本文档风哥主要介绍Oracle数据库空闲缓冲区等待(Free Buffer Waits)相关知识,包括空闲缓冲区等待的概念、原因、影响、规划、配置、管理、监控、优化等内容,由风哥教程参考Oracle官方文档Performance内容编写,适合DBA人员在学习和测试中使用,如果要应用于生产环境则需要自行确认。
Part01-基础概念与理论知识
1.1 空闲缓冲区等待的概念
Oracle数据库空闲缓冲区等待(Free Buffer Waits)是指当会话需要从磁盘读取数据块到缓冲区缓存时,发现没有可用的空闲缓冲区,需要等待DBWR进程将脏块写入磁盘以释放缓冲区。这种等待通常发生在缓冲区缓存不足或DBWR进程无法及时写入脏块时。更多视频教程www.fgedu.net.cn
- 发生在需要读取数据块但缓冲区缓存已满时
- 通常与DBWR进程和缓冲区缓存相关
- 在缓冲区缓存不足时更容易发生
- 会导致会话等待时间增加
- 影响系统整体性能
1.2 空闲缓冲区等待的原因
Oracle数据库空闲缓冲区等待的原因:
- 缓冲区缓存不足:缓冲区缓存过小,无法容纳足够的数据块
- DBWR进程繁忙:DBWR进程无法及时将脏块写入磁盘
- 大量全表扫描:大量全表扫描导致缓冲区缓存被快速填满
- I/O子系统瓶颈:磁盘I/O性能不足,导致写入缓慢
- 检查点过于频繁:检查点操作过于频繁,影响DBWR性能
1.3 空闲缓冲区等待的影响
Oracle数据库空闲缓冲区等待的影响:
- 会话等待时间增加:会话需要等待空闲缓冲区才能读取数据块
- 系统性能下降:大量的空闲缓冲区等待会导致系统性能下降
- 响应时间增加:SQL语句执行时间增加
- I/O负载增加:频繁的磁盘读写增加I/O负载
- 并发性能降低:影响系统的并发处理能力
Part02-生产环境规划与建议
2.1 空闲缓冲区等待规划
Oracle数据库空闲缓冲区等待规划要点:
1. 分析系统架构
2. 评估数据访问模式
3. 分析缓冲区缓存使用情况
4. 设计缓冲区缓存策略
5. 优化DBWR配置
6. 配置相关参数
7. 测试和验证
8. 监控和优化
# 适用场景
– 大量数据读取系统
– 全表扫描频繁系统
– 缓冲区缓存不足系统
– I/O密集型系统
# 不适用场景
– 只读系统
– 小型数据库
– 内存充足系统
2.2 空闲缓冲区等待设计
Oracle数据库空闲缓冲区等待设计建议:
– 基于系统规模设计
– 基于数据访问模式设计
– 最小化空闲缓冲区等待
– 最大化系统性能
– 合理配置参数
# 缓冲区缓存设计
– 根据数据量设计缓冲区缓存大小
– 使用多个缓冲区池
– 配置KEEP和RECYCLE池
– 优化缓冲区缓存命中率
# 设计步骤
1. 分析系统需求
2. 评估数据访问模式
3. 设计缓冲区缓存策略
4. 优化DBWR配置
5. 配置相关参数
6. 测试性能效果
7. 调整配置
2.3 空闲缓冲区等待最佳实践
Oracle数据库空闲缓冲区等待最佳实践:
- 增加缓冲区缓存:根据系统需求增加缓冲区缓存大小
- 优化DBWR进程:增加DBWR进程数量,提高脏块写入效率
- 使用多个缓冲区池:使用KEEP和RECYCLE池优化缓冲区使用
- 优化SQL语句:减少全表扫描,提高SQL效率
- 优化I/O子系统:使用高速存储设备,提高I/O性能
- 监控空闲缓冲区等待:定期监控空闲缓冲区等待情况,及时发现和处理问题
Part03-生产环境项目实施方案
3.1 空闲缓冲区等待配置
3.1.1 配置缓冲区缓存
SQL> SHOW PARAMETER db_cache_size;
NAME TYPE VALUE
———————————— ———– ——————————
db_cache_size big integer 0
SQL> SHOW PARAMETER db_keep_cache_size;
NAME TYPE VALUE
———————————— ———– ——————————
db_keep_cache_size big integer 0
SQL> SHOW PARAMETER db_recycle_cache_size;
NAME TYPE VALUE
———————————— ———– ——————————
db_recycle_cache_size big integer 0
# 调整缓冲区缓存大小
SQL> ALTER SYSTEM SET db_cache_size = 4G SCOPE=BOTH;
System altered.
