什么是 SSTable？
在 BigTable/GFS 和 Cassandra 术语中，SSTable 的定义是什么？
In BigTable/GFS and Cassandra terminology, what is the definition of a SSTable?
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Sorted Strings Table（借自google）是一个键/值字符串对的文件，按键排序
Sorted Strings Table (borrowed from google) is a file of key/value string pairs, sorted by keys
“SSTable 提供了从键到值的持久、有序的不可变映射，其中键和值都是任意字节字符串。提供的操作用于查找与指定键关联的值，并迭代一个中的所有键/值对。在内部，每个 SSTable 包含一系列块（通常每个块的大小为 64KB，但这是可配置的），块索引（存储在 SSTable 的末尾）用于定位索引加载的块；当 SSTable 打开时，可以通过一次磁盘查找来执行查找：我们首先通过在内存索引中执行二分搜索来找到适当的块，然后从磁盘中读取适当的块。可以完全映射到内存中，这使我们能够在不接触磁盘的情况下执行查找和扫描。”
"An SSTable provides a persistent,ordered immutable map from keys to values, where both keys and values are arbitrary byte strings. Operations are provided to look up the value associated with a specified key, and to iterate over all key/value pairs in a specified key range. Internally, each SSTable contains a sequence of blocks (typically each block is 64KB in size, but this is configurable). A block index (stored at the end of the SSTable) is used to locate blocks; the index is loaded into memory when the SSTable is opened. A lookup can be performed with a single disk seek: we first find the appropriate block by performing a binary search in the in-memory index, and then reading the appropriate block from disk. Optionally, an SSTable can be completely mapped into memory, which allows us to perform lookups and scans without touching disk."
SSTable（英文：Sorted Strings Table）是一个键/值字符串的文件
对，按键排序。
SSTable 提供了一个持久的、有序的、不可变的映射，从键到
值，其中键和值都是任意字节字符串。
在内部，每个 SSTable 包含一系列块（通常
每个块的大小为 64KB，但这是可配置的）。
SSTable (engl. Sorted Strings Table) is a file of key/value string
pairs, sorted by keys.
An SSTable provides a persistent,ordered immutable map from keys to
values, where both keys and values are arbitrary byte strings.
Internally, each SSTable contains a sequence of blocks (typically
each block is 64KB in size, but this is configurable).
一个tablet以SSTables的形式存储。
SSTable（直接映射到GFS）是基于键值的不可变存储。它存储数据块，每个数据块大小为 64KB。
定义：
A tablet is stored in the form of SSTables.
SSTable (directly mapped to GFS) is key-value based immutable storage. It stores chunks of data, each is of 64KB.
Definitions:
SSTable 的意思是基于键值对的“排序字符串表”。在 Cassandra 中，SSTable 是不可变的并按键排序。
SSTable means "sorted string table" based on key-value pair.In Cassandra, SSTables are immutable and sorted by keys.