cPanelCon 2014: InnoDB Anatomy
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cPanelCon 2014: InnoDB Anatomy
- 3. The InnoDB Engine Introduction to InnoDB • Currently the default in MySQL (as of 5.5) • Referential/Structural Integrity • Consistent data • Transactional
- 4. InnoDB is atomic in that its transactions have only two possible outcomes - complete fully, or fail completely. • SUCCESS: • FAILURE: All changes committed. All changes rolled back. InnoDB Anatomy – ACID Compliance ATOMICITY COMMITROLLBACK Unchanged Data Changes Applied
- 5. • Data stays consistent before, during, and after a transaction. • No conflict of “versions” • Successful transactions end with a commit. InnoDB maintains optimism. Introduction to InnoDB Structure CONSISTENCY Valid State Work performed Still a valid state
- 6. • Transactions cannot interact with each other • Adjustable level of isolation.* • Row-level locking Introduction to InnoDB Structure ISOLATION * Isolation level changed via the transaction-isolation configuration option.
- 7. • Atomic transactions keep data durable. • Changes are permanent once committed. • Doublewrite buffer helps to recover from crashes that occur during page writes. Introduction to InnoDB Structure DURABILITY
- 8. Explore the InnoDB structure within the file system, and at its lower levels, to find out how it can affect database operations. InnoDB Anatomy The Goal
- 9. Understanding the Physical Structure of Data in InnoDB
- 10. InnoDB Anatomy – Physical Structure Physical File Structure DataDirectory (Default:/var/lib/mysql) Ibdata1 System Tablespace ib_logfile0 Redo/Transaction Log File ib_logfile1 Redo/Transaction Log File Database Folder table.ibd Tablespace File table.frm Format File
- 11. InnoDB Anatomy – Physical Structure InnoDB Tablespace Page Extent Segment Inode File System Partition Disk Block Allocation Unit File Inode
- 12. InnoDB Anatomy – Physical Structure InnoDB Tablespace Page Extent Segment Inode User Records (Index Pages)… First Inode Page Number File Header Insert Buffer Bitmap IBD File
- 13. InnoDB Anatomy – Physical Structure InnoDB Tablespace Page Extent Segment Inode PAGE… FIL Header - 38 bytes … FIL Trailer - 8 bytes 16384
- 14. InnoDB Anatomy – Physical Structure InnoDB Tablespace Page Extent Segment Inode 1MB Page 1 … Page 64 EXTENT
- 15. InnoDB Anatomy – Physical Structure InnoDB Tablespace Page Extent Segment Inode Extent Extent … SEGMENT
- 16. InnoDB Anatomy – Physical Structure InnoDB Tablespace Page Extent Segment Inode File segment ID Extent Data (Free/Partial/Full Listing)
- 17. InnoDB Anatomy – Physical Structure Tablespace Segment Segment Segment Segment Extent Extent Extent Extent Extent Extent Extent
- 18. SYSTEM TABLESPACE ibdata File InnoDB Anatomy – System Tablespace Undo Logs Rollback Segment Rollback Segment Data Dictionary SYS_TABLES SYS_INDEXES SYS_COLUMNS SYS_FIELDS Doublewrite Buffer Block 1 (64 Pages) Block 2 (64 Pages) Change Buffer Insert Buffering Update Buffering Purge Buffering Undo Log Space Rollback Segment …
- 19. InnoDB Anatomy – System Tablespace Separate your Undo Logs (5.6+ only) innodb_undo_logs; innodb_undo_tablespaces; innodb_undo_directory I/O to the undo logs is random, instead of sequential like some other areas of InnoDB. Because of this, it makes sense to separate your Undo Log tablespaces out from the system tablespace onto a disk that handles random reads and writes more effectively, such as a SSD. Use the Information Schema, or innodb_table_monitor, to view the Data Dictionary table data. In 5.6, the information_schema database contains INNODB_SYS* tables that allow you to view data dictionary information directly in MySQL. Alternatively, you can create a table called “innodb_table_monitor” to dump the data dictionary into the MySQL error logs. How can you use this?
- 20. InnoDB Anatomy – System Tablespace Do you need the Doublewrite buffer? innodb_doublewrite With Doublewrite buffer enabled, there is a 5-10% impact on I/O. If you operate on a transactional file system, you disable this to avoid this impact. Customizing Change Buffering for your Workload innodb_change_buffering Change buffering, by default in 5.5+, encompasses insert, update, and delete buffering. If your workload consists almost entirely of one or the other, it can make sense to limit this down to only one type of buffering. How can you use this?
