Doublewrite Buffer

Storage area where InnoDB write pages flushed from buffer pool before writing the pages to their proper positions in the InnoDB data files.
Implemented to recover from half written pages.
In case of OS error , storage issue , unexpected mysqld process exit in the middle of a page write , InnoDB can find a good copy of page from doublewrite buffer during crash recovery.
Data is written twice , doublewrite buffer does not require twice as much IO overhead.
Prior to MYSQL 8.0.20 , doublewrite buffer was part of InnoDB system tablespace ibdata1.
As of MYSQL 8.0.20 , the doublewrite buffer storage area is located in doublewrite files.
innodb_doublewrite - system variable - enable by default.
innodb_doublewrite_dir - Defines where InnoDB create doublewrite files.
id_16384_0.dblwr &id_16384_1.dblwr - 16384 - 16kb (innodb_page_size).
Doublewrite files should be placed on a fastest media available.

When to disable ?