6.6 Disk Issues

• Disk seeks are a big performance bottleneck. This problem becomes more apparent when the amount of data starts to grow so large that effective caching becomes impossible . For large databases where you access data more or less randomly , you can be sure that you will need at least one disk seek to read and a couple of disk seeks to write things. To minimize this problem, use disks with low seek times.

• Increase the number of available disk spindles (and thereby reduce the seek overhead) by either symlinking files to different disks or striping the disks:

  • Using symbolic links

    This means that, for MyISAM tables, you symlink the index file and/or data file from their usual location in the data directory to another disk (that may also be striped). This makes both the seek and read times better, assuming that the disk is not used for other purposes as well. See Section 6.6.1, "Using Symbolic Links."

  • Striping

    Striping means that you have many disks and put the first block on the first disk, the second block on the second disk, and the N th block on the ( N mod number_of_disks ) disk, and so on. This means if your normal data size is less than the stripe size (or perfectly aligned), you will get much better performance. Striping is very dependent on the operating system and the stripe size, so benchmark your application with different stripe sizes. See Section 6.1.5, "Using Your Own Benchmarks."

    The speed difference for striping is very dependent on the parameters. Depending on how you set the striping parameters and number of disks, you may get differences measured in orders of magnitude. You have to choose to optimize for random or sequential access.

• For reliability you may want to use RAID 0+1 (striping plus mirroring), but in this case, you will need 2* N drives to hold N drives of data. This is probably the best option if you have the money for it! However, you may also have to invest in some volume- management software to handle it efficiently .

• A good option is to vary the RAID level according to how critical a type of data is. For example, store semi-important data that can be regenerated on a RAID 0 disk, but store really important data such as host information and logs on a RAID 0+1 or RAID N disk. RAID N can be a problem if you have many writes , due to the time required to update the parity bits.

• On Linux, you can get much more performance by using hdparm to configure your disk's interface. (Up to 100% under load is not uncommon.) The following hdparm options should be quite good for MySQL, and probably for many other applications:

  
  

   hdparm -m 16 -d 1 

  
  

  Note that performance and reliability when using this command depends on your hardware, so we strongly suggest that you test your system thoroughly after using hdparm . Please consult the hdparm man page for more information. If hdparm is not used wisely, filesystem corruption may result, so back up everything before experimenting!

• You can also set the parameters for the filesystem that the database uses:

  If you don't need to know when files were last accessed (which is not really useful on a database server), you can mount your filesystems with the -o noatime option. That skips updates to the last access time in inodes on the filesystem, which avoids some disk seeks.

  On many operating systems, you can set a filesystem to be updated asynchronously by mounting it with the -o async option. If your computer is reasonably stable, this should give you more performance without sacrificing too much reliability. (This flag is on by default on Linux.)

6.6.1 Using Symbolic Links

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space or increase the speed of your system by spreading your tables to a different disk.

The recommended way to do this is to just symlink databases to a different disk. Symlink tables only as a last resort.

6.6.1.1 Using Symbolic Links for Databases on Unix

On Unix, the way to symlink a database is to first create a directory on some disk where you have free space and then create a symlink to it from the MySQL data directory.

 shell>  mkdir /dr1/databases/test  shell>  ln -s /dr1/databases/test   /path/to/datadir  

MySQL doesn't support linking one directory to multiple databases. Replacing a database directory with a symbolic link will work fine as long as you don't make a symbolic link between databases. Suppose that you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1 :

 shell>  cd   /path/to/datadir  shell>  ln -s db1 db2  

Now, for any table tbl_a in db1 , there also appears to be a table tbl_a in db2 . If one client updates db1.tbl_a and another client updates db2.tbl_a , there will be problems.

If you really need to do this, you can change one of the source files. The file to modify depends on your version of MySQL. For MySQL 4.0 and up, look for the following statement in the mysys/my_symlink.c file:

 if (!(MyFlags & MY_RESOLVE_LINK)      (!lstat(filename,&stat_buff) && S_ISLNK(stat_buff.st_mode))) 

Before MySQL 4.0, look for this statement in the mysys/mf_format.c file:

 if (flag & 32  (!lstat(to,&stat_buff) && S_ISLNK(stat_buff.st_mode))) 

Change the statement to this:

 if (1) 

On Windows, you can use internal symbolic links to directories by compiling MySQL with -DUSE_SYMDIR . This allows you to put different databases on different disks. See Section 6.6.1.3, "Using Symbolic Links for Databases on Windows."

