In an SSD, a write operation can only be done when the page is already erased. However, the unit of read/write operations are a page, while the unit of erase operation is a block. That means for a disk write, a naive implementation needs to read the whole block, erase the block, then write updated data back to the block, which is unacceptable. Furthermore, blocks should wear out uniformly, otherwise, the SSD would lose capacity.
To tackle these problems, SSD introduces Flash Translation Layer (FTL) which helps to build an illusion of random access device. To achieve this, FTL employs an approach very similar to LSM trees. Writes are always written to new, already erased pages, while in the background, garbage collects (GC) outdated data. FTL needs to keep a map from the user’s logical address to physical address on SSD, both in-memory and persistently.
So to answer your question, why are sequential writes are faster than random writes on SSDs? Because the address map table is smaller since new data is consecutive in larger chunks. Garbage Collection is simpler and only metadata needs to be updated. Erasing a block is required anyway.
In an SSD, a write operation can only be done when the page is already erased. However, the unit of read/write operations are a page, while the unit of erase operation is a block. That means for a disk write, a naive implementation needs to read the whole block, erase the block, then write updated data back to the block, which is unacceptable. Furthermore, blocks should wear out uniformly, otherwise, the SSD would lose capacity.
To tackle these problems, SSD introduces Flash Translation Layer (FTL) which helps to build an illusion of random access device. To achieve this, FTL employs an approach very similar to LSM trees. Writes are always written to new, already erased pages, while in the background, garbage collects (GC) outdated data. FTL needs to keep a map from the user’s logical address to physical address on SSD, both in-memory and persistently.
So to answer your question, why are sequential writes are faster than random writes on SSDs? Because the address map table is smaller since new data is consecutive in larger chunks. Garbage Collection is simpler and only metadata needs to be updated. Erasing a block is required anyway.