# 配置KEEP池
SQL> ALTER SYSTEM SET db_keep_cache_size = 1G SCOPE=BOTH;
System altered.
# 配置RECYCLE池
SQL> ALTER SYSTEM SET db_recycle_cache_size = 512M SCOPE=BOTH;
System altered.
# 查看缓冲区缓存统计信息
SQL> SELECT
name,
block_size,
current_size,
buffers
FROM v$buffer_pool;
NAME BLOCK_SIZE CURRENT_SIZE BUFFERS
——————– ———- ———— ———-
DEFAULT 8192 4096 524288
KEEP 8192 1024 131072
RECYCLE 8192 512 65536
3.1.2 配置DBWR进程
SQL> SHOW PARAMETER db_writer_processes;
NAME TYPE VALUE
———————————— ———– ——————————
db_writer_processes integer 1
# 调整DBWR进程数量
SQL> ALTER SYSTEM SET db_writer_processes = 4 SCOPE=SPFILE;
System altered.
# 重启数据库使设置生效
SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP;
# 查看DBWR进程状态
SQL> SELECT
name,
description
FROM v$bgprocess
WHERE paddr <> ’00’
AND name LIKE ‘DBW%’;
NAME DESCRIPTION
—– ——————————
DBW0 db writer process 0
DBW1 db writer process 1
DBW2 db writer process 2
DBW3 db writer process 3
3.1.3 配置表使用KEEP池
SQL> ALTER TABLE employees STORAGE (BUFFER_POOL KEEP);
Table altered.
# 将索引放入KEEP池
SQL> ALTER INDEX emp_id_idx STORAGE (BUFFER_POOL KEEP);
Index altered.
# 查看表的缓冲区池配置
SQL> SELECT
table_name,
buffer_pool
FROM user_tables
WHERE table_name = ‘EMPLOYEES’;
TABLE_NAME BUFFER_POOL
——————– ———–
EMPLOYEES KEEP
# 将表放入RECYCLE池
SQL> ALTER TABLE temp_table STORAGE (BUFFER_POOL RECYCLE);
Table altered.
3.2 空闲缓冲区等待管理
3.2.1 管理空闲缓冲区等待
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 1000 5000 5.0
# 查看会话级别的空闲缓冲区等待
SQL> SELECT
s.sid,
s.username,
sw.event,
sw.wait_class,
sw.seconds_in_wait
FROM v$session s,
v$session_wait sw
WHERE s.sid = sw.sid
AND s.status = ‘ACTIVE’
AND sw.event = ‘free buffer waits’;
SID USERNAME EVENT WAIT_CLASS SECONDS_IN_WAIT
— ————- —————————— ——————– —————
123 SCOTT free buffer waits Configuration 10
124 HR free buffer waits Configuration 5
# 查看缓冲区缓存使用情况
SQL> SELECT
name,
physical_reads,
db_block_gets,
consistent_gets,
(1 – (physical_reads / (db_block_gets + consistent_gets))) * 100 AS hit_ratio
FROM v$buffer_pool_statistics
WHERE name = ‘DEFAULT’;
NAME PHYSICAL_READS DB_BLOCK_GETS CONSISTENT_GETS HIT_RATIO
——————– ————– ————- ————— ———–
DEFAULT 10000 100000 900000 99.0
3.2.2 分析空闲缓冲区等待
SQL> SELECT
name,
physical_reads,
db_block_gets,
consistent_gets,
(1 – (physical_reads / (db_block_gets + consistent_gets))) * 100 AS hit_ratio
FROM v$buffer_pool_statistics;
NAME PHYSICAL_READS DB_BLOCK_GETS CONSISTENT_GETS HIT_RATIO
——————– ————– ————- ————— ———–
DEFAULT 10000 100000 900000 99.0
KEEP 100 5000 45000 99.8
RECYCLE 1000 500 4500 80.0
# 查看脏块数量
SQL> SELECT
status,
COUNT(*)
FROM v$bh
WHERE dirty = ‘Y’
GROUP BY status;
STATUS COUNT(*)
——— ———-
xcu 50
free 0
# 查看缓冲区缓存中的对象
SQL> SELECT
o.object_name,
o.object_type,
COUNT(*) AS buffer_count
FROM v$bh b,
dba_objects o
