PostgreSQL 如何精确计算表膨胀(fsm,数据块layout讲解) - PostgreSQL table exactly bloat monitor use freespace map data
背景
对PostgreSQL数据库有一定了解的朋友都知道PostgreSQL的UPDATE, DELETE操作是通过新增tuple版本提供高并发处理的. 这样带来一个问题是需要经常vacuum 表, 回收老版本占用的存储空间. 只有回收的空间才能被重复利用, 如果回收不及时将会造成表的膨胀效应.
那么怎么知道数据表有没有膨胀呢?
方法举例如下 :
一. 一般的做法是监控dead tuple和live tuple的比例.
但是在执行完vacuum后, dead tuple会清除掉, 所以膨胀多少看不出来. 这种方法不适合监控表膨胀, 倒是适合监控dead tuple的比例(反推出是否存在长事务或者未开启autovacuum , 或者IO过于繁忙, DML操作频繁, 并且acuum不及时等等情况).
例如 :
创建测试表 :
digoal=# create table deadtuple_teste (id int, info text);
CREATE TABLE
插入测试数据 :
digoal=# insert into deadtuple_teste select generate_series(1,100000),md5(random()::text);
INSERT 0 100000
查看当前表的空间占用情况, 已占用834个page :
digoal=# select relpages from pg_class where relname='deadtuple_teste';
relpages
----------
834
(1 row)
查看live tuple和dead tuple : 当前没有dead tuple.
digoal=# select relname,n_live_tup,n_dead_tup from pg_stat_all_tables where relname='deadtuple_teste';
relname | n_live_tup | n_dead_tup
-----------------+------------+------------
deadtuple_teste | 100000 | 0
(1 row)
删除29999条记录.
digoal=# delete from deadtuple_teste where id<30000;
DELETE 29999
再次查看live tuple和dead tuple. 还是没有dead tuple, 原因是数据库开启了auto vacuum, 到达一定阈值后会自动触发, 回收掉了.
digoal=# select relname,n_live_tup,n_dead_tup from pg_stat_all_tables where relname='deadtuple_teste';
relname | n_live_tup | n_dead_tup
-----------------+------------+------------
deadtuple_teste | 70001 | 0
(1 row)
如下 :
digoal=# select relname,n_live_tup,n_dead_tup,last_autoanalyze,last_autovacuum from pg_stat_all_tables where relname='deadtuple_teste';
relname | n_live_tup | n_dead_tup | last_autoanalyze | last_autovacuum
-----------------+------------+------------+-------------------------------+-------------------------------
deadtuple_teste | 70001 | 0 | 2013-06-28 08:28:24.745492+08 | 2013-06-28 08:28:24.586153+08
(1 row)
再次查看空间占用, 依然是834个page, 所以使用这种方法没法监控数据表的膨胀.
digoal=# select relpages from pg_class where relname='deadtuple_teste';
relpages
----------
834
(1 row)
二. 另一种方法是通过计算行平均占用空间计算出live tuple占用的空间, 计算出膨胀.
这种方法不适合行的占用空间偏离非常大的情况, 如某些行占用1KB, 有的可能占用10KB. 最后导致空间不好估算.
测试依然使用以上环境,
但是为了数据准确, 需要将page的头信息, tuple的头信息, item信息记录在内.
Table 58-2. Overall Page Layout
| Item | Description |
|---|---|
| PageHeaderData | 24 bytes long. Contains general information about the page, including free space pointers. |
| ItemIdData | Array of (offset,length) pairs pointing to the actual items. 4 bytes per item. |
| Free space | The unallocated space. New item pointers are allocated from the start of this area, new items from the end. |
| Items | The actual items themselves. |
| Special space | Index access method specific data. Different methods store different data. Empty in ordinary tables. |
每个 page 头包含24字节固定长度. 每条tuple对应一条ItemIdData, 4字节.
信息如下 :
Table 58-3. PageHeaderData Layout
| Field | Type | Length | Description |
|---|---|---|---|
| pd_lsn | XLogRecPtr | 8 bytes | LSN: next byte after last byte of xlog record for last change to this page |
| pd_tli | uint16 | 2 bytes | TimeLineID of last change (only its lowest 16 bits) |
| pd_flags | uint16 | 2 bytes | Flag bits |
| pd_lower | LocationIndex | 2 bytes | Offset to start of free space |
| pd_upper | LocationIndex | 2 bytes | Offset to end of free space |
| pd_special | LocationIndex | 2 bytes | Offset to start of special space |
| pd_pagesize_version | uint16 | 2 bytes | Page size and layout version number information |
| pd_prune_xid | TransactionId | 4 bytes | Oldest unpruned XMAX on page, or zero if none |
tuple头部则占用27字节. 如下.
