KingbaseES 等待事件之DataFileRead
等待事件含义
IO:DataFileRead等待事件发生在会话连接等待后端进程从存储中读取所需页面，原因是该页面在共享内存中不可用或无法找到。
所有查询和数据操作（DML）操作都访问缓冲池中的页面，语句包括SELECT、UPDATE和DELETE等。例如，UPDATE可以从表或索引中读取页面。如果请求或更新的页面不在共享缓冲池中，则此读取可能导致IO:DataFileRead事件。
因为共享缓冲池是有限的，如果业务繁忙，它可能会被填满。在这种情况下，对不在内存中的页面的请求会迫使数据库从磁盘中读取块。如果IO:DataFileRead事件频繁发生，则共享缓冲池可能太小，无法容纳工作负载。对于读取大量结果集的SELECT查询来说，这个问题非常严重。
等待事件增加的原因
案例：IO:DataFileRead事件频繁导致的性能问题
背景：
某用户电子商务网站近期遇到了数据库性能瓶颈，用户反馈查询响应变慢，特别是在促销活动期间。通过监控工具发现IO:DataFileRead等待事件频繁发生，且等待时间持续增加。
问题分析：
连接峰值：促销活动期间，用户访问量激增，导致数据库连接数迅速上升，多个会话同时尝试读取大量的数据页面，增加了IO:DataFileRead的等待时间。
执行全表扫描的SELECT语句：通过查询sys_stat_statements视图，发现有几个SELECT查询正在对大型表执行顺序扫描，这些查询没有有效的谓词筛选条件，导致读取了大量的数据页面。
服务器资源匮乏：促销活动期间，由于用户访问量增大，服务器的CPU和磁盘I/O资源都被大量消耗，进一步加剧了IO:DataFileRead等待事件的发生。
案例解决方案：
优化查询：针对那些执行顺序扫描的SELECT查询进行了优化，添加了合适的谓词筛选条件和索引，减少了读取的数据页面数量。同时，对于那些无法优化的查询，建议业务团队考虑对表进行分区或使用其他数据结构（如物化视图）来减轻对单一大表的访问压力。
调整共享缓冲池大小：根据系统的实际负载情况，适当增加了共享缓冲池的大小，以便能够容纳更多的数据页面，减少了从磁盘读取数据页面的次数。注意不要消耗过多服务器内存，原则上shared_buffers内存占用不超过操作系统的四分之一。
限制连接数：为了减轻数据库的连接压力，建议在数据库层面限制最大连接数，并建议业务团队在应用程序层面也进行连接池管理，避免在促销活动期间产生过多的数据库连接。此举既可以减轻服务器压力，又可以避免连接峰值出现。
优化服务器资源：建议对服务器的硬件进行升级，增加内存，增加CPU和磁盘I/O的处理能力，提高系统的整体性能。
效果：
经过上述优化措施的实施，IO:DataFileRead等待事件的频率和等待时间都显著降低，数据库的性能得到了明显的提升。在后续的促销活动期间，用户反馈查询响应速度明显加快，系统能够稳定地支持大量的用户访问。
等待事件增加的其他原因
1、用于大型数据集的 CTAS 和 CREATE INDEX
CTAS是CREATE TABLE AS SELECT语句。如果使用大型数据集作为源运行CTAS，或在大型表上创建索引，则可能会发生IO:DataFileRead事件。创建索引时，数据库可能需要使用顺序扫描读取整个对象。当页面不在内存中时，生成此等待事件。
2、多个vacuum进程同时运行
vacuum进程可以手动或自动触发。当对一个表进行大量更新或删除操作时，由于maintenance_work_mem空间不足，vacuum操作会消耗IO，IO:DataFileRead等待事件会增加。表回收空间后，用于IO:DataFileRead的vacuum时间会减少。
3、引入大量数据
当业务变更时，应用程序接收大量数据时，ANALYZE操作可能会更频繁地发生。IO:DataFileRead等待事件会增加。
解决方法
检查等待事件的查询的谓词筛选条件
如何确定正在生成IO:DataFileRead等待事件的特定查询，可以使用以下方法找到特定查询：
例如扩展sys_stat_statements提供的视图，或者通过KWR报告中的top sql选项。
sys_stat_all_tables视图中 seq_scan，seq_tup_read数量很多。
sys_statio_all_tables视图中会显示heap_blks_read,idx_blks_read,toast_blks_read,tidx_blks_read正在增加。
在top sql中找到可疑的sql并运行EXPLAIN命令。在输出中，识别使用的扫描类型。注意顺序扫描并不一定表明表有问题。但是那些大量结果集使用顺序扫描的查询自然会产生更多的IO:DataFileRead事件，这时需要对sql调优。
将vacuum维护影响降至最低
vacuum和analyze等维护操作非常重要，不建议关闭，以下方法可以最大限度地减少这些影响：
- 在非高峰时段手动运行维护操作。此方法可防止数据库达到自动操作的阈值，或针对性设置表级vacuum触发阈值，分散autovacuum进程工作时间。
- 对于非常大的表，请考虑对表进行分区。分区可减少维护操作的开销。数据库仅需要维护子分区即可。
- 引入大量数据时，请考虑禁用自动分析功能。
当以下公式成立时，autovacuum功能会自动触发。
sys_stat_user_tables.n_dead_tup > (sys_class.reltuples x autovacuum_vacuum_scale_factor) + autovacuum_vacuum_threshold
1、查找占用不必要空间的表
若查找占用过多空间的表，请运行以下查询。查询仅返回有关用户角色有权读取的那些表的信息。
WITH report AS (
