WL#4060: Online Backup: kernel updates for beta release
Affects: Server-6.0 — Status: Complete — Priority: Low
Implement these missing features of the backup kernel. Numbers in braces are estimates of developement time. - Use the backup stream library (WL#4063) [3d] Benefit: Backup stream library defines the format in which backup image is written. - Thread shutdown handling [2d] Banefit: Otherwise can crash or leak memory when backup thread is killed. - Disable foreign key constraints when restoring tables. [1d] Benefit: Restore will work even if there are foreign key constraints. Note: This has been moved to WL#4150 : Online Backup : Disable Foreign Key Constraint on Restore
Charset support --------------- After small research on the subject, we use the following assumptions (please email dev-backup if you find them wrong or incomplete): 1. All identifiers (table/db names etc) processed by parser are stored iusing system_charset_info encoding (e.g. in the LEX_STRING structures obtained from parser). 2. When server needs to identify a table when talking to a storage engine (e.g. in open() call) it uses a path string where table and database names are encoded using special charset. This path string for a given table/database name can be created using build_table_filename() function (defined in sql_table.cc). The function assumes that the input table and database names are encoded using system_charset_info. Based on that: - In backup kernel, all identifiers will be stored encoded with system_charset_info. - The same encoding will be used when saving strings in a backup image (note that name of the system charset is stored in the image's header). - When reading backup image on a server with different system charset than the one used during backup, strings will be translated to the restore server's system charset. - For any table passed to backup/restore driver, the driver will have access to the internal representation of table's name as a path, the same as used in storage engines. Limitations ----------- 1. Beta kernel handles only databases and tables. 2. Beta kernel will not store in backup image's catalogue names of all charsets needed to restore the image. If a server on which we restore will have all required charsets then the restore operation will work normally, but if some charset is missing, then RESTORE will quit with error. In future we can store info about all required charsets in the catalogue and detect missing charsets before starting restore operation. 3. Beta kernel will not store any user and privilege info in the backup image.
Table name mangling ------------------- Per HLS: backup/restore driver should have access to the internal representation of table name. For that purpose new methods will be added to Table_ref class: size_t Table_ref::internal_name(Iname_buf buf); size_t Table_ref::internal_name(char *buf, size_t buflen); Type Table_ref::Iname_buf will be a char array of size appropriate for storing internal table name representation (FN_REFLEN?). So it can be used as follows: Table_ref t; Table_ref::Iname_buf tname; size_t len= t.internal_name(tname); DBUG_ASSERT(tname[len] == '\0'); Alternatively, user can decide itself what buffer size to use: Table_ref t; char tname; size_t len= t.internal_name(tname,1024); Note: table name mangling will be implemented as a fix for BUG#33023. Implementation of the catalogue services API -------------------------------------------- The catalogue services API required by the backup stream library will be implemented using Image_info class. An instance of this class will store descriptions of backup image items in dynamically allocated entries which are instances of classes designed to store each type of item: Image_info::Table_item - for tables Image_info::Db_item - for databases All these classes will inherit from Image_info::Item class and implement its virtual methods for: - accessing meta-data of an item (CREATE statement and/or other data) - creating a corresponding object using the meta-data read from archive. These methods will be used to implement these catalogue services: bcat_get_item_create_query() bcat_get_item_create_data() bcat_create_item() The Item_info class will also have methods for adding items of various types to the catalogue. These will be used to implement. bcat_add_item() catalogue service which is used during restore operation to store inofo about items stored in the backup image. Backup stream library browses contents of a catalogue using iterators. These will be implemented as specializations of Image_info::Iterator class which provides basic iterator operations (creating, proceeding to next item and accessing the current item). The following iterators will be implemented: Image_info::Db_iterator - iterates over all databases Image_info::Ditem_iterator - iterates over all tables of a given database These classes will be used to implement bcat_iterator_get() bcat_iterator_next() bcat_iterator_free() bcat_db_iterator_get() bcat_db_iterator_next() bcat_db_iterator_free() Note: backup stream library uses the following types of global iterators: BSTREAM_IT_DB - implemented with Image_info::Db_iterator class. BSTREAM_IT_GLOBAL - as above - the only (dynamic) global items we support are databases. BSTREAM_IT_PERDB - implemented with Image_info::Ditem_iterator class. BSTREAM_IT_PERTABLE - implemented as null iterator which returns no entries (we don't support per-table items in this version). A limitation of this version is that we don't handle charsets and users in the image catalogue (they will still be present and used in the CREATE statements used to restore items). However, as a minimum we must store in the catalogue the common charset used to encode all remaining strings (we will use UTF-8) and the default charset of the server on which backup was created. We will store no user names in the catalogue and ignore (with warning) any user names read during restore. BSTREAM_IT_CHARSET - implemented as two element iterator returning "UTF-8" charset followed by the server's default charset. BSTREAM_IT_USER - implemented as a null iterator returning no data. Implementation of the Image_info class -------------------------------------- Image_info class will implement backup catalogue and also stored the information of the backup image header. For the later it will inherit from bstream_header_info structure and store header info there. Additionally, for each snapshot included in the image, a pointer to instance of Snapshot_info object will be stored in Image_info::m_snap array. This instance will be used to store info about tables belonging