When having a MyISAM table with many partitions the backup process seems to want to open all the related MYD and MYI partition files and keep them open simultaneously until the full table has been copied.

Unlike with InnoDB and Aria tables, where a proper "Too many open files" message (or similar) is logged, with MyISAM things fail with just:

Error on file ./test/t1#P#p513.MYD open during `test`.`t1` table copy

How to reproduce: create a partitioned MyISAM table as:

CREATE TABLE `t1` (

  `id` bigint(20) NOT NULL AUTO_INCREMENT,

  PRIMARY KEY (`id`)

) ENGINE=MyISAM

 PARTITION BY HASH (`id`)

 PARTITIONS 600;

then try to run mariabackup with an open files limit too small for this:

mariabackup --backup --user=root --open-files-limit=500