WHERE b.objd = o.data_object_id
AND b.status != ‘free’
GROUP BY o.object_name,
o.object_type
ORDER BY buffer_count DESC
FETCH FIRST 10 ROWS ONLY;
OBJECT_NAME OBJECT_TYPE BUFFER_COUNT
——————– —————— ————
EMPLOYEES TABLE 1000
EMP_ID_IDX INDEX 500
DEPARTMENTS TABLE 200
3.3 空闲缓冲区等待监控
3.3.1 监控空闲缓冲区等待
SQL> SELECT
event,
wait_class,
COUNT(*)
FROM v$session_wait
WHERE event = ‘free buffer waits’
GROUP BY event, wait_class;
EVENT WAIT_CLASS COUNT(*)
—————————— ——————– ——–
free buffer waits Configuration 5
# 使用AWR报告监控空闲缓冲区等待
SQL> @$ORACLE_HOME/rdbms/admin/awrrpt.sql
# 输入报告类型:html
# 输入开始快照ID:100
# 输入结束快照ID:101
# 输入报告文件名:awrrpt.html
# 查看AWR报告中的空闲缓冲区等待部分
# Top 5 Wait Events
~~~~~~~~~~~~~~~~~~~
Avg
wait % DB
Event Waits Time(s) (ms) time Wait Class
———————————— ———– ———– —— —— ———-
free buffer waits 1000 5000 500 40.0 Configuration
db file scattered read 500 2500 500 20.0 User I/O
3.3.2 监控空闲缓冲区等待历史
SQL> SELECT
h.snap_id,
h.event_name,
SUM(h.total_waits) AS total_waits,
SUM(h.time_waited_micro) / 1000000 AS time_waited_seconds
FROM dba_hist_system_event h
WHERE h.snap_id BETWEEN 100 AND 101
AND h.event_name = ‘free buffer waits’
GROUP BY h.snap_id, h.event_name
ORDER BY h.snap_id;
SNAP_ID EVENT_NAME TOTAL_WAITS TIME_WAITED_SECONDS
——- —————————— ———– ——————
100 free buffer waits 500 25
101 free buffer waits 500 25
# 查看会话空闲缓冲区等待历史
SQL> SELECT
h.session_id,
h.event_name,
COUNT(*) AS wait_count,
SUM(h.wait_time + h.time_waited) / 100 AS total_wait_time_seconds
FROM dba_hist_active_sess_history h
WHERE h.sample_time BETWEEN SYSDATE – 1/24 AND SYSDATE
AND h.event_name = ‘free buffer waits’
GROUP BY h.session_id, h.event_name
ORDER BY total_wait_time_seconds DESC
FETCH FIRST 10 ROWS ONLY;
SESSION_ID EVENT_NAME WAIT_COUNT TOTAL_WAIT_TIME_SECONDS
———- —————————— ———– ———————-
123 free buffer waits 100 500
124 free buffer waits 50 250
Part04-生产案例与实战讲解
4.1 空闲缓冲区等待实施案例
在某企业的生产环境中,需要实施空闲缓冲区等待监控和优化,提高数据库性能。
– 数据库版本:Oracle 19c
– 系统规模:中等规模,日交易量100万
– 问题:数据库性能下降,空闲缓冲区等待严重
# 实施方案
1. 监控空闲缓冲区等待
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 1000 5000 5.0
2. 分析缓冲区缓存命中率
SQL> SELECT
name,
physical_reads,
db_block_gets,
consistent_gets,
(1 – (physical_reads / (db_block_gets + consistent_gets))) * 100 AS hit_ratio
FROM v$buffer_pool_statistics;
NAME PHYSICAL_READS DB_BLOCK_GETS CONSISTENT_GETS HIT_RATIO
——————– ————– ————- ————— ———–
DEFAULT 10000 100000 900000 90.0
KEEP 0 0 0 0
RECYCLE 0 0 0 0
3. 查看脏块数量
SQL> SELECT
status,
COUNT(*)
FROM v$bh
WHERE dirty = ‘Y’
GROUP BY status;
STATUS COUNT(*)
——— ———-
xcu 50
free 0
4. 查看缓冲区缓存中的对象
SQL> SELECT
o.object_name,
o.object_type,
COUNT(*) AS buffer_count
FROM v$bh b,
dba_objects o
WHERE b.objd = o.data_object_id
AND b.status != ‘free’
GROUP BY o.object_name,
o.object_type
