db_backup function

Prototype

void db_backup(
SQLCA * sqlca,
int op,
int file_num,
unsigned long page_num,
struct sqlda * sqlda);

Authorization

Must be connected as a user with DBA authority, REMOTE DBA authority (SQL Remote), or BACKUP authority.

Description

BACKUP statement is recommended

Although this function provides one way to add backup features to an application, the recommended way to do this task is to use the BACKUP statement. See BACKUP statement.

The action performed depends on the value of the op parameter:

DB_BACKUP_START Must be called before a backup can start. Only one backup can be running per database at one time against any given database server. Database checkpoints are disabled until the backup is complete (db_backup is called with an op value of DB_BACKUP_END). If the backup cannot start, the SQLCODE is SQLE_BACKUP_NOT_STARTED. Otherwise, the SQLCOUNT field of the sqlca is set to the database page size. Backups are processed one page at a time.
The file_num, page_num, and sqlda parameters are ignored.
DB_BACKUP_OPEN_FILE Open the database file specified by file_num, which allows pages of the specified file to be backed up using DB_BACKUP_READ_PAGE. Valid file numbers are 0 through DB_BACKUP_MAX_FILE for the root database files, and 0 through DB_BACKUP_TRANS_LOG_FILE for the transaction log file. If the specified file does not exist, the SQLCODE is SQLE_NOTFOUND. Otherwise, SQLCOUNT contains the number of pages in the file, SQLIOESTIMATE contains a 32-bit value (POSIX time_t) that identifies the time that the database file was created, and the operating system file name is in the sqlerrmc field of the SQLCA.
The page_num and sqlda parameters are ignored.
DB_BACKUP_READ_PAGE Read one page of the database file specified by file_num. The page_num should be a value from 0 to one less than the number of pages returned in SQLCOUNT by a successful call to db_backup with the DB_BACKUP_OPEN_FILE operation. Otherwise, SQLCODE is set to SQLE_NOTFOUND. The sqlda descriptor should be set up with one variable of type DT_BINARY or DT_LONG_BINARY pointing to a buffer. The buffer should be large enough to hold binary data of the size returned in the SQLCOUNT field on the call to db_backup with the DB_BACKUP_START operation.
DT_BINARY data contains a two-byte length followed by the actual binary data, so the buffer must be two bytes longer than the page size.

Application must save buffer

This call makes a copy of the specified database page into the buffer, but it is up to the application to save the buffer on some backup media.
DB_BACKUP_READ_RENAME_LOG This action is the same as DB_BACKUP_READ_PAGE, except that after the last page of the transaction log has been returned, the database server renames the transaction log and starts a new one.
If the database server is unable to rename the log at the current time (for example in version 7.0.x or earlier databases there may be incomplete transactions), the SQLE_BACKUP_CANNOT_RENAME_LOG_YET error is set. In this case, do not use the page returned, but instead reissue the request until you receive SQLE_NOERROR and then write the page. Continue reading the pages until you receive the SQLE_NOTFOUND condition.

The SQLE_BACKUP_CANNOT_RENAME_LOG_YET error may be returned multiple times and on multiple pages. In your retry loop, you should add a delay so as not to slow the server down with too many requests.

When you receive the SQLE_NOTFOUND condition, the transaction log has been backed up successfully and the file has been renamed. The name for the old transaction file is returned in the sqlerrmc field of the SQLCA.

You should check the sqlda->sqlvar[0].sqlind value after a db_backup call. If this value is greater than zero, the last log page has been written and the log file has been renamed. The new name is still in sqlca.sqlerrmc, but the SQLCODE value is SQLE_NOERROR.

You should not call db_backup again after this, except to close files and finish the backup. If you do, you get a second copy of your backed up log file and you receive SQLE_NOTFOUND.
DB_BACKUP_CLOSE_FILE Must be called when processing of one file is complete to close the database file specified by file_num.
The page_num and sqlda parameters are ignored.
DB_BACKUP_END Must be called at the end of the backup. No other backup can start until this backup has ended. Checkpoints are enabled again.
The file_num, page_num and sqlda parameters are ignored.
DB_BACKUP_PARALLEL_START Starts a parallel backup. Like DB_BACKUP_START, only one backup can be running against a database at one time on any given database server. Database checkpoints are disabled until the backup is complete (until db_backup is called with an op value of DB_BACKUP_END). If the backup cannot start, you receive SQLE_BACKUP_NOT_STARTED. Otherwise, the SQLCOUNT field of the sqlca is set to the database page size.
The file_num parameter instructs the database server to rename the transaction log and start a new one after the last page of the transaction log has been returned. If the value is non-zero then the transaction log is renamed or restarted. Otherwise, it is not renamed and restarted. This parameter eliminates the need for the DB_BACKUP_READ_RENAME_LOG operation, which is not allowed during a parallel backup operation.