Table 58-4. HeapTupleHeaderData Layout
| Field | Type | Length | Description |
|---|---|---|---|
| t_xmin | TransactionId | 4 bytes | insert XID stamp |
| t_xmax | TransactionId | 4 bytes | delete XID stamp |
| t_cid | CommandId | 4 bytes | insert and/or delete CID stamp (overlays with t_xvac) |
| t_xvac | TransactionId | 4 bytes | XID for VACUUM operation moving a row version |
| t_ctid | ItemPointerData | 6 bytes | current TID of this or newer row version |
| t_infomask2 | uint16 | 2 bytes | number of attributes, plus various flag bits |
| t_infomask | uint16 | 2 bytes | various flag bits |
| t_hoff | uint8 | 1 byte | offset to user data |
因此1条记录真实的占用空间应该是ItemIdData+tupleHead+tupledata=4+27+?字节.
第一个例子的长度如下 :
digoal=# select pg_column_size(id),pg_column_size('info') from deadtuple_teste limit 1;
pg_column_size | pg_column_size
----------------+----------------
4 | 5
(1 row)
一条tuple的空间是4+27+4+5=40字节.
采样10000条计算平均值 :
digoal=# select avg(c1+c2) from (select pg_column_size(id) c1,pg_column_size('info') c2 from deadtuple_teste limit 1000) t;
avg
--------------------
9.0000000000000000
(1 row)
最终得到的单条tuple的空间占用是40字节.
live tuple占用的空间如下 :
digoal=# select n_live_tup*40 from pg_stat_all_tables where relname='deadtuple_teste';
?column?
----------
2800040
(1 row)
page头占用的空间如下 :
digoal=# select 24*relpages from pg_class where relname='deadtuple_teste';
?column?
----------
20016
(1 row)
表的真实空间占用如下 :
digoal=# select pg_relation_size('deadtuple_teste'::regclass);
pg_relation_size
------------------
6832128
(1 row)
膨胀比例 ;
(6832128-2800040-20016)/6832128=0.58723607051858513189.
如果表中有字段使用了TOAST存储, 那么使用这种方法计算膨胀将更加复杂.
digoal=# select reltoastrelid::regclass from pg_class where relname='deadtuple_teste';
reltoastrelid
-------------------------
pg_toast.pg_toast_33317
(1 row)
三. 最后的绝招来了, 使用freespace map来计算膨胀是最赤裸的方法, 因为PostgreSQL 本身也是使用freespace map来获取定位所需空间的数据块的.
自从8.4以后, 每个数据库对象有自己的fsm文件, 详细介绍可参考.
《PostgreSQL Free Space Map Principle》
fsm中存储的并不是每个表page的详细剩余字节数, 而是使用1个字节来存储1个page的剩余空间, 因此一个表的page被拆分成了256份, 剩余空间200代表这个page剩余blocksize*(200/256).
fsm page内部使用binary tree存储, 1个fsm的page默认为8KB, 所以理论上可以存储8192/2-1个表page的剩余空间信息. 但是除掉fsm头信息实际上一个fsm page只能存储4069个表对象的page剩余空间信息.
fsm pages本身也是树结构, 一个数据库对象最大为2^32-1个page组成. 所以只要3级就足以记录一个对象的page剩余空间信息 :
4069^3 > (2^32-1)
fsm page内部结构图例 :
以下为第三级的fsm page内部结构举例 :
level
1
2
4
8
16
32
64
128
256
512 以上略, 逐渐收缩. 直到顶端.
1024 4 ...
2048 4 2 ...
4069 3 4 0 2 .... <- This level represents heap pages (如果是第二级或者第一级的fsm page, 这里表示child fsm page的信息)
(上面的3,4,0,2)每个数字代表heap page的剩余空间, 1个字节表示, 后面会打印fsm page的内容, 一目了然.
fsm pages 结构图例 :
0 <-- page 0 at level 2 (root page)
0 <-- page 0 at level 1
0 <-- page 0 at level 0
1 <-- page 1 at level 0
2 <-- ...
...
4069
1 <-- page 1 at level 1
4069
...
2
...
4069*2
3
...
4069*3
4069
....
4069*4069
1
....
....