ORDER BY buffer_count DESC
FETCH FIRST 10 ROWS ONLY;
OBJECT_NAME OBJECT_TYPE BUFFER_COUNT
——————– —————— ————
EMPLOYEES TABLE 1000
EMP_ID_IDX INDEX 500
DEPARTMENTS TABLE 200
5. 优化措施
– 增加缓冲区缓存大小
SQL> ALTER SYSTEM SET db_cache_size = 4G SCOPE=BOTH;
– 增加DBWR进程数量
SQL> ALTER SYSTEM SET db_writer_processes = 4 SCOPE=SPFILE;
– 配置KEEP池
SQL> ALTER SYSTEM SET db_keep_cache_size = 1G SCOPE=BOTH;
– 将常用表放入KEEP池
SQL> ALTER TABLE employees STORAGE (BUFFER_POOL KEEP);
SQL> ALTER INDEX emp_id_idx STORAGE (BUFFER_POOL KEEP);
– 重启数据库使设置生效
SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP;
6. 验证优化效果
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 100 500 5.0
# 实施效果
– 空闲缓冲区等待减少90%
– 数据库性能提高50%
– SQL执行时间减少70%
– 系统稳定性提高
4.2 空闲缓冲区等待优化案例
在某金融机构的生产环境中,需要优化空闲缓冲区等待,提高数据库性能。
– 数据库版本:Oracle 19c
– 系统规模:大规模,日交易量1000万
– 问题:空闲缓冲区等待严重,影响数据库性能
# 优化方案
1. 监控空闲缓冲区等待
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 5000 25000 5.0
2. 分析缓冲区缓存命中率
SQL> SELECT
name,
physical_reads,
db_block_gets,
consistent_gets,
(1 – (physical_reads / (db_block_gets + consistent_gets))) * 100 AS hit_ratio
FROM v$buffer_pool_statistics;
NAME PHYSICAL_READS DB_BLOCK_GETS CONSISTENT_GETS HIT_RATIO
——————– ————– ————- ————— ———–
DEFAULT 50000 500000 4500000 90.0
KEEP 0 0 0 0
RECYCLE 0 0 0 0
3. 查看脏块数量
SQL> SELECT
status,
COUNT(*)
FROM v$bh
WHERE dirty = ‘Y’
GROUP BY status;
STATUS COUNT(*)
——— ———-
xcu 500
free 0
4. 查看缓冲区缓存中的对象
SQL> SELECT
o.object_name,
o.object_type,
COUNT(*) AS buffer_count
FROM v$bh b,
dba_objects o
WHERE b.objd = o.data_object_id
AND b.status != ‘free’
GROUP BY o.object_name,
o.object_type
ORDER BY buffer_count DESC
FETCH FIRST 10 ROWS ONLY;
OBJECT_NAME OBJECT_TYPE BUFFER_COUNT
——————– —————— ————
TRANSACTIONS TABLE 5000
TRANS_ID_IDX INDEX 2500
ACCOUNTS TABLE 1000
5. 优化措施
– 增加缓冲区缓存大小
SQL> ALTER SYSTEM SET db_cache_size = 8G SCOPE=BOTH;
– 增加DBWR进程数量
SQL> ALTER SYSTEM SET db_writer_processes = 8 SCOPE=SPFILE;
– 配置KEEP池
SQL> ALTER SYSTEM SET db_keep_cache_size = 2G SCOPE=BOTH;
– 配置RECYCLE池
SQL> ALTER SYSTEM SET db_recycle_cache_size = 1G SCOPE=BOTH;
– 将常用表放入KEEP池
SQL> ALTER TABLE transactions STORAGE (BUFFER_POOL KEEP);
SQL> ALTER INDEX trans_id_idx STORAGE (BUFFER_POOL KEEP);
SQL> ALTER TABLE accounts STORAGE (BUFFER_POOL KEEP);
– 将临时表放入RECYCLE池
SQL> ALTER TABLE temp_table STORAGE (BUFFER_POOL RECYCLE);
– 重启数据库使设置生效
SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP;
6. 验证优化效果
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 500 2500 5.0
# 优化效果
– 空闲缓冲区等待减少90%
– 数据库性能提高60%
– SQL执行时间减少80%
– 系统稳定性提高
4.3 空闲缓冲区等待问题处理
在某电商网站的生产环境中,空闲缓冲区等待导致数据库性能下降,需要处理。
– 数据库性能突然下降
– 系统负载增加
– 空闲缓冲区等待严重
# 分析步骤
1. 监控空闲缓冲区等待
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 2000 10000 5.0
2. 分析缓冲区缓存命中率
SQL> SELECT
name,
physical_reads,
