Question

I’m using a modified php mysql backup script I found on the net to backup my sql databases but I kept running into an issue with mysql_fetch_row and I was wondering if there was a way to fix it.

I commented the line with the memory error.

<?php
ini_set('memory_limit','4000M');
$ho = "host";
$us = "username";
$pa = "password";
$da='dbname';

backup_tables($ho,$us,$pa,$da);


/* backup the db OR just a table */
function backup_tables($host,$user,$pass,$name,$tables = '*')
{
    $link = mysql_connect($host,$user,$pass);
    mysql_select_db($name,$link);

    //get all of the tables
    if($tables == '*')
    {
        $tables = array();
        $result = mysql_query('SHOW TABLES');
        while($row = mysql_fetch_row($result))
        {
            $tables[] = $row[0];
        }
    }
    else
    {
        $tables = is_array($tables) ? $tables : explode(',',$tables);
    }

    $filename = 'db-backup-'.time().'-'.(md5(implode(',',$tables))).'.sql';
    $handle = fopen($filename,'w');

    //cycle through
    foreach($tables as $table)
    {
        $result = mysql_query('SELECT * FROM '.$table);
        $num_fields = mysql_num_fields($result);

        fwrite( $handle, 'DROP TABLE '.$table.';' );

        $row2 = mysql_fetch_row(mysql_query('SHOW CREATE TABLE '.$table));
        fwrite( $handle, "\n\n".$row2[1].";\n\n" );

        for ($i = 0; $i < $num_fields; $i++)
        {
            //this is the line with the error******
            while($row = mysql_fetch_row($result))
            {
                fwrite( $handle, 'INSERT INTO '.$table.' VALUES(' );
                for($j=0; $j<$num_fields; $j++)
                {
                    $row[$j] = addslashes($row[$j]);
                    $row[$j] = ereg_replace("\n","\\n",$row[$j]);
                    if (isset($row[$j])) { fwrite( $handle, '"'.$row[$j].'"' ) ; } 
                    else { fwrite( $handle, '""' ); }

                    if ($j<($num_fields-1)) { fwrite( $handle, ',' ); }
            }
            fwrite( $handle, ");\n" );
       }
     }
    fwrite( $handle, "\n\n\n" );
}

    //save file

    fclose($handle);
}
?>

and this is the error:

Fatal error: Out of memory (allocated 1338507264) (tried to allocate 35 bytes) in /home/user/backupSQL/backupmodified.php on line 47

I know there are better ways to do a backup but this meets all my requirements for my system and I only run into the memory problem with my ridiculously huge databases.

Thanks for the help.

-PHP/MySQL newbie

ps. heres the link i used http://www.htmlcenter.com/blog/back-up-your-mysql-database-with-php/

edit: mysqldump works fine when backing up these large databases, but the large dbs are the most modified ones and I cant have my dbs lock while dumping when someone needs to work on them. That’s why I resorted to this script.

Answer 1

As the other posters have mentioned, you would be better off using a ready-made tool, but let’s talk about your script for now.

If you have a ridiculously large table, then there’s just no way to allocate that much memory. You’ll have to split your data set into multiple chunks. Use a query like:

$result = mysql_query('SELECT * FROM '.$table.' LIMIT '.$start.', 10000');

Then just iterate with $start or do a while loop to check if you’re still getting results. Like this, you’ll get the data in chunks of 10000 records so it’ll surely fit in your memory.

There’s still a problem, though – consistency. What if some rows change while you’re still fetching chunks? Well, you could wrap your SELECT loop in a transaction and lock the table while you’re getting the data. I know you’re trying to avoid locks, but locking a table is better than an entire database. This still won’t ensure inter-table consistency, but it’s the best that can be done under the conditions you set.

EDIT: Here is the code for the loop. I implemented the while solution. There is an equivalent for loop, but the comparison between the two is outside the scope of this post.

foreach($tables as $table)
{
    fwrite( $handle, 'DROP TABLE '.$table.';' );

    $row2 = mysql_fetch_row(mysql_query('SHOW CREATE TABLE '.$table));
    fwrite( $handle, "\n\n".$row2[1].";\n\n" );

    $start = 0;
    do {
        $result = mysql_query( 'SELECT * FROM '.$table.' LIMIT '.$start.', 10000' );
        $start += 10000;
        $num_rows = mysql_num_rows( $result );

        while( $row = mysql_fetch_row( $result ) ) {
            $line = 'INSERT INTO '.$table.' VALUES(';
            foreach( $row as $value ) {
                $value = addslashes( $value );
                $value = ereg_replace( "\n","\\n", $value );
                $line .= '"'.$value.'",';
            }
            $line = substr( $line, 0, -1 ); // cut the final ','
            $line .= ');\n';
            fwrite( $handle, $line );
        }
    } while( $num_rows !== 0 );
}