Understanding SSTables and MemTables

What is an SSTable?

A Sorted String Table (SSTable) is a simple yet powerful file format that stores key-value pairs in sorted order. It's a fundamental building block in many modern databases like LevelDB, Cassandra, and HBase.

Key Characteristics:

Immutable: Once written, SSTables don't change
Sorted: Keys are maintained in sorted order
Sequential: Optimized for sequential read/write operations
Efficient: Great for large datasets and bulk operations

Why Write to Memory First?

Modern databases use a two-step process: first writing to memory (MemTable) before flushing to disk (SSTable). Here's why:

Speed: Memory operations are much faster than disk writes
Batching: Multiple writes can be batched together before hitting disk
Sorting: Data can be efficiently sorted in memory
Write Optimization: Reduces disk I/O and wear

In-Memory Segment (MemTable)

The MemTable is a memory-resident data structure that maintains sorted key-value pairs. When it reaches a certain size, it's flushed to disk as an SSTable.

SSTable on Disk

SSTables store data in sorted order on disk. They're immutable, which means once written, they don't change. This property makes them excellent for read operations and long-term storage.