[This post refers to innodb_ruby version 0.8.8 as of February 3, 2014.]

In On learning InnoDB: A journey to the core I introduced a new library and command-line tool in the innodb_ruby project. Later on in A quick introduction to innodb_ruby I walked through installation and a few quick demos of the innodb_space command-line tool.

In my last post, Page management in InnoDB space files, I described InnoDB’s extent, file segment, and free space management structures. Now I will provide a few demonstrations of using innodb_space to examine those structures in real tables.

A minimal, empty table

I created an empty table (the schema doesn’t matter) to illustrate the “minimal” state of InnoDB’s page management structures. The space-page-type-regions mode will summarize the type of all contiguous regions of the same page type:

$ innodb_space -f test/e.ibd space-page-type-regions start end count type 0 0 1 FSP_HDR 1 1 1 IBUF_BITMAP 2 2 1 INODE 3 3 1 INDEX 4 5 2 FREE (ALLOCATED)

The table has allocated the standard pages for IBD space files: FSP_HDR , IBUF_BITMAP , INODE , and an INDEX page for the root of the (empty) index. There are also two FREE (ALLOCATED) pages which are unused.

The space-lists mode can be used to summarize the extent descriptor and inode lists in the space:

$ innodb_space -f test/e.ibd space-lists name length f_page f_offset l_page l_offset free 0 0 0 0 0 free_frag 1 0 158 0 158 full_frag 0 0 0 0 0 full_inodes 0 0 0 0 0 free_inodes 1 2 38 2 38

Only the free_frag extent descriptor list has any entries, and only a single extent is in it. The free_inodes list has the one INODE page seen above in it.

The contents of the free_frag list can be examined with the space-list-iterate mode, which will print a graphic illustrating the usage of pages within all extents in an extent list (“ # ” means the page is used, “ . ” means the page is free):

$ innodb_space -f test/e.ibd -L free_frag space-list-iterate start_page page_used_bitmap 0 ####............................................................

The file segments in all indexes in the space can be summarized with the space-indexes mode:

$ innodb_space -f test/e.ibd space-indexes id root fseg used allocated fill_factor 16 3 internal 1 1 100.00% 16 3 leaf 0 0 0.00%

Only the internal file segment has any pages allocated, and it only has a single page allocated. The index-fseg-internal-lists mode will summarize the extent lists in the internal file segment:

$ innodb_space -f test/e.ibd -p 3 index-fseg-internal-lists name length f_page f_offset l_page l_offset free 0 0 0 0 0 not_full 0 0 0 0 0 full 0 0 0 0 0

All three lists are empty, because this empty table has not allocated any full extents. So where did the 1 page that is used go? It’s a “fragment” page, and those can be listed with the index-fseg-internal-frag-pages mode:

$ innodb_space -f test/e.ibd -p 3 index-fseg-internal-frag-pages page index level data free records 3 16 0 0 16252 0

That’s the minimal state of things — mostly empty bookkeeping structures, and a single INDEX page. Let’s take a look at a table with some real data in it.

A table with one million rows

In A quick introduction to innodb_ruby, I created a table with 1 million rows in it. We’ll use the same table in the examples here.

There are a total of 2,165 pages, with the majority of them type INDEX as a typical table would be:

$ innodb_space -f test/t.ibd space-page-type-regions start end count type 3 37 35 INDEX 38 63 26 FREE (ALLOCATED) 64 2188 2125 INDEX 2189 2239 51 FREE (ALLOCATED) 2240 2240 1 INDEX 2241 2303 63 FREE (ALLOCATED) 2304 2304 1 INDEX 2305 2367 63 FREE (ALLOCATED) 2368 2368 1 INDEX 2369 2431 63 FREE (ALLOCATED) 2432 2432 1 INDEX 2433 2495 63 FREE (ALLOCATED) 2496 2496 1 INDEX 2497 2687 191 FREE (ALLOCATED)

Note that there are a few gaps of ALLOCATED (free) pages in between blocks of INDEX pages. InnoDB does not guarantee that it uses free pages sequentially, and many optimizations around bulk data loading will cause pages to be used out of order. (More on page splitting and these optimizations in a future post.)

Looking at the space’s lists, there are actually a few extents in free , as well as the usual one extent in free_frag :

$ innodb_space -f test/t.ibd space-lists name length f_page f_offset l_page l_offset free 2 0 1758 0 1798 free_frag 1 0 158 0 158 full_frag 0 0 0 0 0 full_inodes 0 0 0 0 0 free_inodes 1 2 38 2 38

The pages in the free extent descriptor list are all free, as expected:

$ innodb_space -f test/t.ibd -L free space-list-iterate 2560 ................................................................ 2624 ................................................................

