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Disk seeks are a huge 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 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 data files 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 7.6.1, “Using Symbolic Links”.
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 (
) disk, and so on. This means if your normal data size is less than the stripe size (or perfectly aligned), you 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 7.1.5, “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 need 2 Ч
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 depend on your hardware, so we strongly suggest that you test your system thoroughly after using
hdparm. Please consult the
hdparm manual 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 do not 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.)
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 different disk.
The recommended way to do this is simply to symlink databases to a different disk. Symlink tables only as a last resort.
On Unix, the way to symlink a database is first to create a directory on some disk where you have free space and then to create a symlink to it from the MySQL data directory.
ln -s /dr1/databases/test
MySQL does not support linking one directory to multiple databases. Replacing a database directory with a symbolic link works as long as you do not 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
ln -s db1 db2
The result is that, or any table
db1, there also appears to be a table
db2. If one client updates
db1.tbl_a and another client updates
db2.tbl_a, problems are likely to occur.
However, if you really need to do this, it is possible by altering the source file
mysys/my_symlink.c, in which you should look for the following statement:
if (!(MyFlags & MY_RESOLVE_LINK) || (!lstat(filename,&stat_buff) && S_ISLNK(stat_buff.st_mode)))
Change the statement to this:
You should not symlink tables on systems that do not have a fully operational
realpath() call. (Linux and Solaris support
realpath()). You can check whether your system supports symbolic links by issuing a
SHOW VARIABLES LIKE 'have_symlink' statement.
Symlinks are fully supported only for
MyISAM tables. For files used by tables for other storage engines, you may get strange problems if you try to use symbolic links.
The handling of symbolic links for
MyISAM tables works as follows:
In the data directory, you always have the table format (
.frm) file, the data (
.MYD) file, and the index (
.MYI) file. The data file and index file can be moved elsewhere and replaced in the data directory by symlinks. The format file cannot.
You can symlink the data file and the index file independently to different directories.
You can instruct a running MySQL server to perform the symlinking by using the
DATA DIRECTORY and
INDEX DIRECTORY options to
CREATE TABLE. See Section 13.1.5, “
CREATE TABLE Syntax”. Alternatively, symlinking can be accomplished manually from the command line using
ln -s if mysqld is not running.
myisamchk does not replace a symlink with the data file or index file. It works directly on the file to which the symlink points. Any temporary files are created in the directory where the data file or index file is located. The same is true for the
OPTIMIZE TABLE, and
REPAIR TABLE statements.
Note: When you drop a table that is using symlinks, both the symlink and the file to which the symlink points are dropped. This is an extremely good reason why you should not run mysqld as the system
root or allow system users to have write access to MySQL database directories.
If you rename a table with
ALTER TABLE ... RENAME and you do not 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 is not 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.
ALTER TABLE ignores the
DATA DIRECTORY and
INDEX DIRECTORY table options.
BACKUP TABLE and
RESTORE TABLE do not respect symbolic links.
.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) produces 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
ln -s tbl1.frm tbl2.frmshell>
ln -s tbl1.MYD tbl2.MYDshell>
ln -s tbl1.MYI tbl2.MYI
Problems result if one thread reads
db1.tbl1 and another thread updates
The query cache is “fooled” (it has no way of knowing that
tbl1 has not been updated, so it returns outdated results).
ALTER statements on
Symbolic links are enabled by default for all Windows servers. This enables 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 database symbolic links work on Unix, although the procedure for setting up the link is different. If you do not need symbolic links, you can disable them using the
On Windows, create 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 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 using this procedure
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, make sure that the server is not running when you move the database directory.
Create a text file
C:\mysql\data\foo.sym that contains the pathname
After this, all tables created in the database
foo are created in
D:\data\foo. Note that the symbolic link is not used if a directory with the same name as the database exists in the MySQL data directory.
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