In an LSM-tree, what happens to new writes in terms of memtable and SSTable?

In an LSM-tree, what happens to new writes in terms of memtable and SSTable?

• A. Writes go into an in-memory memtable; when full, they are flushed to a disk-based SSTable.
• B. Data is written only to disk as a single monolithic table.
• C. SSTables are kept in memory and never merged.
• D. Memtables are always stored as SSTables immediately.

Answer: (A)

The main idea being tested is how writes are buffered and persisted in an LSM-tree. Writes are first placed in an in-memory structure called a memtable. This lets the system absorb new data quickly and avoids expensive random writes to disk. When the memtable becomes full, its contents are flushed to disk as a new SSTable, which is an immutable on-disk file containing sorted key–value pairs. After the flush, a fresh memtable starts collecting new writes. Over time, multiple SSTables exist on disk and are periodically merged through compaction to keep reads efficient and storage organized. This design, with a memtable in memory that is flushed to disk as SSTables, is what makes the write path in an LSM-tree effective. So, the correct description matches: writes go into an in-memory memtable; when full, they are flushed to a disk-based SSTable.

The main idea being tested is how writes are buffered and persisted in an LSM-tree. Writes are first placed in an in-memory structure called a memtable. This lets the system absorb new data quickly and avoids expensive random writes to disk. When the memtable becomes full, its contents are flushed to disk as a new SSTable, which is an immutable on-disk file containing sorted key–value pairs. After the flush, a fresh memtable starts collecting new writes. Over time, multiple SSTables exist on disk and are periodically merged through compaction to keep reads efficient and storage organized. This design, with a memtable in memory that is flushed to disk as SSTables, is what makes the write path in an LSM-tree effective.

So, the correct description matches: writes go into an in-memory memtable; when full, they are flushed to a disk-based SSTable.