- 21. InnoDB in Memory and on Disk Memory Buffer Pool Insert Buffer Log Buffer Additional Memory Disk System Tablespace Doublewrite Buffer Transaction Log Files Insert Buffer Undo Logs Rollback Segment Data Dictionary Undo Buffering Indexing Thread Processing Tablespace Files
- 22. InnoDB Anatomy – Pages Page Headers/Trailers Name Byte Length Offset Description FIL_PAGE_SPACE 4 0 Space ID FIL_PAGE_OFFSET 4 4 Page Number FIL_PAGE_PREV 4 8 Previous Page (in key order) FIL_PAGE_NEXT 4 12 Next Page (in key order) FIL_PAGE_LSN 8 16 LSN of page’s latest log record FIL_PAGE_TYPE 2 24 Page Type FIL_PAGE_FILE_FLUSH_LSN 8 26 Flushed-up-to LSN (only in space ID 0, page 0) FIL_PAGE_ARCH_LOG_NO 4 34 Latest archived LSN (only in space ID 0, page 0) FIL Header (38) FIL Trailer (8) Name Byte Length Offset Description FIL_PAGE_END_LSN 8 16376 Low 4 bytes: Checksum, Last 4 bytes: FIL_PAGE_LSN storage/innobase/include/fil0fil.h
- 23. InnoDB Anatomy - Demonstration Changing values directly At the byte level, these values can be changed directly in many situations to “trick” InnoDB in one way or another. One good example of this is to get around a page checksum failure. You can change the stored checksum to match the calculated checksum, bypassing the crash and often allowing you sufficient access to your records. How can you use this? InnoDB: Page checksum 2047964429, prior-to-4.0.14-form checksum 4196043695 InnoDB: stored checksum 1873408413, prior-to-4.0.14-form stored checksum 1946395024 # printf '%Xn' 2047964429; printf '%Xn' 4196043695 7A11750D Primary calculated checksum FA1A8BAF “Old-style” calculated checksum # expr 16384 * 6 Example Page 6 98304 Starting byte offset for Page 6 Writing the primary calculated checksum over the stored value of page 6: # printf ‘x7Ax11x75x0D’ | dd of=table.ibd bs=1 seek=98304 count=4 conv=notrunc Writing the “old-style” calculated checksum over the stored value of page 6: # printf ‘x7Ax11x75x0D’ | dd of=table.ibd bs=1 seek=98304 count=4 conv=notrunc
- 24. •Stored in 2 files by default (ib_logfile0/1) •Treated as single file •Circular buffer LOG BLOCK Header (12) Log Records Trailer (4) … LOG BLOCK Header (12) Log Records Trailer (4) … LOG BLOCK Header (12) Log Records Trailer (4) … ib_logfile0ib_logfile1 InnoDB Anatomy – Redo Logs Structure •Log blocks are 512 bytes •Each block contains checkpoint data The Logical Log FileThe Redo Logs
- 25. InnoDB Anatomy – Redo Logs Optimized log file size innodb_log_file_size Larger size means less checkpoint flushing required, reducing I/O impact. Balance with expected recovery time required as a result of the size (less of an issue in 5.6). General Formula: (Current LSN – LSN 60 seconds later) * 60 / 1024 / 1024 Optimized log buffer size innodb_log_buffer_size Log buffer allows transactions to move forward without having to write the log to disk before commit. Increased size allows larger transactions to run without requiring writes to disk before a commit is performed. How can you use this?
- 26. InnoDB Anatomy – Index Pages INDEX Pages - B+Tree Structure •Efficient method of storing data on disk in a tree format. •Actual records stored in leaf pages (level 0). •Root-level pages exist at the top of the tree structure. •Non-leaf pages contain only pointers to leaf pages. Level 0 Level 1 Level 2 Root Non-Leaf Leaf Leaf Non-Leaf Leaf
- 27. InnoDB Anatomy – Index Pages B+Tree Structure – Basic Index Example Root Node Customer IDs 1-500 Non-Leaf 1-250 Non-Leaf 251-500 Leaf Node 251-260 Leaf Node 261-270 Leaf Node 1-10 Leaf Node 11-20 …
- 28. InnoDB Anatomy – Index Pages INDEX Pages •Not physically in order •User data “grows down” •Page directory “grows up” FIL Header (38) … Page Directory FIL Trailer (8) INDEX Header (36) FSEG Header (20) System Records (26) User Data … EMPTY
- 29. InnoDB Anatomy – Index Pages INDEX Page Header (after FIL Header) Name Byte Length Offset Description PAGE_N_DIR_SLOTS 2 38 + 0 Number of Slots in Page Directory PAGE_HEAP_TOP 2 38 + 2 Pointer to Record Heap Top PAGE_N_HEAP 2 38 + 4 Number of Records in Heap PAGE_FREE 2 38 + 6 Pointer to start of page’s free-record list PAGE_GARBAGE 2 38 + 8 Number of bytes in “deleted” records PAGE_LAST_INSERT 2 38 + 10 Pointer to last inserted record, or NULL if this has been reset – eg. by a delete. PAGE_DIRECTION 2 38 + 12 Last Insert direction, PAGE_LEFT, PAGE_RIGHT … PAGE_N_DIRECTION 2 38 + 14 Consecutive inserts in the same direction PAGE_N_RECS 2 38 + 16 Number of user records on the page PAGE_MAX_TRX_ID 8 38 + 18 Highest ID of transaction that may have modified a record on the page. PAGE_LEVEL 2 38 + 26 Level of node in index tree PAGE_INDEX_ID 4 38 + 28 Index ID that page belongs to storage/innobase/include/page0page.h
- 30. InnoDB Anatomy – Demonstration Demonstration Determining page level on an INDEX page First, find your page’s start byte: # expr 16384 * 3 49152 The offset of the PAGE_LEVEL value is 26 after the FIL Header (38): # expr 49152 + 38 + 26 49216 The byte-length is 2 # xxd –ps –s 49216 –l 2 customer.ibd 0001 Page Level: 1 /var/lib/mysql/testdb/
- 32. Conclusion • Jeremy Cole • http://blog.jcole.us/ • https://github.com/jeremycole/ • Percona • http://www.percona.com/files/percona-live/justin- innodb-internals.pdf • MySQL Internals Documentation & Source • http://dev.mysql.com/doc/internals/en/innodb.html • https://launchpad.net/mysql Sources and Thanks
Editor's Notes
- - Referential integrity = ensuring validity via adherence to constraints and restrictions Consistent data = enforced via checksum matching, by default. Transactional = grouping series of operations into a single, logical, atomic unit of work.
