questions on undo log

Hi, Guys

When reading innodb code, I'm little confused about the relationship
between innodb buffer pool and undo log:

From my understanding, in disk tablespace files, undo log can be
divided into two parts: rollback segments (1023 slots per segment) and
the undo space pointered by slot.

So my question is which part is cached in innodb buffer pool?  the
slot? or also the undo space?

BTW. If anyone knows any available doc to describe the internal of
innodb undo log mechanism, that should be very helpful to me. :-)

Many thanks.  - Xuekun

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