MyISAM Key Buffer Usage

For MyISAM one of the most important variables is the Key Buffer.  The Key Buffer is sometimes called the Key Cache. It's used as a buffer for the indices of MyISAM tables. There is some overhead in the buffer depending on the configured key block size.

The official way to calculate the key buffer usage as documented in the MySQL Reference manual:

1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)

 This will return the factor, so you have to multiply it with 100 to get the percentage. The Key_blocks_unused is used instead of the more obvious Key_blocks_used. This is due to the fact that Key_blocks_used is the maximum number of key blocks ever used. It will not return to 0 after a FLUSH TABLES.

This calculation does not take the overhead in account. The key buffer efficiency can be calculated if the key buffer is empty or (has been) completely full.

If the the key buffer is full:
key_buffer_coefficient = key_cache_block_size/(key_buffer_size/Key_blocks_used)

If the the key buffer is empty:
key_buffer_coefficient = key_cache_block_size/(key_buffer_size/Key_blocks_unused)

The 'fixed' formula:

1 - ((Key_blocks_unused * key_cache_block_size) / (key_buffer_size * key_buffer_coefficient))

The result is shown below:

The overhead in the key buffer will be allocated on startup, so the larger overhead for a larger key buffer will result in more memory usage, even if MyISAM is only used for the internal mysql tables.

Working with IP's in MySQL and MariaDB - Part 2

Use DNS directly from your database

mysql> SELECT lookup('localhost');
+---------------------+
| lookup('localhost') |
+---------------------+
| 127.0.0.1           |
+---------------------+
1 row in set (0.00 sec)

mysql> SELECT reverse_lookup('127.0.0.1');
+-----------------------------+
| reverse_lookup('127.0.0.1') |
+-----------------------------+
| localhost                   |
+-----------------------------+
1 row in set (0.00 sec)

This is how you install these functions.

1. Build udf_example.so which is in your mysql source. (make udf_example.so)
2. Copy the udf_example.so file from your build directory to your plugin_dir.
3. Create the lookup and reverse_lookup functions

mysql> CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';

Query OK, 0 rows affected (0.00 sec) 

mysql> CREATE FUNCTION reverse_lookup RETURNS STRING SONAME 'udf_example.so';
Query OK, 0 rows affected (0.00 sec)

I've created a feature request for IPv6 support to these functions.

Working with IP's in MySQL and MariaDB

For MySQL it's a best practice to store IP addresses in a INT column rather than in a VARCHAR. This allows for more compact storage. This best practice is only for MySQL as PostgreSQL does have data types like inet and cidr.

MySQL is equiped with the INET_ATON() and INET_NTOA() functions since version 3.23.15. In MySQL 5.6.3 the INET6_ATON() and INET6_NTOA() functions were addes to finaly add IPv6 address support. To make it easier to work with IPv4 and IPv6 addresses the IS_IPV4_COMPAT(), IS_IPV4_MAPPED(), IS_IPV4() and IS_IPV6() functions were added. The difference between the new and old functions is that the old functions use INT UNSIGNED to store IP addresses and the new function uses VARBINARY(16) for IPv6 addresses and VARBINARY(4) for IPv4 addresses.

Here are some examples about how you could do nifty things with IP addresses in MySQL.

Store hosts and networks in MySQL

CREATE TABLE `hosts` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`hostname` varchar(255) DEFAULT NULL,
`domainname` varchar(255) DEFAULT NULL,
`ip` int(10) unsigned NOT NULL,
PRIMARY KEY (`id`),
KEY `ip` (`ip`)
)

CREATE TABLE `networks` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`network` int(10) unsigned NOT NULL,
`mask` int(10) unsigned NOT NULL,
`name` varchar(255) DEFAULT '',
PRIMARY KEY (`id`),
KEY `network` (`network`),
KEY `mask` (`mask`)
)

Insert and retrieve entries

mysql> INSERT INTO `hosts`(`hostname`,`domainname`,`ip`) VALUES('test6','example.com',INET_ATON('192.168.1.2'));
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM `hosts` WHERE `id`=LAST_INSERT_ID();
+----+----------+-------------+------------+
| id | hostname | domainname  | ip         |
+----+----------+-------------+------------+
|  6 | test6    | example.com | 3232235778 |
+----+----------+-------------+------------+
1 row in set (0.00 sec)

mysql> SELECT hostname,domainname,INET_NTOA(ip) ip FROM `hosts` WHERE `id`=LAST_INSERT_ID();
+----------+-------------+-------------+
| hostname | domainname  | ip          |
+----------+-------------+-------------+
| test6    | example.com | 192.168.1.2 |
+----------+-------------+-------------+
1 row in set (0.00 sec)

Convert a subnetmask to CIDR notation.

mysql> SELECT INET_ATON('255.255.255.0'),BIT_COUNT(INET_ATON('255.255.255.0'));
+----------------------------+---------------------------------------+
| INET_ATON('255.255.255.0') | BIT_COUNT(INET_ATON('255.255.255.0')) |
+----------------------------+---------------------------------------+
|                 4294967040 |                                    24 |
+----------------------------+---------------------------------------+
1 row in set (0.00 sec)

mysql> CREATE FUNCTION mask_to_cidr (mask CHAR(15)) RETURNS INT(2) DETERMINISTIC RETURN BIT_COUNT(INET_ATON(mask));
Query OK, 0 rows affected (0.00 sec)

mysql> SET @mask='255.255.252.0';
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @mask,mask_to_cidr(@mask);
+---------------+---------------------+
| @mask         | mask_to_cidr(@mask) |
+---------------+---------------------+
| 255.255.252.0 |                  22 |
+---------------+---------------------+
1 row in set (0.00 sec)