- Core InnoDB Concepts after this
- Transaction/Redo terms interchangeable when referring to the ib_logfiles.
- Tablespaces: Divide the data – innodb’s way of holding data for individual tables Pages: 16K data sections Extents: Units of allocation, stores groups of data (pages). Extents hold up to 64 pages each. Segments: Divisions of data within the tablespaces Inodes: Contain attributes and pointers to other sections of data.
- Tablespace: At least 3 initial header pages, each holding the standard page structure. Headers contain values about what to expect from the tablespace. Page: SEGMENT ACTS AS A DIVISION OF THE TABLESPACE – LOGICAL GROUP OF EXTENTS
- Tablespace: At least 3 initial header pages, each holding the standard page structure. Headers contain values about what to expect from the tablespace. Page: SEGMENT ACTS AS A DIVISION OF THE TABLESPACE – LOGICAL GROUP OF EXTENTS
- Tablespace: At least 3 initial header pages, each holding the standard page structure. Headers contain values about what to expect from the tablespace. Page: SEGMENT ACTS AS A DIVISION OF THE TABLESPACE – LOGICAL GROUP OF EXTENTS
- Tablespace: At least 3 initial header pages, each holding the standard page structure. Headers contain values about what to expect from the tablespace. Page: SEGMENT ACTS AS A DIVISION OF THE TABLESPACE – LOGICAL GROUP OF EXTENTS
- Tablespace: At least 3 initial header pages, each holding the standard page structure. Headers contain values about what to expect from the tablespace. Page: SEGMENT ACTS AS A DIVISION OF THE TABLESPACE – LOGICAL GROUP OF EXTENTS
- Additional log files can be used and/or relocated. Currently only one “group” supported Reliance on the log file allows InnoDB to delay flushes/writes to disk.
- Undo logs are composed of rollback segments (128), each able to support 1023 transactions; in total can support up to 128K concurrent transactions- increased from pre-5.5 value of just a single segment of 1023 transactions.
- Undo logs can be split off to be handled elsewhere, such as with an SSD, for optimal perfromance
- ZFS is an example of a transactional file system; makes sure writes are atomic
- Additional log files can be used and/or relocated. Log buffer flushed once per second MySQL 5.6 adjusts for log file size changes performed while offline. Currently only one “group” supported Reliance on the log file allows InnoDB to delay flushes/writes to disk.
- Additional log files can be used and/or relocated. Page structure does not apply here (blocks of 512 bytes instead). Total size of logical log file can be up to 512GB (file size * files in group) Reliance on the log file allows InnoDB to delay flushes/writes to disk. MySQL 5.6 adjusts for log file size changes performed while offline. Log buffer flushed once per second
- Allows for a consistent, determinable amount of reads to access any record in an index. Root page is a “starting point” for accessing the tree, tells it where to look and how far it will need to go (root page’s level indicates how far down it takes to get to 0, hence the bottom->top numbering) Tree can be as small as a single root page, or as big as millions of pages in a multi-level structure Everything is an index in InnoDB This is how all user records are stored Not stored physically in order, but use pointers In a single 4-level index tree, you have the potential for 814 billion rows/25.9TiB of data.
- “Non-leaf Nodes” also referred to as “Internal Nodes” Root page is a “starting point” for accessing the tree, tells it where to look and how far it will need to go (root page’s level indicates how far down it takes to get to 0, hence the bottom->top numbering) Everything is an index in InnoDB In a single 4-level index tree, you have the potential for 814 billion rows/25.9TiB of data.
- /* Directions of cursor movement */ #define PAGE_LEFT 1 #define PAGE_RIGHT 2 #define PAGE_SAME_REC 3 #define PAGE_SAME_PAGE 4 #define PAGE_NO_DIRECTION 5
- New tablespaces: undo and temporary tables (non-compressed)