SELECT schemaname
,tblname
,n_dead_tup
,n_live_tup
,block_size*tblpages AS real_size
,(tblpages-est_tblpages)*block_size AS extra_size
,CASE WHEN tblpages - est_tblpages > 0
THEN 100 * (tblpages - est_tblpages)/tblpages::float
ELSE 0
END AS extra_ratio, fillfactor, (tblpages-est_tblpages_ff)*block_size AS bloat_size
,CASE WHEN tblpages - est_tblpages_ff > 0
THEN 100 * (tblpages - est_tblpages_ff)/tblpages::float
ELSE 0
END AS bloat_ratio
,is_na
FROM (
SELECT ceil( reltuples / ( (block_size-page_hdr)/tpl_size ) ) + ceil( toasttuples / 4 ) AS est_tblpages
,ceil( reltuples / ( (block_size-page_hdr)*fillfactor/(tpl_size*100) ) ) + ceil( toasttuples / 4 ) AS est_tblpages_ff
,tblpages
,fillfactor
,block_size
,tblid
,schemaname
,tblname
,n_dead_tup
,n_live_tup
,heappages
,toastpages
,is_na
FROM (
SELECT ( 4 + tpl_hdr_size + tpl_data_size + (2*ma)
- CASE WHEN tpl_hdr_size%ma = 0 THEN ma ELSE tpl_hdr_size%ma END
- CASE WHEN ceil(tpl_data_size)::int%ma = 0 THEN ma ELSE ceil(tpl_data_size)::int%ma END
) AS tpl_size
,block_size - page_hdr AS size_per_block
,(heappages + toastpages) AS tblpages
,heappages
,toastpages
,reltuples
,toasttuples
,block_size
,page_hdr
,tblid
,schemaname
,tblname
,fillfactor
,is_na
,n_dead_tup
,n_live_tup
FROM (
SELECT tbl.oid AS tblid
,ns.nspname AS schemaname
,tbl.relname AS tblname
,tbl.reltuples AS reltuples
,tbl.relpages AS heappages
,coalesce(toast.relpages, 0) AS toastpages
,coalesce(toast.reltuples, 0) AS toasttuples
,psat.n_dead_tup AS n_dead_tup
,psat.n_live_tup AS n_live_tup
,24 AS page_hdr
,current_setting('block_size')::numeric AS block_size
,coalesce(substring( array_to_string(tbl.reloptions, ' ') FROM 'fillfactor=([0-9]+)')::smallint, 100) AS fillfactor
,CASE WHEN version()~'mingw32' OR version()~'64-bit|x86_64|ppc64|ia64|amd64' THEN 8 ELSE 4 END AS ma
,23 + CASE WHEN MAX(coalesce(null_frac,0)) > 0 THEN ( 7 + count(*) ) / 8 ELSE 0::int END AS tpl_hdr_size
,sum( (1-coalesce(s.null_frac, 0)) * coalesce(s.avg_width, 1024) ) AS tpl_data_size
,bool_or(att.atttypid = 'pg_catalog.name'::regtype) OR count(att.attname) <> count(s.attname) AS is_na
FROM pg_attribute AS att
JOIN pg_class AS tbl ON (att.attrelid = tbl.oid)
JOIN pg_stat_all_tables AS psat ON (tbl.oid = psat.relid)
JOIN pg_namespace AS ns ON (ns.oid = tbl.relnamespace)
LEFT JOIN pg_stats AS s ON (s.schemaname=ns.nspname AND s.tablename = tbl.relname AND s.inherited=false AND s.attname=att.attname)
LEFT JOIN pg_class AS toast ON (tbl.reltoastrelid = toast.oid)
