BIO Lists

• The 'BIO' is a structure containing an I/O request (read or write) for a block device.
• During traversal of the device-mapper tree, a single I/O might have to be split (e.g. for a stripe, or on the boundary of a physical extent), cloned (e.g. for mirroring), etc.
• The device-mapper has an internal I/O request struct, which manages the mapping of the BIOs in this manner. It has one original BIO, and one or more internally-generated BIOs.
• The original BIO is only being completed when all the BIOs generated for it were completed.
• Note: to avoid copying the data fields, a BIO points to memory pages - several BIO-s may point to the same (or different) pages. Thus, we only need to copy the page lists, not the actually data of the pages.

Originally written by guy keren