And convert a CIDR mask back to dot-decimal notation

mysql> CREATE FUNCTION cidr_to_mask (cidr INT(2)) RETURNS CHAR(15) DETERMINISTIC RETURN INET_NTOA(CONV(CONCAT(REPEAT(1,cidr),REPEAT(0,32-cidr)),2,10));
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT cidr_to_mask(22);
+------------------+
| cidr_to_mask(22) |
+------------------+
| 255.255.252.0    |
+------------------+
1 row in set (0.00 sec)

Use the & operator to find the network address using an IP and mask.

mysql> SELECT INET_NTOA(INET_ATON('255.255.255.0') & INET_ATON('192.168.2.3'));
+------------------------------------------------------------------+
| INET_NTOA(INET_ATON('255.255.255.0') & INET_ATON('192.168.2.3')) |
+------------------------------------------------------------------+
| 192.168.2.0                                                      |
+------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SET @netmask='255.255.255.0';
Query OK, 0 rows affected (0.00 sec)

mysql> SET @ipaddr = '192.168.2.6';Query OK, 0 rows affected (0.00 sec)

mysql> SELECT CONV(INET_ATON(@ipaddr),10,2) ip,CONV(INET_ATON(@netmask),10,2) mask,CONV(INET_ATON(@ipaddr) & INET_ATON(@netmask),10,2) network\G
*************************** 1. row ***************************
ip: 11000000101010000000001000000110
mask: 11111111111111111111111100000000
network: 11000000101010000000001000000000
1 row in set (0.00 sec)

This can be used to join a table with host ip's with a table of networks. (Remember the netmasks table in Solaris and/or NIS?)

mysql> SELECT CONCAT(h.hostname,'.',h.domainname) fqdn, INET_NTOA(h.ip) ip, INET_NTOA(n.mask & h.ip) network, INET_NTOA(n.mask) mask FROM hosts h INNER JOIN networks n ON (n.mask & h.ip) = n.network;
+-------------------+-------------+-------------+---------------+
| fqdn              | ip          | network     | mask          |
+-------------------+-------------+-------------+---------------+
| test1.example.com | 192.168.0.1 | 192.168.0.0 | 255.255.255.0 |
| test2.example.com | 192.168.0.2 | 192.168.0.0 | 255.255.255.0 |
| test3.example.com | 192.168.0.3 | 192.168.0.0 | 255.255.255.0 |
| test4.example.com | 10.0.0.1    | 10.0.0.0    | 255.0.0.0     |
| test5.example.com | 10.0.0.2    | 10.0.0.0    | 255.0.0.0     |
+-------------------+-------------+-------------+---------------+
5 rows in set (0.00 sec)

With the ~ operator the mask can be inverted and then be used to find the broadcast address using the XOR operator |.

mysql> SELECT CONV(INET_ATON(@ipaddr),10,2) ip,CONV(INET_ATON(@netmask),10,2) mask,CONV(INET_ATON(@ipaddr) & INET_ATON(@netmask),10,2) network, CONV(CONV(SUBSTRING(CONV(~INET_ATON('255.255.255.0'),10,2),-32),2,10) | INET_ATON(@ipaddr),10,2) broadcast\G
*************************** 1. row ***************************
ip: 11000000101010000000001000000110
mask: 11111111111111111111111100000000
network: 11000000101010000000001000000000
broadcast: 11000000101010000000001011111111
1 row in set (0.00 sec)

You could use a view to make it easier for users to select the rows they wanted without using functions. But that will result in a full scan if you search using an IP. MySQL does not optimize this by first converting the search value to an INT but converts all rows to a dot-decimal notation and then compares it. MariaDB has a nifty feature called persistent virtual columns which allow indexing. For the example below this means that instead of scanning all four rows it just uses the index to find the row we need.

mysql> CREATE TABLE `iptest`(`ip_n` int(11) UNSIGNED NOT NULL, `ip_a` CHAR(15) AS (INET_NTOA(`ip_n`)) PERSISTENT, PRIMARY KEY(`ip_n`), KEY `ip_a` (`ip_a`));
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO `iptest`(`ip_n`) VALUES (INET_ATON('192.168.1.1')),(INET_ATON('192.168.1.2')),(INET_ATON('192.168.1.3')),(INET_ATON('192.168.1.4'));
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM `iptest`;
+------------+-------------+
| ip_n       | ip_a        |
+------------+-------------+
| 3232235777 | 192.168.1.1 |
| 3232235778 | 192.168.1.2 |
| 3232235779 | 192.168.1.3 |
| 3232235780 | 192.168.1.4 |
+------------+-------------+
4 rows in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM `iptest` WHERE `ip_a`='192.168.1.2';
+----+-------------+--------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | iptest | ref  | ip_a          | ip_a | 16      | const |    1 | Using where |
+----+-------------+--------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql> CREATE VIEW v_iptest AS SELECT ip_n,INET_NTOA(ip_n) ip_a FROM `iptest`;
Query OK, 0 rows affected (0.00 sec)

mysql> EXPLAIN SELECT * FROM `v_iptest` WHERE `ip_a`='192.168.1.2';
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | iptest | index | NULL          | PRIMARY | 4       | NULL |    4 | Using where; Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)