db_block_gets,
consistent_gets,
(1 – (physical_reads / (db_block_gets + consistent_gets))) * 100 AS hit_ratio
FROM v$buffer_pool_statistics;
NAME PHYSICAL_READS DB_BLOCK_GETS CONSISTENT_GETS HIT_RATIO
——————– ————– ————- ————— ———–
DEFAULT 20000 200000 1800000 90.0
KEEP 0 0 0 0
RECYCLE 0 0 0 0
3. 查看脏块数量
SQL> SELECT
status,
COUNT(*)
FROM v$bh
WHERE dirty = ‘Y’
GROUP BY status;
STATUS COUNT(*)
——— ———-
xcu 200
free 0
4. 查看缓冲区缓存中的对象
SQL> SELECT
o.object_name,
o.object_type,
COUNT(*) AS buffer_count
FROM v$bh b,
dba_objects o
WHERE b.objd = o.data_object_id
AND b.status != ‘free’
GROUP BY o.object_name,
o.object_type
ORDER BY buffer_count DESC
FETCH FIRST 10 ROWS ONLY;
OBJECT_NAME OBJECT_TYPE BUFFER_COUNT
——————– —————— ————
ORDERS TABLE 2000
ORD_ID_IDX INDEX 1000
CUSTOMERS TABLE 500
# 问题原因
– 缓冲区缓存不足
– DBWR进程数量不足
– 缺少KEEP池配置
– 大量全表扫描
# 解决方案
1. 增加缓冲区缓存大小
SQL> ALTER SYSTEM SET db_cache_size = 4G SCOPE=BOTH;
2. 增加DBWR进程数量
SQL> ALTER SYSTEM SET db_writer_processes = 4 SCOPE=SPFILE;
3. 配置KEEP池
SQL> ALTER SYSTEM SET db_keep_cache_size = 1G SCOPE=BOTH;
4. 将常用表放入KEEP池
SQL> ALTER TABLE orders STORAGE (BUFFER_POOL KEEP);
SQL> ALTER INDEX ord_id_idx STORAGE (BUFFER_POOL KEEP);
SQL> ALTER TABLE customers STORAGE (BUFFER_POOL KEEP);
5. 优化SQL语句
SQL> CREATE INDEX ord_customer_idx ON orders(customer_id);
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(‘SCOTT’, ‘ORDERS’);
6. 重启数据库使设置生效
SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP;
7. 验证问题解决
SQL> SELECT
event,
wait_class,
total_waits,
time_waited,
average_wait
FROM v$system_event
WHERE event = ‘free buffer waits’;
EVENT WAIT_CLASS TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
—————————— ——————– ———– ———– ————
free buffer waits Configuration 200 1000 5.0
# 解决效果
– 空闲缓冲区等待减少90%
– 数据库性能恢复正常
– 系统负载降低
– SQL执行时间减少80%
Part05-风哥经验总结与分享
5.1 空闲缓冲区等待管理经验
Oracle数据库空闲缓冲区等待管理经验:
- 增加缓冲区缓存:根据系统需求增加缓冲区缓存大小
- 优化DBWR进程:增加DBWR进程数量,提高脏块写入效率
- 使用多个缓冲区池:使用KEEP和RECYCLE池优化缓冲区使用
- 优化SQL语句:减少全表扫描,提高SQL效率
- 优化I/O子系统:使用高速存储设备,提高I/O性能
- 监控空闲缓冲区等待:定期监控空闲缓冲区等待情况,及时发现和处理问题
- 持续优化:根据分析结果,持续优化数据库性能
5.2 空闲缓冲区等待检查清单
– [ ] 监控空闲缓冲区等待情况
– [ ] 分析缓冲区缓存命中率
– [ ] 查看脏块数量
– [ ] 查看缓冲区缓存中的对象
– [ ] 增加缓冲区缓存
– [ ] 优化DBWR进程
– [ ] 使用多个缓冲区池
– [ ] 优化SQL语句
– [ ] 优化I/O子系统
– [ ] 验证优化效果
# 空闲缓冲区等待问题处理流程
1. 发现数据库性能问题
2. 监控空闲缓冲区等待
3. 分析缓冲区缓存命中率
4. 查看脏块数量
5. 查看缓冲区缓存中的对象
6. 识别空闲缓冲区等待原因
7. 制定解决方案
8. 实施解决方案
9. 验证问题解决
10. 总结经验,优化配置
5.3 空闲缓冲区等待管理工具
Oracle数据库空闲缓冲区等待管理常用工具:
- v$system_event:查看系统级别的空闲缓冲区等待
- v$session_wait:查看会话级别的空闲缓冲区等待
- v$buffer_pool_statistics:查看缓冲区池统计信息
- v$bh:查看缓冲区缓存中的数据块
- dba_hist_system_event:查看空闲缓冲区等待历史
- dba_hist_active_sess_history:查看会话空闲缓冲区等待历史
- AWR报告:分析数据库性能
- ASH报告:分析活动会话历史
- Oracle Enterprise Manager:图形化监控和管理
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