方式"}},[s._v("#")]),s._v(" 压缩方式")]),s._v(" "),n("p",[s._v("当时间序列写入并按照指定的类型编码为二进制数据后，IoTDB 会使用压缩技术对该数据进行压缩，进一步提升空间存储效率。虽然编码和压缩都旨在提升存储效率，但编码技术通常只适合特定的数据类型（如二阶差分编码只适合与 INT32 或者 INT64 编码，存储浮点数需要先将他们乘以 10m 以转换为整数），然后将它们转换为二进制流。压缩方式（SNAPPY）针对二进制流进行压缩，因此压缩方式的使用不再受数据类型的限制。")]),s._v(" "),n("h2",{attrs:{id:"基本压缩方式"}},[n("a",{staticClass:"header-anchor",attrs:{href:"#基本压缩方式"}},[s._v("#")]),s._v(" 基本压缩方式")]),s._v(" "),n("p",[s._v("IoTDB 允许在创建一个时间序列的时候指定该列的压缩方式。现阶段 IoTDB 支持以下几种压缩方式：")]),s._v(" "),n("ul",[n("li",[s._v("UNCOMPRESSED（不压缩）")]),s._v(" "),n("li",[s._v("SNAPPY 压缩")]),s._v(" "),n("li",[s._v("LZ4 压缩")]),s._v(" "),n("li",[s._v("GZIP 压缩")])]),s._v(" "),n("p",[s._v("压缩方式的指定语法详见本文 "),n("RouterLink",{attrs:{to:"/zh/UserGuide/Master/Reference/SQL-Reference.html"}},[s._v("SQL 参考文档")]),s._v("。")],1),s._v(" "),n("h2",{attrs:{id:"旋转门压缩"}},[n("a",{staticClass:"header-anchor",attrs:{href:"#旋转门压缩"}},[s._v("#")]),s._v(" 旋转门压缩")]),s._v(" "),n("p",[s._v("旋转门压缩（SDT）算法是一种有损压缩算法。SDT 的计算复杂度较低，并使用线性趋势来表示大量数据。")]),s._v(" "),n("p",[s._v("在 IoTDB 中，SDT 在刷新到磁盘时会压缩并丢弃数据。")]),s._v(" "),n("p",[s._v("IoTDB 允许您在创建时间序列时指定 SDT 的属性，并支持以下三个属性：")]),s._v(" "),n("ul",[n("li",[s._v("CompDev （Compression Deviation，压缩偏差）")])]),s._v(" "),n("p",[s._v("CompDev 是 SDT 中最重要的参数，它表示当前样本与当前线性趋势之间的最大差值。CompDev 设置的值需要大于 0。")]),s._v(" "),n("ul",[n("li",[s._v("CompMinTime （Compression Minimum Time Interval，最小压缩时间间隔）")])]),s._v(" "),n("p",[s._v("CompMinTime 是测量两个存储的数据点之间的时间距离的参数，用于减少噪声。\n如果当前点和最后存储的点之间的时间间隔小于或等于其值，则无论压缩偏差如何，都不会存储当前点。\n默认值为 0，单位为毫秒。")]),s._v(" "),n("ul",[n("li",[s._v("CompMaxTime （Compression Maximum Time Interval，最大压缩时间间隔）")])]),s._v(" "),n("p",[s._v("CompMaxTime 是测量两个存储的数据点之间的时间距离的参数。\n如果当前点和最后一个存储点之间的时间间隔大于或等于其值，\n无论压缩偏差如何，都将存储当前点。\n默认值为 9,223,372,036,854,775,807，单位为毫秒。")]),s._v(" "),n("p",[s._v("支持的数据类型：")]),s._v(" "),n("ul",[n("li",[s._v("INT32（整型）")]),s._v(" "),n("li",[s._v("INT64（长整型）")]),s._v(" "),n("li",[s._v("FLOAT（单精度浮点数）")]),s._v(" "),n("li",[s._v("DOUBLE（双精度浮点数）")])]),s._v(" "),n("p",[s._v("SDT 的指定语法详见本文 "),n("RouterLink",{attrs:{to:"/zh/UserGuide/Master/Reference/SQL-Reference.html"}},[s._v("SQL 参考文档")]),s._v("。")],1),s._v(" "),n("p",[s._v("以下是使用 SDT 压缩的示例。")]),s._v(" "),n("div",{staticClass:"language- line-numbers-mode"},[n("pre",{pre:!0,attrs:{class:"language-text"}},[n("code",[s._v("IoTDB> CREATE TIMESERIES root.sg1.d0.s0 WITH DATATYPE=INT32,ENCODING=PLAIN,LOSS=SDT,COMPDEV=2\n")])]),s._v(" "),n("div",{staticClass:"line-numbers-wrapper"},[n("span",{staticClass:"line-number"},[s._v("1")]),n("br")])]),n("p",[s._v("刷入磁盘和 SDT 压缩之前，结果如下所示：")]),s._v(" "),n("div",{staticClass:"language- line-numbers-mode"},[n("pre",{pre:!0,attrs:{class:"language-text"}},[n("code",[s._v("IoTDB> SELECT s0 FROM root.sg1.d0\n+-----------------------------+--------------+\n| Time|root.sg1.d0.s0|\n+-----------------------------+--------------+\n|2017-11-01T00:06:00.001+08:00| 1|\n|2017-11-01T00:06:00.002+08:00| 1|\n|2017-11-01T00:06:00.003+08:00| 1|\n|2017-11-01T00:06:00.004+08:00| 1|\n|2017-11-01T00:06:00.005+08:00| 1|\n|2017-11-01T00:06:00.006+08:00| 1|\n|2017-11-01T00:06:00.007+08:00| 1|\n|2017-11-01T00:06:00.015+08:00| 10|\n|2017-11-01T00:06:00.016+08:00| 20|\n|2017-11-01T00:06:00.017+08:00| 1|\n|2017-11-01T00:06:00.018+08:00| 30|\n+-----------------------------+--------------+\nTotal line number = 11\nIt costs 