4069*4069*4069
一个二级节点,4069个三级节点可以记录大表的所有page信息呢?
(注意图中有误, 4070应该改成4069)
32MB的fsm page可以记录126.38GB的数据库对象的free space map信息.
创建测试表 :
digoal=# create table fsm_test(id int, info text);
CREATE TABLE
插入测试数据
digoal=# insert into fsm_test select generate_series(1,1000000),'test';
INSERT 0 1000000
查看对象大小 :
digoal=# select pg_relation_size('fsm_test');
pg_relation_size
------------------
44285952
(1 row)
表对应的文件
digoal=# select pg_relation_filepath('fsm_test');
pg_relation_filepath
----------------------
base/16385/33311
(1 row)
fsm文件加_fsm后缀.
digoal=# select * from pg_stat_file('base/16385/33311_fsm');
size | access | modification | change | creation | isdir
-------+------------------------+------------------------+------------------------+----------+-------
32768 | 2013-06-27 21:59:21+08 | 2013-06-27 21:59:21+08 | 2013-06-27 21:59:21+08 | | f
(1 row)
fsm文件共计4个page, 所以有2个是三级page(2,3). 计算时也只需要统计三级page的信息.
digoal=# select 32768/8192;
?column?
----------
4
(1 row)
fsm文件跟随对象文件名. 所以在truncate对象后fsm会重新生成. 但是必须要第一次vacuum后fsm才会出现, 否则是没有的.
digoal=# truncate fsm_test;
TRUNCATE TABLE
digoal=# insert into fsm_test select generate_series(1,1000000),'test';
INSERT 0 1000000
digoal=# select pg_relation_filepath('fsm_test');
pg_relation_filepath
----------------------
base/16385/33325
(1 row)
新fsm文件名 :
digoal=# select * from pg_stat_file('base/16385/33325_fsm');
size | access | modification | change | creation | isdir
-------+------------------------+------------------------+------------------------+----------+-------
32768 | 2013-06-28 09:55:16+08 | 2013-06-28 09:55:16+08 | 2013-06-28 09:55:16+08 | | f
(1 row)
查看freespace map page的内容可以通过pageinspect插件 :
digoal=# create extension pageinspect;
CREATE EXTENSION
digoal=# select * from fsm_page_contents(get_raw_page('fsm_test','fsm',0));
fsm_page_contents
-------------------
fp_next_slot: 0 +
(1 row)
digoal=# select * from fsm_page_contents(get_raw_page('fsm_test','fsm',1));
fsm_page_contents
-------------------
fp_next_slot: 0 +
(1 row)
digoal=# select * from fsm_page_contents(get_raw_page('fsm_test','fsm',2));
fsm_page_contents
-------------------
fp_next_slot: 0 +
(1 row)
digoal=# select * from fsm_page_contents(get_raw_page('fsm_test','fsm',3));
fsm_page_contents
-------------------
fp_next_slot: 0 +
(1 row)
表明fsm_test表目前没有剩余空间.
直接从get_raw_page取数据 :
digoal=# select * from get_raw_page('fsm_test','fsm',3);
\x000000000000000000000000180000200020042000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000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删除fsm_test的数据, 只留最后一行.
digoal=# delete from fsm_test where id<1000000;
DELETE 999999
digoal=# delete from fsm_test where id<1000000;
DELETE 999999
digoal=# vacuum verbose analyze fsm_test ;
INFO: vacuuming "public.fsm_test"
INFO: "fsm_test": found 0 removable, 0 nonremovable row versions in 0 out of 5406 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: vacuuming "pg_toast.pg_toast_33311"
INFO: index "pg_toast_33311_index" now contains 0 row versions in 1 pages
DETAIL: 0 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_toast_33311": found 0 removable, 0 nonremovable row versions in 0 out of 0 pages
DETAIL: 0 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: analyzing "public.fsm_test"
INFO: "fsm_test": scanned 5406 of 5406 pages, containing 1 live rows and 0 dead rows; 1 rows in sample, 1 estimated total rows
VACUUM
vacuum后空间回收并可以重复利用, 但是还占用了磁盘空间.