to that image and define methods for processing the snapshot. To access items stored in the catalogue Image_info will provide the following methods: Db_item* get_db(uint pos) - return pointer to Db_item describing db stored at position pos in the catalogue, NULL if position is empty. Table_item* get_table(unit k, unsigned long n) - return pointer to Table_item describing n-th table stored in k-th snapshot, NULL if k or n are invalid. To populate catalogue with items the following methods will be used: Db_item* add_db(const Db_ref &db, uint pos) - create Db_item at position pos in the catalogue and store info about db there. Returns pointer to the newly created entry or NULL if operation failed. Table_item* add_table(const Table_ref &t, uint k, unsigned long n) - create Table_item for table t stored in k-th snapshot at position n. Returns pointer to the newly created entry or NULL on error. The Image_info class will manage memory for the Table_item and Db_item object instances. However, it will assume that the memory used by the Db_ref and Table_ref instances is managed externally and persists for the life span of the Image_info object. That is, the memory used for strings with table/db names is managed externally and these strings should exist as long as the catalogue exists. See below for description of how the memory is managed. This is how various item types will be stored in the catalogue: BSTREAM_IT_CHARSET - we store only 2 first charsets: the common string charset and the servers default charset. These will be stored in members Image_info::common_cset and Image_info::default_cset as pointers to the charset structure. BSTREAM_IT_USER - in this version we don't store user info in the catalogue - upon restore the names read from the image will be ignored. BSTREAM_IT_PRIVILEGE - in this version we don't store privileges in the image. BSTREAM_IT_DB - database items will be stored in a Image_info::m_db member which is a dynamic array of Image_info::Db_item objects. BSTREAM_IT_TABLE - n-th table of k-th snapshot will be stored at position n in an dynamic array m_tables of the Snapshot_info object instance pointed by m_snap[k]. Memory management ----------------- During BACKUP/RESTORE process we need this memory: - Memory for buffers for getting data from backup drivers. One buffer per driver is used, allocated inside Block_writer instance created as a part of a Scheduler::Pump object used for that driver. Memory is dallocated in pump's destructor. Size of the buffer is defined by DATA_BUFFER_SIZE constant (1Mb currently). - Memory for buffers for sending data to restore drivers. Single buffer is reused for all drivers (as data blocks are read sequentially from a stream). The buffer is allocated inside backup stream library (bstream_rd_data_chunk() function) using bstream_alloc() function. It is deallocated when backup stream is closed. - Memory to store Db_item instances for each database stored in the catalogue. These are stored in the m_dbs member of Image_info object. It is an instance of Image_info::Databases class which uses DYNAMIC_ARRAY for storage (wrapped by Dynamic_array<Db_item> class). - Memory for the strings storing each database name (backup). In case when explicit list of databases is given as in BACKUP DATABASE db1,db2,...,dbn TO ... the names of these databases are stored in the query string and are returned from parser as LEX_STRING values. Kernel uses these strings without copying them and allocating any new memory for them. In case of BACKUP DATABASE * ... , the names are read from I_S.DB table and copied to a String instance (which allocates memory to store the string). All String instances used are appended to Backup_info::name_strings list. When Backup_info object is deleted, the strings are freed. Note: this design is far from being elegant or optimal, but it works and should be good enough for beta. - Memory for the strings storing each database name (restore). Upon restore, database names are read from a backup stream. The memory for storing them is allocated by backup stream library (in bstream_rd_string() function) using bstream_alloc(). Note that memory is not freed by the library and kernel needs to do that after the restore process is finished (see below). - Memory to store Table_item instances for each table stored in the catalogue. For tables belonging to k-th snapshot, the Table_item instances are stored inside Snapshot_info::m_tables member of class Tables. The latter uses DYNAMIC_ARRAY for storage like Image_info::Databases. - Memory for the strings containing each table's name (backup). The same as for databases - memory is allocated by String instances which are appended to Backup_info::name_strings list and deleted in ~Backup_info. - Memory for the strings containing each table's name (restore). The same as for databases, memory is allocated by backup stream library. Memory (de)allocator for backup stream library ---------------------------------------------- Backup stream library (de)allocates memory using service functions: void* bstream_alloc(unsigned long int howmuch) void bstream_free(void*) However, not every memory allocated is freed by the library. Thus the implementation must free any left-over memory. The functions are implemented using class Mem_allocator with methods alloc() and free(). An instance of this class is created before any backup/restore operation is started (in execute_backup_command()) and destroyed when operation is completed. This is done with functions: prepare_stream_memory() free_stream_memory() This assumes only one BACKUP/RESTORE operation active at a time (otherwise we couldn't use single Mem_allocator instance). All memory blocks allocated with Mem_allocator are stored in a linked list, so that in the destructor we can iterate over them and free them. When a block is freed explicitly, it is removed from the list. Implementation -------------- There are three patches commited which adapt backup kernel to use the stream library. First patch reorganizes source tree a bit, changing names of some source and header files. Second patch introduces necessary changes and fixes problems in the backup stream library. Third patch modifies the backup kernel. 1. source tree chagnes: <http://lists.mysql.com/commits/38880> 2. stream library patches: <http://lists.mysql.com/commits/38882> 3. backup kernel modifications: <http://lists.mysql.com/commits/38893> Here is another patch which ensures that there is only single BACKUP/RESTORE operation running at a time: <http://lists.mysql.com/commits/39012>
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