MySQL Reference Manual :: 14.1.3.2 Dynamic Table Characteristics

All string columns are dynamic except those with a length less than four.

Each record is preceded by a bitmap that indicates which columns contain the empty string (for string columns) or zero (for numeric columns). Note that this does not include columns that contain NULL values. If a string column has a length of zero after trailing space removal, or a numeric column has a value of zero, it is marked in the bitmap and not saved to disk. Non-empty strings are saved as a length byte plus the string contents.

Much less disk space usually is required than for fixed-length tables.

Each record uses only as much space as is required. However, if a record becomes larger, it is split into as many pieces as are required, resulting in record fragmentation. For example, if you update a row with information that extends the row length, the row becomes fragmented. In this case, you may have to run OPTIMIZE TABLE or myisamchk -r from time to time to get better performance. Use myisamchk -ei to obtain table statistics.

More difficult than static-format tables to reconstruct after a crash, because a record may be fragmented into many pieces and a link (fragment) may be missing.

The expected row length for dynamic-sized records is calculated using the following expression:

3
+ (number of columns + 7) / 8
+ (number of char columns)
+ (packed size of numeric columns)
+ (length of strings)
+ (number of NULL columns + 7) / 8

There is a penalty of 6 bytes for each link. A dynamic record is linked whenever an update causes an enlargement of the record. Each new link is at least 20 bytes, so the next enlargement probably goes in the same link. If not, there is another link. You may check how many links there are with myisamchk -ed. All links may be removed with myisamchk -r.