这个时候查看fsm就不一样了.
digoal=# select * from get_raw_page('fsm_test','fsm',3);
\x00000000000000000000000018000020002004200000000000000000f4f400e8f40000e8e8f40000000000e8e8e8e8e8f400000000000000000000e8e8e8e8e8e8e8e8e8e8f4000000000000000000000000000000000000000000e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8f400000000000000000000000000000000000000000000000000000000000000000000000000000000000000e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8f40000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8f400000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8f40000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8f40000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8e8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分层后信息如下 :
1. 头信息 :
\x00000000000000000000000018000020002004200000000000000000
2. 树信息如下 :
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最后一级表示heap page的剩余空间, 刚好4069个字节. 往上是2048, 1024, 512, 256, 128, 64, 32, 16, 8, 4, 2, 1;
所以在计算剩余空间的时候,也只需要计算最后的4096个字节, 同时去除1级和2级fsm page.
本例的计算方法如下 :
do language plpgsql $$
declare
v_fsm_node_id int;
v_fsm_pageid int;
v_fsm_pages int;
v_rel_pages int;
v_fsm_file text;
v_block_size int;
x int;
res int8 := 0;
begin
-- 查出block_size
select setting::int into v_block_size from pg_settings where name='block_size';
-- 查出fsm文件名
select pg_relation_filepath('fsm_test')||'_fsm' into v_fsm_file;
-- 查处fsm占用多少个page
select size/v_block_size into v_fsm_pages from pg_stat_file(v_fsm_file);
-- 查出表占用多少个page
select relpages into v_rel_pages from pg_class where relname='fsm_test';
raise notice '%', v_rel_pages;
-- 查处剩余空间
for v_fsm_pageid in 2..(v_fsm_pages-1) loop
for v_fsm_node_id in 8249..16385 by 2 loop
execute $_$select (x'$_$||substring(textin(byteaout(get_raw_page('fsm_test','fsm',v_fsm_pageid))),v_fsm_node_id,2)||$_$')::int$_$ into x;
-- raise notice '%', x;
res := res+x;
end loop;
end loop;
raise notice 'res:%', res;
end;
$$;
结果
NOTICE: 5406
NOTICE: res:1254204
DO
因此这个表当前可用的剩余空间占比如下 :
digoal=# select 1254204/(5406*256.0);
-[ RECORD 1 ]--------------------
?column? | 0.90625867092119866815
这个结果和pgstattuple得到的结果一致.
digoal=# select * from pgstattuple('fsm_test');
-[ RECORD 1 ]------+---------
table_len | 44285952
tuple_count | 1
tuple_len | 33
tuple_percent | 0
dead_tuple_count | 0
dead_tuple_len | 0
dead_tuple_percent | 0
free_space | 40134544
free_percent | 90.63
所以监控膨胀用pgstattuple这个插件就可以了, 前面使用freespace map查看只是让大家了解里面的细节.
注意, 在计算膨胀前最好先analyze一下表, 这样得到的结果更精确. 当然如果开启了autovacuum的话就没必要了哦
四. 其实使用pg_freespacemap这个插件的话也可以达到同样的目的.
这里省略不讲, 有兴趣的朋友可参考 :
《PostgreSQL Improve fsm & vm output》
http://www.postgresql.org/docs/devel/static/pgfreespacemap.html
五. bucardo发布postgres_check中包含的查询膨胀的SQL
查询表膨胀TOP5
SELECT
current_database() AS db, schemaname, tablename, reltuples::bigint AS tups, relpages::bigint AS pages, otta,
ROUND(CASE WHEN otta=0 OR sml.relpages=0 OR sml.relpages=otta THEN 0.0 ELSE sml.relpages/otta::numeric END,1) AS tbloat,
CASE WHEN relpages < otta THEN 0 ELSE relpages::bigint - otta END AS wastedpages,
CASE WHEN relpages < otta THEN 0 ELSE bs*(sml.relpages-otta)::bigint END AS wastedbytes,
CASE WHEN relpages < otta THEN $$0 bytes$$::text ELSE (bs*(relpages-otta))::bigint || $$ bytes$$ END AS wastedsize,