WHERE att.attnum > 0
AND NOT att.attisdropped
AND tbl.relkind = 'r'
GROUP BY tbl.oid, ns.nspname, tbl.relname, tbl.reltuples, tbl.relpages, toastpages, toasttuples, fillfactor, block_size, ma, n_dead_tup, n_live_tup
ORDER BY schemaname, tblname
) AS s
) AS s2
) AS s3
ORDER BY bloat_size DESC
)
SELECT *
FROM report
WHERE bloat_ratio != 0
-- AND schemaname = 'public'
-- AND tblname = 'pgbench_accounts'
;
-- WHERE NOT is_na
-- AND tblpages*((pst).free_percent + (pst).dead_tuple_percent)::float4/100 >= 1
2、查找消耗不必要空间的索引
若要查找占用不必要空间的索引，请运行以下查询。
SELECT current_database(), nspname AS schemaname, tblname, idxname, bs*(relpages)::bigint AS real_size,
bs*(relpages-est_pages)::bigint AS extra_size,
100 * (relpages-est_pages)::float / relpages AS extra_ratio,
fillfactor, bs*(relpages-est_pages_ff) AS bloat_size,
100 * (relpages-est_pages_ff)::float / relpages AS bloat_ratio,
is_na
-- , 100-(sub.pst).avg_leaf_density, est_pages, index_tuple_hdr_bm,
-- maxalign, pagehdr, nulldatawidth, nulldatahdrwidth, sub.reltuples, sub.relpages
-- (DEBUG INFO)
FROM (
SELECT coalesce(1 +
ceil(reltuples/floor((bs-pageopqdata-pagehdr)/(4+nulldatahdrwidth)::float)), 0
-- ItemIdData size + computed avg size of a tuple (nulldatahdrwidth)
) AS est_pages,
coalesce(1 +
ceil(reltuples/floor((bs-pageopqdata-pagehdr)*fillfactor/(100*(4+nulldatahdrwidth)::float))), 0
) AS est_pages_ff,
bs, nspname, table_oid, tblname, idxname, relpages, fillfactor, is_na
-- , stattuple.pgstatindex(quote_ident(nspname)||'.'||quote_ident(idxname)) AS pst,
-- index_tuple_hdr_bm, maxalign, pagehdr, nulldatawidth, nulldatahdrwidth, reltuples
-- (DEBUG INFO)
FROM (
SELECT maxalign, bs, nspname, tblname, idxname, reltuples, relpages, relam, table_oid, fillfactor,
( index_tuple_hdr_bm +
maxalign - CASE -- Add padding to the index tuple header to align on MAXALIGN
WHEN index_tuple_hdr_bm%maxalign = 0 THEN maxalign
ELSE index_tuple_hdr_bm%maxalign
END
+ nulldatawidth + maxalign - CASE -- Add padding to the data to align on MAXALIGN
WHEN nulldatawidth = 0 THEN 0
WHEN nulldatawidth::integer%maxalign = 0 THEN maxalign
ELSE nulldatawidth::integer%maxalign
END
)::numeric AS nulldatahdrwidth, pagehdr, pageopqdata, is_na
-- , index_tuple_hdr_bm, nulldatawidth -- (DEBUG INFO)
FROM (
SELECT
i.nspname, i.tblname, i.idxname, i.reltuples, i.relpages, i.relam, a.attrelid AS table_oid,
current_setting('block_size')::numeric AS bs, fillfactor,
CASE -- MAXALIGN: 4 on 32bits, 8 on 64bits (and mingw32 ?)
