GROUP BY queries from MyISAM tables consistently fails with 'TABLE FULL' for some lengths of fields being grouped but succeeds for longer lengths. Does not appear to be hitting operating system limits.
For example, with a source MyISAM table 'test.input' of 10m records defined as field a=int(11) auto-increment primary key (values 1-10,000,000) and field b defined as varchar(128) not null and randomly filled, then the following fail but any other length (1-127) runs OK:
Replacing the length with @len causes the crash to occur with values of @len that are 1 lower than the lengths above: i.e. @len=61, 62, 79 and 80.
While the query is running, a pair of files (.MAI, .MAD) are being created in /tmp; the query crashes at the point that the .MAI file is about the exceed 512Mb. For other lengths the .MAI file exceeds 512Mb without crashing. /tmp has over 404Gb free space available.
None of the conditions documented at http://dev.mysql.com/doc/refman/5.5/en/table-size-limit.html apply in this case - plenty of disk space, query runs OK with longer values being grouped, not using InnoDB, file size is suitable for pointer size, etc.
The solution listed elsewhere for specifying MAX_ROWS is for when explicitly creating an Aria table; MariaDB is implicitly creating the table so cannot specify MAX_ROWS. In any cases, 10m rows is not excessive.
Table input created and populated as: