如果有看过之前我分析的ziplist压缩列表的分析的话，理解这个我觉得不是什么特别的难题。ziplist压缩列表和zipmap都采用了动态分配字节的做法表示长度，比如通过固定的字节表示节省了不少的空间。同样带来的问题就是复杂的指针移动，和字符位置移动。但总的来说，一定是利大于弊了，要不然设计者也不会这么做。ziplist保存的使用一个列表，zipmap就保存的则是一个个键值对，通过key:value key:value的形式连着。下面我给出zipmap的结构构成，zipmap其实也就是一个超级长的字符串。
里面涉及了几个变量zmlen，len,free，下面给出完整的解释："foo" "bar" "hello" "world"
/* String -> String Map data structure optimized for size. * This file implements a data structure mapping strings to other strings * implementing an O(n) lookup data structure designed to be very memory * efficient. * * The Redis Hash type uses this data structure for hashes composed of a small * number of elements, to switch to a hash table once a given number of * elements is reached. * * Given that many times Redis Hashes are used to represent objects composed * of few fields, this is a very big win in terms of used memory. * * zipmap压缩表和ziplist十分类似，都做到了内存操作效率比较高的 * -------------------------------------------------------------------------- * * Copyright (c) 2009-2010, Salvatore Sanfilippo* All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are met: * * * Redistributions of source code must retain the above copyright notice, * this list of conditions and the following disclaimer. * * Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * Neither the name of Redis nor the names of its contributors may be used * to endorse or promote products derived from this software without * specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE * POSSIBILITY OF SUCH DAMAGE. */ /* Memory layout of a zipmap, for the map "foo" => "bar", "hello" => "world": * *"foo" "bar" "hello" "world" * * is 1 byte length that holds the current size of the zipmap. * When the zipmap length is greater than or equal to 254, this value * is not used and the zipmap needs to be traversed to find out the length. * 占有着1个字节，所以他的最多可代表的数量是254,当zipmap中的元素记录超过这个数时， * 那只能从前往后后遍历算大小了，和ziplist是不一样的。 * * is the length of the following string (key or value). * lengths are encoded in a single value or in a 5 bytes value. * If the first byte value (as an unsigned 8 bit value) is between 0 and * 252, it's a single-byte length. If it is 253 then a four bytes unsigned * integer follows (in the host byte ordering). A value of 255 is used to * signal the end of the hash. The special value 254 is used to mark * empty space that can be used to add new key/value pairs. * 代表了后面字符串key 或 value的值的长度，长度一般被编码1个字节或5个