“Understanding MySQLLocking Issues”

Contents

Server Variables Query Cache Locking Issues Internal Locking External Locking Concurrent Inserts Connection Pooling

Server VariablesThe MySQL server maintains many system

variables that indicate how it is configured.• Each system variable has its default value.• We can dynamically change these settings

using SET command.• SHOW VARIABLES• Currently there are 291 variables defined in

MySQL 5.1.6• SHOW VARIABLES LIKE 'have_query_cache';• SHOW STATUS LIKE 'Table%';

Query Cache• The have_query_cache server system variable

indicates whether the query cache is available.

• SHOW VARIABLES LIKE 'have_query_cache';• It totally depends upon query_cache_type and

query_cache_size• query_cache_type is of 0,1 and 2 (On

DEMAND)• You can set max query_cache_size (1MB) as

well as query_cache_min_res_unit (4KB)• SHOW STATUS LIKE '%qcache%';

Locking Issues• SHOW STATUS LIKE 'Table%';

To achieve a very high lock speed, MySQL uses table locking (instead of page, row, or column locking) for all storage engines except InnoDB and NDBCLUSTER.

• Internal Locking• External Locking• Concurrent Inserts

Internal LockingPerformed within the MySQL server itself to

manage contention for table contents by multiple sessions.

MySQL grants table write locks as follows:

-If there are no locks on the table, put a write lock on it.Otherwise, put the lock request in the write lock queue.

MySQL grants table read locks as follows:

-If there are no write locks on the table, put a read lock on it.Otherwise, put the lock request in the read lock queue.

External Locking• External locking is used in situations where

a single process such as the MySQL server cannot be assumed to be the only process that requires access to tables.

• If you run multiple servers that use the same database directory, each server must have external locking enabled.

• With external locking in effect, each process that requires access to a table acquires a file system lock for the table files before proceeding to access the table.

External Locking• External locking affects server performance

because the server must sometimes wait for other processes before it can access tables.

• The easiest way to satisfy these conditions is to always use --external-locking together with

• skip_external_locking = OFF • SET delay-key-write=OFF • SET query-cache-size=0.

Concurrent Inserts• The MySQL storage engine supports

concurrent inserts to reduce contention between readers and writers for a given table

• An INSERT statement can be executed to add rows to the end of the table at the same time that SELECT statements are reading rows from the table.

• If there are multiple INSERT statements, they are queued and performed in sequence, concurrently with the SELECT statements.

Concurrent Inserts- concurrent_insert = 0, 1, 2

0) If concurrent_insert is set to 0, concurrent inserts are disabled.

1) By default, the variable is set to 1 and concurrent inserts are handled.

2) If the variable is set to 2, concurrent inserts at the end of the table are permitted even for tables that have deleted rows(holes).

Connection Pooling• By default, connection pooling is enabled in

ADO.NET, Unless you explicitly disable it.• Only connections with the same

configuration can be pooled. ADO.NET keeps several pools at the same time, one for each configuration.

• Connections are separated into pools by connection string, and by Windows identity when integrated security is used.

• A connection pool is created for each unique connection string.

Connection Pooling- Connections are added to the pool as needed, up to

the maximum pool size specified (100 is the default).

- Connections are released back into the pool when they are closed or disposed.

- If MinPoolSize is either not specified in the connection string or is specified as 0, the connections in the pool will be closed after a period of inactivity.

- However, if the specified MinPoolSize is greater than zero, the connection pool is not destroyed until the AppDomain is unloaded and the process ends.

Take Away