CHAR and VARCHAR lengths for multibyte characters

Question

I have a table with ~100mln rows. One of the columns is a VARCHAR(64) with utf8mb4 encoding for storing user nicknames (others are few integers).

The data in that column has MAX(LENGTH()) == 42 and MAX(CHAR_LENGTH()) == 24 (I can attach an image of distribution of lengths if needed). ~90% of rows only use ASCII characters.

From the MySQL docs,

CHAR(30) can hold up to 30 characters. [...] a CHAR(255) column can exceed 768 bytes

So does it mean it can store N characters, even multibyte? How about a varchar, does the specified size mean bytes or characters?

My table is mostly write-intensive (INSERT O.D.K. UPDATE). Is it a good idea to switch to constant-length field (especially with such not-very-long strings) versus having them as a varying-length field, probably stored somewhere else on the disk?

Byte length distribution:

Char_length distribution:

Answer 1

So does it mean it can store N characters, even multibyte? How about a varchar, does the specified size mean bytes or characters?

Yes, the length is measured in characters, not bytes. See also the docs for BINARY/VARBINARY data types

Is it a good idea to switch to constant-length field (especially with such not-very-long strings) versus having them as a varying-length field, probably stored somewhere else on the disk?

No. Constant length fields will always occupy the maximum possible space, while varchars will usually occupy much less space. Less space means more rows per disk page, thus less page reads/writes and less memory needed.

Also see this question for CHAR/VARCHAR comparison