6.6.1.2 Using Symbolic Links for Tables on Unix

Before MySQL 4.0, you should not symlink tables unless you are very careful with them. The problem is that if you run ALTER TABLE , REPAIR TABLE , or OPTIMIZE TABLE on a symlinked table, the symlinks will be removed and replaced by the original files. This happens because these statements work by creating a temporary file in the database directory and replacing the original file with the temporary file when the statement operation is complete.

You should not symlink tables on systems that don't have a fully working realpath() call. (At least Linux and Solaris support realpath() ). You can check whether your system supports symbolic links by issuing a SHOW VARIABLES LIKE 'have_symlink' statement.

In MySQL 4.0, symlinks are fully supported only for MyISAM tables. For other table types, you will probably get strange problems if you try to use symbolic links on files in the operating system with any of the preceding statements.

The handling of symbolic links for MyISAM tables in MySQL 4.0 works the following way:

• In the data directory, you will always have the table definition file, the data file, and the index file. The data file and index file can be moved elsewhere and replaced in the data directory by symlinks. The definition file cannot.

• You can symlink the data file and the index file independently to different directories.

• The symlinking can be done manually from the command line with ln -s if mysqld is not running. With SQL, you can instruct the server to perform the symlinking by using the DATA DIRECTORY and INDEX DIRECTORY options to CREATE TABLE .

• myisamchk will not replace a symlink with the data file or index file. It works directly on the file a symlink points to. Any temporary files are created in the directory where the data file or index file is located.

• When you drop a table that is using symlinks, both the symlink and the file the symlink points to are dropped. This is a good reason why you should not run mysqld as root or allow users to have write access to the MySQL database directories.

• If you rename a table with ALTER TABLE ... RENAME and you don't move the table to another database, the symlinks in the database directory are renamed to the new names and the data file and index file are renamed accordingly .

• If you use ALTER TABLE ... RENAME to move a table to another database, the table is moved to the other database directory. The old symlinks and the files to which they pointed are deleted. In other words, the new table will not be symlinked.

• If you are not using symlinks, you should use the --skip-symbolic-links option to mysqld to ensure that no one can use mysqld to drop or rename a file outside of the data directory.

SHOW CREATE TABLE doesn't report if a table has symbolic links prior to MySQL 4.0.15. This is also true for mysqldump , which uses SHOW CREATE TABLE to generate CREATE TABLE statements.

Table symlink operations that are not yet supported:

• ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.

• BACKUP TABLE and RESTORE TABLE don't respect symbolic links.

• The .frm file must never be a symbolic link (as indicated previously, only the data and index files can be symbolic links). Attempting to do this (for example, to make synonyms) will produce incorrect results. Suppose that you have a database db1 under the MySQL data directory, a table tbl1 in this database, and in the db1 directory you make a symlink tbl2 that points to tbl1 :

  
  

   shell>  cd   /path/to/datadir   /db1  shell>  ln -s tbl1.frm tbl2.frm  shell>  ln -s tbl1.MYD tbl2.MYD  shell>  ln -s tbl1.MYI tbl2.MYI  

  
  

  Now there will be problems if one thread reads db1.tbl1 and another thread updates db1.tbl2 :

  • The query cache will be fooled (it will believe tbl1 has not been updated so will return out-of-date results).

  • ALTER statements on tbl2 will also fail.

6.6.1.3 Using Symbolic Links for Databases on Windows

Beginning with MySQL 3.23.16, the mysqld-max and mysql-max-nt servers for Windows are compiled with the -DUSE_SYMDIR option. This allows you to put a database directory on a different disk by setting up a symbolic link to it. This is similar to the way that symbolic links work on Unix, although the procedure for setting up the link is different.

As of MySQL 4.0, symbolic links are enabled by default. If you don't need them, you can disable them with the skip-symbolic-links option:

 [mysqld] skip-symbolic-links 

Before MySQL 4.0, symbolic links are disabled by default. To enable them, you should put the following entry in your my.cnf or my.ini file:

 [mysqld] symbolic-links 

On Windows, you make a symbolic link to a MySQL database by creating a file in the data directory that contains the path to the destination directory. The file should be named db_name .sym , where db_name is the database name .

Suppose that the MySQL data directory is C:\mysql\data and you want to have database foo located at D:\data\foo . Set up a symlink like this:

1. Make sure that the D:\data\foo directory exists by creating it if necessary. If you already have a database directory named foo in the data directory, you should move it to D:\data . Otherwise, the symbolic link will be ineffective . To avoid problems, the server should not be running when you move the database directory.

2. Create a file C:\mysql\data\foo.sym that contains the pathname D:\data\foo\ .

After that, all tables created in the database foo will be created in D:\data\foo . Note that the symbolic link will not be used if a directory with the database name exists in the MySQL data directory.