iname, ituples::bigint AS itups, ipages::bigint AS ipages, iotta,
ROUND(CASE WHEN iotta=0 OR ipages=0 OR ipages=iotta THEN 0.0 ELSE ipages/iotta::numeric END,1) AS ibloat,
CASE WHEN ipages < iotta THEN 0 ELSE ipages::bigint - iotta END AS wastedipages,
CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta) END AS wastedibytes,
CASE WHEN ipages < iotta THEN $$0 bytes$$ ELSE (bs*(ipages-iotta))::bigint || $$ bytes$$ END AS wastedisize,
CASE WHEN relpages < otta THEN
CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta::bigint) END
ELSE CASE WHEN ipages < iotta THEN bs*(relpages-otta::bigint)
ELSE bs*(relpages-otta::bigint + ipages-iotta::bigint) END
END AS totalwastedbytes
FROM (
SELECT
nn.nspname AS schemaname,
cc.relname AS tablename,
COALESCE(cc.reltuples,0) AS reltuples,
COALESCE(cc.relpages,0) AS relpages,
COALESCE(bs,0) AS bs,
COALESCE(CEIL((cc.reltuples*((datahdr+ma-
(CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma END))+nullhdr2+4))/(bs-20::float)),0) AS otta,
COALESCE(c2.relname,$$?$$) AS iname, COALESCE(c2.reltuples,0) AS ituples, COALESCE(c2.relpages,0) AS ipages,
COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS iotta -- very rough approximation, assumes all cols
FROM
pg_class cc
JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname <> $$information_schema$$
LEFT JOIN
(
SELECT
ma,bs,foo.nspname,foo.relname,
(datawidth+(hdr+ma-(case when hdr%ma=0 THEN ma ELSE hdr%ma END)))::numeric AS datahdr,
(maxfracsum*(nullhdr+ma-(case when nullhdr%ma=0 THEN ma ELSE nullhdr%ma END))) AS nullhdr2
FROM (
SELECT
ns.nspname, tbl.relname, hdr, ma, bs,
SUM((1-coalesce(null_frac,0))*coalesce(avg_width, 2048)) AS datawidth,
MAX(coalesce(null_frac,0)) AS maxfracsum,
hdr+(
SELECT 1+count(*)/8
FROM pg_stats s2
WHERE null_frac<>0 AND s2.schemaname = ns.nspname AND s2.tablename = tbl.relname
) AS nullhdr
FROM pg_attribute att
JOIN pg_class tbl ON att.attrelid = tbl.oid
JOIN pg_namespace ns ON ns.oid = tbl.relnamespace
LEFT JOIN pg_stats s ON s.schemaname=ns.nspname
AND s.tablename = tbl.relname
AND s.inherited=false
AND s.attname=att.attname,
(
SELECT
(SELECT current_setting($$block_size$$)::numeric) AS bs,
CASE WHEN SUBSTRING(SPLIT_PART(v, $$ $$, 2) FROM $$#"[0-9]+.[0-9]+#"%$$ for $$#$$)
IN ($$8.0$$,$$8.1$$,$$8.2$$) THEN 27 ELSE 23 END AS hdr,
CASE WHEN v ~ $$mingw32$$ OR v ~ $$64-bit$$ THEN 8 ELSE 4 END AS ma
FROM (SELECT version() AS v) AS foo
) AS constants
WHERE att.attnum > 0 AND tbl.relkind=$$r$$
GROUP BY 1,2,3,4,5
) AS foo
) AS rs
ON cc.relname = rs.relname AND nn.nspname = rs.nspname
LEFT JOIN pg_index i ON indrelid = cc.oid
LEFT JOIN pg_class c2 ON c2.oid = i.indexrelid
) AS sml order by wastedbytes desc limit 5
查询索引膨胀TOP5
SELECT
current_database() AS db, schemaname, tablename, reltuples::bigint AS tups, relpages::bigint AS pages, otta,
ROUND(CASE WHEN otta=0 OR sml.relpages=0 OR sml.relpages=otta THEN 0.0 ELSE sml.relpages/otta::numeric END,1) AS tbloat,
CASE WHEN relpages < otta THEN 0 ELSE relpages::bigint - otta END AS wastedpages,
CASE WHEN relpages < otta THEN 0 ELSE bs*(sml.relpages-otta)::bigint END AS wastedbytes,
CASE WHEN relpages < otta THEN $$0 bytes$$::text ELSE (bs*(relpages-otta))::bigint || $$ bytes$$ END AS wastedsize,
