小型哈希对象根据 hash-max-ziplist-entries 和 hash-max-ziplist-value 参数的值进行压缩列表编码。这是一种简单的数据序列化方式。
压缩列表定义如下（摘自 Redis 源代码）：
/* The ziplist is a specially encoded dually linked list that is designed
* to be very memory efficient. It stores both strings and integer values,
* where integers are encoded as actual integers instead of a series of
* characters. It allows push and pop operations on either side of the list
* in O(1) time. However, because every operation requires a reallocation of
* the memory used by the ziplist, the actual complexity is related to the
* amount of memory used by the ziplist.
*
* ----------------------------------------------------------------------------
*
* ZIPLIST OVERALL LAYOUT:
* The general layout of the ziplist is as follows:
* <zlbytes><zltail><zllen><entry><entry><zlend>
*
* <zlbytes> is an unsigned integer to hold the number of bytes that the
* ziplist occupies. This value needs to be stored to be able to resize the
* entire structure without the need to traverse it first.
*
* <zltail> is the offset to the last entry in the list. This allows a pop
* operation on the far side of the list without the need for full traversal.
*
* <zllen> is the number of entries.When this value is larger than 2**16-2,
* we need to traverse the entire list to know how many items it holds.
*
* <zlend> is a single byte special value, equal to 255, which indicates the
* end of the list.
*
* ZIPLIST ENTRIES:
* Every entry in the ziplist is prefixed by a header that contains two pieces
* of information. First, the length of the previous entry is stored to be
* able to traverse the list from back to front. Second, the encoding with an
* optional string length of the entry itself is stored.
*
* The length of the previous entry is encoded in the following way:
* If this length is smaller than 254 bytes, it will only consume a single
* byte that takes the length as value. When the length is greater than or
* equal to 254, it will consume 5 bytes. The first byte is set to 254 to
* indicate a larger value is following. The remaining 4 bytes take the
* length of the previous entry as value.
*
* The other header field of the entry itself depends on the contents of the
* entry. When the entry is a string, the first 2 bits of this header will hold
* the type of encoding used to store the length of the string, followed by the
* actual length of the string. When the entry is an integer the first 2 bits
* are both set to 1. The following 2 bits are used to specify what kind of
* integer will be stored after this header. An overview of the different
* types and encodings is as follows:
*
* |00pppppp| - 1 byte
* String value with length less than or equal to 63 bytes (6 bits).
* |01pppppp|qqqqqqqq| - 2 bytes
* String value with length less than or equal to 16383 bytes (14 bits).
* |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
* String value with length greater than or equal to 16384 bytes.
* |11000000| - 1 byte
* Integer encoded as int16_t (2 bytes).
* |11010000| - 1 byte
* Integer encoded as int32_t (4 bytes).
* |11100000| - 1 byte
* Integer encoded as int64_t (8 bytes).
* |11110000| - 1 byte
* Integer encoded as 24 bit signed (3 bytes).
* |11111110| - 1 byte
* Integer encoded as 8 bit signed (1 byte).
* |1111xxxx| - (with xxxx between 0000 and 1101) immediate 4 bit integer.
* Unsigned integer from 0 to 12. The encoded value is actually from
* 1 to 13 because 0000 and 1111 can not be used, so 1 should be
* subtracted from the encoded 4 bit value to obtain the right value.
* |11111111| - End of ziplist.
*
* All the integers are represented in little endian byte order.
*/
哈希对象中的每个项目都表示为ziplist中的键值对(2个连续条目)。键和值都可以存储为简单字符串或整数。这种格式在内存中更加紧凑，因为它节省了很多指针(每个8字节)，这些指针需要实现动态数据结构(如真正的哈希表)。
缺点是当在ziplist上应用HSET/HGET操作时，实际上是O(N)的。这就是为什么必须保持ziplist尽可能小的原因。当ziplist数据适合L1 CPU缓存时，相应的算法足够快，尽管它们具有线性复杂度。
您可能需要参考以下链接以获取更多信息:
Redis 10x more memory usage than data
Redis Data Structure Space Requirements
这些答案涉及其他数据结构(如集合、列表或排序集合)，但概念完全相同。
