- 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.
- Introduce the option :autoshrink attribute to be
added to innodb_data_file_path variable to allow
the shrinking of system tablespace during startup process.
Steps for shrinking the system tablespace:
1) Find the last used extent in system tablespace
by iterating through the BITMAP in extent descriptor pages
2) If the last used extent is lesser than user specified size
then set desired target size to user specified size.
3) Store the page contents of "to be modified" extent
descriptor pages, latches the "to be modified"
extent descriptor pages and check for buffer pool
memory availability
4) Make checkpoint to flush all pages in buffer pool, so
that pages in flush list doesn't have to use doublewrite
buffer and disable doublewrite buffer during shrinking process
5) Update the FSP_SIZE and FSP_FREE_LIMIT in header page
6) Remove the "to be truncated" pages from FSP_FREE and
FSP_FREE_FRAG list
7) Reset the bitmap in the last descriptor pages for the
"to be truncated" pages.
8) In case of multiple files, calculate the truncated last
file size and do the truncation in last file
9) Check whether mini-transaction log size doesn't exceed
the minimum value of innodb_log_buffer_size which is 2MB.
In that case, replace the modified buffer pool pages with
the page old content.
11) Commit the mini-transaction for shrinking the tablespace
and enable/disable the doublewrite buffer depends on user
specified value.
recv_sys_t::apply(): Handle the truncation of system tablespace
only if the recovered tablespace size is lesser than actual
existing size.
