(mysql.info.gz) Multiple key caches
Info Catalog
(mysql.info.gz) Shared key cache
(mysql.info.gz) MyISAM key cache
(mysql.info.gz) Midpoint insertion
7.4.6.2 Multiple Key Caches
...........................
Shared access to the key cache improves performance but does not
eliminate contention among threads entirely. They still compete for
control structures that manage access to the key cache buffers. To
reduce key cache access contention further, MySQL 4.1.1 offers the
feature of multiple key caches. This allows you to assign different
table indexes to different key caches.
When there can be multiple key caches, the server must know which cache
to use when processing queries for a given `MyISAM' table. By default,
all `MyISAM' table indexes are cached in the default key cache. To
assign table indexes to a specific key cache, use the `CACHE INDEX'
statement.
For example, the following statement assigns indexes from the tables
`t1', `t2', and `t3' to the key cache named `hot_cache':
mysql> CACHE INDEX t1, t2, t3 IN hot_cache;
+---------+--------------------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+---------+--------------------+----------+----------+
| test.t1 | assign_to_keycache | status | OK |
| test.t2 | assign_to_keycache | status | OK |
| test.t3 | assign_to_keycache | status | OK |
+---------+--------------------+----------+----------+
* If the server has been built with the `ISAM' storage engine
enabled, `ISAM' tables use the key cache mechanism. However, `ISAM'
indexes use only the default key cache and cannot be reassigned to a
different cache.
The key cache referred to in a `CACHE INDEX' statement can be created
by setting its size with a `SET GLOBAL' parameter setting statement or
by using server startup options. For example:
mysql> SET GLOBAL keycache1.key_buffer_size=128*1024;
To destroy a key cache, set its size to zero:
mysql> SET GLOBAL keycache1.key_buffer_size=0;
Key cache variables are structured system variables that have a name and
components. For `keycache1.key_buffer_size', `keycache1' is the cache
Structured System Variables:: for a description of the syntax used for
referring to structured key cache system variables.
By default, table indexes are assigned to the main (default) key cache
created at the server startup. When a key cache is destroyed, all
indexes assigned to it are reassigned to the default key cache.
For a busy server, we recommend a strategy that uses three key caches:
* A hot key cache that takes up 20% of the space allocated for all
key caches. This is used for tables that are heavily used for
searches but that are not updated.
* A cold key cache that takes up 20% of the space allocated for all
key caches. This is used for medium-sized intensively modified
tables, such as temporary tables.
* A warm key cache that takes up 60% of the key cache space. This
is the default key cache, to be used by default for all other
tables.
One reason the use of three key caches is beneficial is that access to
one key cache structure does not block access to the others. Queries
that access tables assigned to one cache do not compete with queries
that access tables assigned to another cache. Performance gains occur
for other reasons as well:
* The hot cache is used only for retrieval queries, so its contents
are never modified. Consequently, whenever an index block needs
to be pulled in from disk, the contents of the cache block chosen
for replacement need not be flushed first.
* For an index assigned to the hot cache, if there are no queries
requiring an index scan, there is a high probability that the index
blocks corresponding to non-leaf nodes of the index B-tree will
remain in the cache.
* An update operation most frequently executed for temporary tables
is performed much faster when the updated node is in the cache and
need not be read in from disk first. If the size of the indexes of
the temporary tables are comparable with the size of cold key
cache, the probability is very high that the updated node will be
in the cache.
`CACHE INDEX' sets up an association between a table and a key cache,
but the association is lost each time the server restarts. If you want
the association to take effect each time the server starts, one way to
accomplish this is to use an option file: Include variable settings
that configure your key caches, and an `init-file' option that names a
file containing `CACHE INDEX' statements to be executed. For example:
key_buffer_size = 4G
hot_cache.key_buffer_size = 2G
cold_cache.key_buffer_size = 2G
init_file=/path/to/data-directory/mysqld_init.sql
The statements in `mysqld_init.sql' will be executed each time the
server starts. It should contain one SQL statement per line. The
following example assigns several tables each to `hot_cache' and
`cold_cache':
CACHE INDEX a.t1, a.t2, b.t3 IN hot_cache
CACHE INDEX a.t4, b.t5, b.t6 IN cold_cache
Info Catalog
(mysql.info.gz) Shared key cache
(mysql.info.gz) MyISAM key cache
(mysql.info.gz) Midpoint insertion
automatically generated byinfo2html