The page_num parameter informs the database server of the maximum size of the client's buffer, in database pages. On the server side, the parallel backup readers try to read sequential blocks of pages—this value lets the server know how large to allocate these blocks: passing a value of N lets the server know that the client is willing to accept at most N database pages at a time from the server. The server may return blocks of pages of less than size N if it is unable to allocate enough memory for blocks of N pages. If the client does not know the size of database pages until after the call to DB_BACKUP_PARALLEL_START, this value can be provided to the server with the DB_BACKUP_INFO operation. This value must be provided before the first call to retrieve backup pages (DB_BACKUP_PARALLEL_READ).

Note

If you are using db_backup to start a parallel backup, db_backup does not create writer threads. The caller of db_backup must receive the data and act as the writer.
DB_BACKUP_INFO This parameter provides additional information to the database server about the parallel backup. The file_num parameter indicates the type of information being provided, and the page_num parameter provides the value. You can specify the following additional information with DB_BACKUP_INFO:
- DB_BACKUP_INFO_PAGES_IN_BLOCK The page_num argument contains the maximum number of pages that should be sent back in one block.
- DB_BACKUP_INFO_CHKPT_LOG This is the client-side equivalent to the WITH CHECKPOINT LOG option of the BACKUP statement. A page_num value of DB_BACKUP_CHKPT_COPY indicates COPY, while the value DB_BACKUP_CHKPT_NOCOPY indicates NO COPY. If this value is not provided it defaults to COPY.
DB_BACKUP_PARALLEL_READ This operation reads a block of pages from the database server. Before invoking this operation, use the DB_BACKUP_OPEN_FILE operation to open all the files that you want to back up. DB_BACKUP_PARALLEL_READ ignores the file_num and page_num arguments.
The sqlda descriptor should be set up with one variable of type DT_LONGBINARY pointing to a buffer. The buffer should be large enough to hold binary data of the size N pages (specified in the DB_BACKUP_START_PARALLEL operation, or in a DB_BACKUP_INFO operation). For more information about this data type, see DT_LONGBINARY in Embedded SQL data types.

The server returns a sequential block of database pages for a particular database file. The page number of the first page in the block is returned in the SQLCOUNT field. The file number that the pages belong to is returned in the SQLIOESTIMATE field, and this value matches one of the file numbers used in the DB_BACKUP_OPEN_FILE calls. The size of the data returned is available in the stored_len field of the DT_LONGBINARY variable, and is always a multiple of the database page size. While the data returned by this call contains a block of sequential pages for a given file, it is not safe to assume that separate blocks of data are returned in sequential order, or that all of one database file's pages are returned before another database file's pages. The caller should be prepared to receive portions of another individual file out of sequential order, or of any opened database file on any given call.

An application should make repeated calls to this operation until the size of the read data is 0, or the value of sqlda->sqlvar[0].sqlind is greater than 0. If the backup is started with transaction log renaming/restarting, SQLERROR could be set to SQLE_BACKUP_CANNOT_RENAME_LOG_YET. In this case, do not use the pages returned, but instead reissue the request until you receive SQLE_NOERROR, and then write the data. The SQLE_BACKUP_CANNOT_RENAME_LOG_YET error may be returned multiple times and on multiple pages. In your retry loop, you should add a delay so the database server is not slowed down by too many requests. Continue reading the pages until either of the first two conditions are met.

The dbbackup utility uses the following algorithm. Note that this is not C code, and does not include error checking.

sqlda->sqld = 1;
sqlda->sqlvar[0].sqltype = DT_LONGBINARY

/* Allocate LONGBINARY value for page buffer. It MUST have */
/* enough room to hold the requested number (128) of database pages */
sqlda->sqlvar[0].sqldata = allocated buffer

/* Open the server files needing backup */
for file_num = 0 to DB_BACKUP_MAX_FILE
  db_backup( ... DB_BACKUP_OPEN_FILE, file_num ... )
  if SQLCODE == SQLE_NO_ERROR
    /* The file exists */
    num_pages = SQLCOUNT
    file_time = SQLE_IO_ESTIMATE
    open backup file with name from sqlca.sqlerrmc
end for

/* read pages from the server, write them locally */
while TRUE
  /* file_no and page_no are ignored */
  db_backup( &sqlca, DB_BACKUP_PARALLEL_READ, 0, 0, &sqlda );

  if SQLCODE != SQLE_NO_ERROR
     break;

  if buffer->stored_len == 0 || sqlda->sqlvar[0].sqlind > 0
     break;

  /* SQLCOUNT contains the starting page number of the block    */
  /* SQLIOESTIMATE contains the file number the pages belong to */
  write block of pages to appropriate backup file
end while

/* close the server backup files */
for file_num = 0 to DB_BACKUP_MAX_FILE
    /* close backup file */
    db_backup( ... DB_BACKUP_CLOSE_FILE, file_num ... )
end for

/* shut down the backup */
db_backup( ... DB_BACKUP_END ... )

/* cleanup */
free page buffer

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