The free_frag extent descriptor list shows a number of “fragment” pages are used as well:

$ innodb_space -f test/t.ibd -L free_frag space-list-iterate start_page page_used_bitmap 0 ######################################..........................

The index file segments show the bulk of the used pages are allocated to the leaf file segment, also as expected (there are only 3 non-leaf internal pages to manage the 2,137 leaf pages in the B+tree):

$ innodb_space -f test/t.ibd space-indexes id root fseg used allocated fill_factor 15 3 internal 3 3 100.00% 15 3 leaf 2162 2528 85.52%

You can also see that the leaf index file segment has more pages allocated than it’s actually using, showing an 85.52% fill factor. This is due to InnoDB’s “segment fill factor” which is fixed at 87.5% in stock MySQL, but is now configurable in Twitter MySQL thanks to a heads up from Facebook filing MySQL Bug 64673.

Since the internal index file segment has only three pages, as expected the file segment lists are all empty:

$ innodb_space -f test/t.ibd -p 3 index-fseg-internal-lists name length f_page f_offset l_page l_offset free 0 0 0 0 0 not_full 0 0 0 0 0 full 0 0 0 0 0

The three used pages are allocated as fragment pages:

$ innodb_space -f test/t.ibd -p 3 index-fseg-internal-frag-pages page index level data free records 3 15 2 26 16226 2 36 15 1 14521 1401 1117 37 15 1 13585 2341 1045

The leaf index file segment lists are pretty busy, with 32 full extents and 6 not full extents:

$ innodb_space -f test/t.ibd -p 3 index-fseg-leaf-lists name length f_page f_offset l_page l_offset free 0 0 0 0 0 not_full 6 0 1518 0 1718 full 33 0 198 0 1478

In addition the leaf index file segment has allocated all 32 fragment pages possible (before any of the full extents above):

$ innodb_space -f test/t.ibd -p 3 index-fseg-leaf-frag-pages page index level data free records 4 15 0 9812 6286 446 5 15 0 15158 860 689 6 15 0 10912 5170 496 7 15 0 10670 5412 485 8 15 0 12980 3066 590 9 15 0 11264 4808 512 10 15 0 4488 11690 204 11 15 0 9680 6418 440 12 15 0 9306 6800 423 13 15 0 9658 6434 439 14 15 0 10032 6062 456 15 15 0 9988 6108 454 16 15 0 9570 6530 435 17 15 0 9130 6978 415 18 15 0 8844 7266 402 19 15 0 11770 4300 535 20 15 0 9020 7092 410 21 15 0 8646 7462 393 22 15 0 9746 6354 443 23 15 0 11066 5014 503 24 15 0 8910 7204 405 25 15 0 11748 4322 534 26 15 0 10978 5094 499 27 15 0 11132 4940 506 28 15 0 9350 6750 425 29 15 0 13508 2526 614 30 15 0 14938 1082 679 31 15 0 14520 1506 660 32 15 0 9086 7016 413 33 15 0 9724 6368 442 34 15 0 10978 5102 499 35 15 0 9504 6592 432

The full extents, as expected, are all full:

$ innodb_space -f test/t.ibd -p 3 -L full index-fseg-leaf-list-iterate start_page page_used_bitmap 64 ################################################################ 128 ################################################################ 192 ################################################################ 256 ################################################################ 320 ################################################################ 384 ################################################################ 448 ################################################################ 512 ################################################################ 576 ################################################################ 640 ################################################################ 704 ################################################################ 768 ################################################################ 832 ################################################################ 896 ################################################################ 960 ################################################################ 1024 ################################################################ 1088 ################################################################ 1152 ################################################################ 1216 ################################################################ 1280 ################################################################ 1344 ################################################################ 1408 ################################################################ 1472 ################################################################ 1536 ################################################################ 1600 ################################################################ 1664 ################################################################ 1728 ################################################################ 1792 ################################################################ 1856 ################################################################ 1920 ################################################################ 1984 ################################################################ 2048 ################################################################ 2112 ################################################################

The not_full extents are all partially filled, as expected:

$ innodb_space -f test/t.ibd -p 3 -L not_full index-fseg-leaf-list-iterate start_page page_used_bitmap 2176 #############................................................... 2240 #............................................................... 2304 #............................................................... 2368 #............................................................... 2432 #............................................................... 2496 #...............................................................

You can see the artifacts of InnoDB’s page split optimizations here: It has taken the first page out of an extent several times (due to page number “hinting” which is dubious at best) in an effort to lay out data in sequential order on disk. A deeper examination of this behavior will come in the future.