WHEN version() ~ 'mingw32' OR version() ~ '64-bit|x86_64|ppc64|ia64|amd64' THEN 8
ELSE 4
END AS maxalign,
/* per page header, fixed size: 20 for 7.X, 24 for others */
24 AS pagehdr,
/* per page btree opaque data */
16 AS pageopqdata,
/* per tuple header: add IndexAttributeBitMapData if some cols are null-able */
CASE WHEN max(coalesce(s.null_frac,0)) = 0
THEN 2 -- IndexTupleData size
ELSE 2 + (( 32 + 8 - 1 ) / 8)
-- IndexTupleData size + IndexAttributeBitMapData size ( max num filed per index + 8 - 1 /8)
END AS index_tuple_hdr_bm,
/* data len: we remove null values save space using it fractionnal part from stats */
sum( (1-coalesce(s.null_frac, 0)) * coalesce(s.avg_width, 1024)) AS nulldatawidth,
max( CASE WHEN a.atttypid = 'pg_catalog.name'::regtype THEN 1 ELSE 0 END ) > 0 AS is_na
FROM pg_attribute AS a
JOIN (
SELECT nspname, tbl.relname AS tblname, idx.relname AS idxname,
idx.reltuples, idx.relpages, idx.relam,
indrelid, indexrelid, indkey::smallint[] AS attnum,
coalesce(substring(
array_to_string(idx.reloptions, ' ')
from 'fillfactor=([0-9]+)')::smallint, 90) AS fillfactor
FROM pg_index
JOIN pg_class idx ON idx.oid=pg_index.indexrelid
JOIN pg_class tbl ON tbl.oid=pg_index.indrelid
JOIN pg_namespace ON pg_namespace.oid = idx.relnamespace
WHERE pg_index.indisvalid AND tbl.relkind = 'r' AND idx.relpages > 0
) AS i ON a.attrelid = i.indexrelid
JOIN pg_stats AS s ON s.schemaname = i.nspname
AND ((s.tablename = i.tblname AND s.attname = pg_catalog.pg_get_indexdef(a.attrelid, a.attnum, TRUE))
-- stats from tbl
OR (s.tablename = i.idxname AND s.attname = a.attname))
-- stats from functionnal cols
JOIN pg_type AS t ON a.atttypid = t.oid
WHERE a.attnum > 0
GROUP BY 1, 2, 3, 4, 5, 6, 7, 8, 9
) AS s1
) AS s2
JOIN pg_am am ON s2.relam = am.oid WHERE am.amname = 'btree'
) AS sub
-- WHERE NOT is_na
ORDER BY 2,3,4;
3、查找符合autovacuum条件的表
若要查找符合自动清除条件的表，请运行以下查询。
--This query shows tables that need vacuuming and are eligible candidates.
--The following query lists all tables that are due to be processed by autovacuum.
-- During normal operation, this query should return very little.
WITH vbt AS (SELECT setting AS autovacuum_vacuum_threshold
FROM sys_settings WHERE name = 'autovacuum_vacuum_threshold')
, vsf AS (SELECT setting AS autovacuum_vacuum_scale_factor
FROM sys_settings WHERE name = 'autovacuum_vacuum_scale_factor')
, fma AS (SELECT setting AS autovacuum_freeze_max_age
FROM sys_settings WHERE name = 'autovacuum_freeze_max_age')
, sto AS (SELECT opt_oid, split_part(setting, '=', 1) as param,
split_part(setting, '=', 2) as value
FROM (SELECT oid opt_oid, unnest(reloptions) setting FROM sys_class) opt)
SELECT
'"'||ns.nspname||'"."'||c.relname||'"' as relation
, sys_size_pretty(sys_table_size(c.oid)) as table_size
, age(relfrozenxid) as xid_age
, coalesce(cfma.value::float, autovacuum_freeze_max_age::float) autovacuum_freeze_max_age
, (coalesce(cvbt.value::float, autovacuum_vacuum_threshold::float) +
coalesce(cvsf.value::float,autovacuum_vacuum_scale_factor::float) * c.reltuples)
as autovacuum_vacuum_tuples
, n_dead_tup as dead_tuples
FROM sys_class c
JOIN sys_namespace ns ON ns.oid = c.relnamespace
JOIN sys_stat_all_tables stat ON stat.relid = c.oid
JOIN vbt on (1=1)
JOIN vsf ON (1=1)
JOIN fma on (1=1)
LEFT JOIN sto cvbt ON cvbt.param = 'autovacuum_vacuum_threshold' AND c.oid = cvbt.opt_oid
LEFT JOIN sto cvsf ON cvsf.param = 'autovacuum_vacuum_scale_factor' AND c.oid = cvsf.opt_oid
LEFT JOIN sto cfma ON cfma.param = 'autovacuum_freeze_max_age' AND c.oid = cfma.opt_oid
WHERE c.relkind = 'r'
AND nspname <> 'sys_catalog'
AND (
age(relfrozenxid) >= coalesce(cfma.value::float, autovacuum_freeze_max_age::float)
or
coalesce(cvbt.value::float, autovacuum_vacuum_threshold::float) +
coalesce(cvsf.value::float,autovacuum_vacuum_scale_factor::float) * c.reltuples <= n_dead_tup
-- or 1 = 1
) ORDER BY age(relfrozenxid) DESC;
解决大量连接
当发现DatabaseConnections出现峰值。这表示数据库的连接数量增加。
建议采用以下方法：
限制应用程序可以对每个实例打开的连接数。考虑使用连接池功能管理数据库连接，请设置合理的连接数量，设置基于实例中CPU可以有效并行化的数量。
对于不使用的空闲连接要及时释放，避免消耗过多服务器内存资源。