0.008s\n")])]),s._v(" "),n("div",{staticClass:"line-numbers-wrapper"},[n("span",{staticClass:"line-number"},[s._v("1")]),n("br"),n("span",{staticClass:"line-number"},[s._v("2")]),n("br"),n("span",{staticClass:"line-number"},[s._v("3")]),n("br"),n("span",{staticClass:"line-number"},[s._v("4")]),n("br"),n("span",{staticClass:"line-number"},[s._v("5")]),n("br"),n("span",{staticClass:"line-number"},[s._v("6")]),n("br"),n("span",{staticClass:"line-number"},[s._v("7")]),n("br"),n("span",{staticClass:"line-number"},[s._v("8")]),n("br"),n("span",{staticClass:"line-number"},[s._v("9")]),n("br"),n("span",{staticClass:"line-number"},[s._v("10")]),n("br"),n("span",{staticClass:"line-number"},[s._v("11")]),n("br"),n("span",{staticClass:"line-number"},[s._v("12")]),n("br"),n("span",{staticClass:"line-number"},[s._v("13")]),n("br"),n("span",{staticClass:"line-number"},[s._v("14")]),n("br"),n("span",{staticClass:"line-number"},[s._v("15")]),n("br"),n("span",{staticClass:"line-number"},[s._v("16")]),n("br"),n("span",{staticClass:"line-number"},[s._v("17")]),n("br"),n("span",{staticClass:"line-number"},[s._v("18")]),n("br")])]),n("p",[s._v("刷入磁盘和 SDT 压缩之后，结果如下所示：")]),s._v(" "),n("div",{staticClass:"language- line-numbers-mode"},[n("pre",{pre:!0,attrs:{class:"language-text"}},[n("code",[s._v("IoTDB> FLUSH\nIoTDB> SELECT s0 FROM root.sg1.d0\n+-----------------------------+--------------+\n| Time|root.sg1.d0.s0|\n+-----------------------------+--------------+\n|2017-11-01T00:06:00.001+08:00| 1|\n|2017-11-01T00:06:00.007+08:00| 1|\n|2017-11-01T00:06:00.015+08:00| 10|\n|2017-11-01T00:06:00.016+08:00| 20|\n|2017-11-01T00:06:00.017+08:00| 1|\n+-----------------------------+--------------+\nTotal line number = 5\nIt costs 0.044s\n")])]),s._v(" "),n("div",{staticClass:"line-numbers-wrapper"},[n("span",{staticClass:"line-number"},[s._v("1")]),n("br"),n("span",{staticClass:"line-number"},[s._v("2")]),n("br"),n("span",{staticClass:"line-number"},[s._v("3")]),n("br"),n("span",{staticClass:"line-number"},[s._v("4")]),n("br"),n("span",{staticClass:"line-number"},[s._v("5")]),n("br"),n("span",{staticClass:"line-number"},[s._v("6")]),n("br"),n("span",{staticClass:"line-number"},[s._v("7")]),n("br"),n("span",{staticClass:"line-number"},[s._v("8")]),n("br"),n("span",{staticClass:"line-number"},[s._v("9")]),n("br"),n("span",{staticClass:"line-number"},[s._v("10")]),n("br"),n("span",{staticClass:"line-number"},[s._v("11")]),n("br"),n("span",{staticClass:"line-number"},[s._v("12")]),n("br"),n("span",{staticClass:"line-number"},[s._v("13")]),n("br")])]),n("p",[s._v("SDT 在刷新到磁盘时进行压缩。 SDT 算法始终存储第一个点，并且不存储最后一个点。")]),s._v(" "),n("p",[s._v("时间范围在 [2017-11-01T00:06:00.001, 2017-11-01T00:06:00.007] 的数据在压缩偏差内，因此被压缩和丢弃。\n之所以存储时间为 2017-11-01T00:06:00.007 的数据点，是因为下一个数据点 2017-11-01T00:06:00.015 的值超过压缩偏差。\n当一个数据点超过压缩偏差时，SDT 将存储上一个读取的数据点，并重新计算上下压缩边界。作为最后一个数据点，不存储时间 2017-11-01T00:06:00.018。")]),s._v(" "),n("h2",{attrs:{id:"压缩比统计信息"}},[n("a",{staticClass:"header-anchor",attrs:{href:"#压缩比统计信息"}},[s._v("#")]),s._v(" 压缩比统计信息")]),s._v(" "),n("p",[s._v("压缩比统计信息文件：data/system/storage_groups/compression_ratio/Ratio-{ratio_sum}-{memtable_flush_time}")]),s._v(" "),n("ul",[n("li",[s._v("ratio_sum: memtable压缩比的总和")]),s._v(" "),n("li",[s._v("memtable_flush_time: memtable刷盘的总次数")])]),s._v(" "),n("p",[s._v("通过 "),n("code",[s._v("ratio_sum / memtable_flush_time")]),s._v(" 可以计算出平均压缩比")])])}),[],!1,null,null,null);n.default=t.exports}}]); |