This is the original, and oldest, storage engine included in MySQL. It does not support transactions. Its design is optimized more for speed and compact data storage, and is much simpler than that of InnoDB. The index structure is essentially a list of offsets within the data file. Inserts are just appends to the data file. However, deletes and updates are not so straightforward, since they can leave holes in, or fragment, the data file.

MyIASM does have some severe scalability limitations—the most important ones being the following:

• Key cache: Mutexes guard the key cache and serialize access to it.
• Table locking: Readers obtain read locks on all tables they need to read. Writers obtain exclusive (write) locks. Essentially, only one session is allowed to update a table at a time, forcing a huge serialization bottleneck for updates.

Thus, it's pretty ineffective for even moderate write loads.