- With the help of MDEV-14795, InnoDB implemented a way to shrink
the InnoDB system tablespace after undo tablespaces have been moved
to separate files (MDEV-29986). There is no way to defragment any
pages of InnoDB system tables. By doing that, shrinking of
system tablespace can be more effective. This patch deals with
defragment of system tables inside ibdata1.
Following steps are done to do the defragmentation of system
tablespace:
1) Make sure that there is no user tables exist in ibdata1
2) Iterate through all extent descriptor pages in system tablespace
and note their states.
3) Find the free earlier extent to replace the lastly used
extents in the system tablespace.
4) Iterate through all indexes of system tablespace and defragment
the tree level by level.
5) Iterate the level from left page to right page and find out
the page comes under the extent to be replaced. If it is then
do step (6) else step(4)
6) Prepare the allocation of new extent by latching necessary
pages. If any error happens then there is no modification of
page happened till step (5).
7) Allocate the new page from the new extent
8) Prepare the associated pages for the block to be modified
9) Prepare the step of freeing of page
10) If any error happens during preparing of associated pages,
freeing of page then restore the page which was modified
during new page allocation
11) Copy the old page content to new page
12) Change the associative pages like left, right and parent page
13) Complete the freeing of old page
Allocation of page from new extent, changing of relative pages,
freeing of page are done by 2 steps. one is prepare which
latches the to be modified pages and checks their validation.
Other is complete(), Do the operation
fseg_validate(): Validate the list exist in inode segment
Defragmentation is enabled only when :autoextend exist in
innodb_data_file_path variable.
