MySQL:Optimizing for Many Tables: 如果你的表太多, 怎办?

8.4.3.1HowMySQLOpensandClosesTables

Whenyou execute a mysqladmin status command,you should see something likethis:

Uptime:426Runningthreads:1Questions:11082

Reloads:1Opentables:12

TheOpentablesvalueof12can be somewhat puzzlingifyou have fewer than12tables.

MySQLismultithreaded,so there may be many clients issuing queriesfora given table simultaneously.Tominimize the problemwithmultiple client sessions having different states on the same table,the tableisopened independentlybyeach concurrent session.Thisuses additional memory but normally increases performance.WithMyISAMtables,one extra file descriptorisrequiredforthe data fileforeach client that has the table open.(Bycontrast,the index file descriptorisshared between all sessions.)

The table_open_cache and max_connections system variables affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. Many operating systems permit you to increase the open-files limit, although the method varies widely from system to system. Consult your operating system documentation to determine whether it is possible to increase the limit and how to do so.

table_open_cache is related to max_connections. For example, for 200 concurrent running connections, specify a table cache size of at least 200 * N, where N is the maximum number of tables per join in any of the queries which you execute. You must also reserve some extra file descriptors for temporary tables and files.Makesure that your operating system can handle the numberofopen file descriptors impliedbythe table_open_cache setting.Iftable_open_cacheissettoo high,MySQLmay runoutoffile descriptorsandexhibit symptoms suchasrefusing connectionsorfailing to perform queries.

Alsotakeintoaccount that theMyISAMstorage engine needs two file descriptorsforeach unique open table.Fora partitionedMyISAMtable,two file descriptors are requiredforeach partitionofthe opened table.(WhenMyISAMopens a partitioned table,it opens every partitionofthistable,whetherornota given partitionisactually used.SeeMyISAMandpartition file descriptor usage.)Toincrease the numberoffile descriptors available toMySQL,setthe open_files_limit system variable.SeeSectionB.4.2.17,"File Not Found and Similar Errors".

The cache of open tables is kept at a level of table_open_cache entries. The server autosizes the cache size at startup. To set the size explicitly, set the table_open_cache system variable at startup. MySQL may temporarily open more tables than this to execute queries, as described later in this section.

MySQL closes an unused table and removes it from the table cache under the following circumstances:

When the cache is full and a thread tries to open a table that is not in the cache.

When the cache contains more than table_open_cache entries and a table in the cache is no longer being used by any threads.Whena table-flushing operation occurs.Thishappenswhensomeone issues a FLUSH TABLES statementorexecutes a mysqladmin flush-tablesormysqladmin refresh command.

Whenthe table cache fills up,the server uses the following procedure to locate a cache entry touse:

Tables not currently in use are released, beginning with the table least recently used.Ifanewtable must be opened,but the cacheisfullandnotables can be released,the cacheistemporarily extendedasnecessary.Whenthe cacheisina temporarily extended stateanda table goesfroma used to unused state,the tableisclosedandreleasedfromthe cache.

AMyISAMtableisopenedforeach concurrent access.Thismeans the table needs to be opened twiceiftwo threads access the same tableorifa thread accesses the table twiceinthe same query(forexample,byjoining the table to itself).Eachconcurrent open requires an entryinthe table cache.Thefirst openofanyMyISAMtable takes two file descriptors:oneforthe data fileandoneforthe index file.Eachadditionaluseofthe table takes only one file descriptorforthe data file.Theindex file descriptorisshared among all threads.

If you are opening a table with the HANDLER tbl_name OPEN statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads and is not closed until the thread calls HANDLER tbl_name CLOSE or the thread terminates. When this happens, the table is put back in the table cache (if the cache is not full). See Section 13.2.4, "HANDLER Statement".Todetermine whether your table cacheistoo small,check theOpened_tablesstatus variable,which indicates the numberoftable-opening operations since the server started:

MySQL>

SHOW GLOBAL STATUS LIKE'Opened_tables';

+---------------+-------+

|Variable_name|Value|

+---------------+-------+

|Opened_tables|2741|

+---------------+-------+

Ifthevalueisvery largeorincreases rapidly,evenwhenyou havenotissued many FLUSH TABLES statements,increase the table_open_cachevalueat server startup.

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