iname, ituples::bigint AS itups, ipages::bigint AS ipages, iotta,
ROUND(CASE WHEN iotta=0 OR ipages=0 OR ipages=iotta THEN 0.0 ELSE ipages/iotta::numeric END,1) AS ibloat,
CASE WHEN ipages < iotta THEN 0 ELSE ipages::bigint - iotta END AS wastedipages,
CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta) END AS wastedibytes,
CASE WHEN ipages < iotta THEN $$0 bytes$$ ELSE (bs*(ipages-iotta))::bigint || $$ bytes$$ END AS wastedisize,
CASE WHEN relpages < otta THEN
CASE WHEN ipages < iotta THEN 0 ELSE bs*(ipages-iotta::bigint) END
ELSE CASE WHEN ipages < iotta THEN bs*(relpages-otta::bigint)
ELSE bs*(relpages-otta::bigint + ipages-iotta::bigint) END
END AS totalwastedbytes
FROM (
SELECT
nn.nspname AS schemaname,
cc.relname AS tablename,
COALESCE(cc.reltuples,0) AS reltuples,
COALESCE(cc.relpages,0) AS relpages,
COALESCE(bs,0) AS bs,
COALESCE(CEIL((cc.reltuples*((datahdr+ma-
(CASE WHEN datahdr%ma=0 THEN ma ELSE datahdr%ma END))+nullhdr2+4))/(bs-20::float)),0) AS otta,
COALESCE(c2.relname,$$?$$) AS iname, COALESCE(c2.reltuples,0) AS ituples, COALESCE(c2.relpages,0) AS ipages,
COALESCE(CEIL((c2.reltuples*(datahdr-12))/(bs-20::float)),0) AS iotta -- very rough approximation, assumes all cols
FROM
pg_class cc
JOIN pg_namespace nn ON cc.relnamespace = nn.oid AND nn.nspname <> $$information_schema$$
LEFT JOIN
(
SELECT
ma,bs,foo.nspname,foo.relname,
(datawidth+(hdr+ma-(case when hdr%ma=0 THEN ma ELSE hdr%ma END)))::numeric AS datahdr,
(maxfracsum*(nullhdr+ma-(case when nullhdr%ma=0 THEN ma ELSE nullhdr%ma END))) AS nullhdr2
FROM (
SELECT
ns.nspname, tbl.relname, hdr, ma, bs,
SUM((1-coalesce(null_frac,0))*coalesce(avg_width, 2048)) AS datawidth,
MAX(coalesce(null_frac,0)) AS maxfracsum,
hdr+(
SELECT 1+count(*)/8
FROM pg_stats s2
WHERE null_frac<>0 AND s2.schemaname = ns.nspname AND s2.tablename = tbl.relname
) AS nullhdr
FROM pg_attribute att
JOIN pg_class tbl ON att.attrelid = tbl.oid
JOIN pg_namespace ns ON ns.oid = tbl.relnamespace
LEFT JOIN pg_stats s ON s.schemaname=ns.nspname
AND s.tablename = tbl.relname
AND s.inherited=false
AND s.attname=att.attname,
(
SELECT
(SELECT current_setting($$block_size$$)::numeric) AS bs,
CASE WHEN SUBSTRING(SPLIT_PART(v, $$ $$, 2) FROM $$#"[0-9]+.[0-9]+#"%$$ for $$#$$)
IN ($$8.0$$,$$8.1$$,$$8.2$$) THEN 27 ELSE 23 END AS hdr,
CASE WHEN v ~ $$mingw32$$ OR v ~ $$64-bit$$ THEN 8 ELSE 4 END AS ma
FROM (SELECT version() AS v) AS foo
) AS constants
WHERE att.attnum > 0 AND tbl.relkind=$$r$$
GROUP BY 1,2,3,4,5
) AS foo
) AS rs
ON cc.relname = rs.relname AND nn.nspname = rs.nspname
LEFT JOIN pg_index i ON indrelid = cc.oid
LEFT JOIN pg_class c2 ON c2.oid = i.indexrelid
) AS sml order by wastedibytes desc limit 5
建议
根据浪费的字节数, 设置合适的autovacuum_vacuum_scale_factor, 大表如果频繁的有更新或删除和插入操作, 建议设置较小的autovacuum_vacuum_scale_factor来降低浪费空间.
同时还需要打开autovacuum, 根据服务器的内存大小, CPU核数, 设置足够大的autovacuum_work_mem 或 autovacuum_max_workers 或 maintenance_work_mem, 以及足够小的 autovacuum_naptime .
同时还需要分析是否对大数据库使用了逻辑备份pg_dump, 系统中是否经常有长SQL, 长事务. 这些都有可能导致膨胀.
使用pg_reorg或者vacuum full可以回收膨胀的空间.
参考: 《PostgreSQL 垃圾回收原理以及如何预防膨胀 - How to prevent object bloat in PostgreSQL》
otta评估出的表实际需要页数, iotta评估出的索引实际需要页数;
bs数据库的块大小;
tbloat表膨胀倍数, ibloat索引膨胀倍数, wastedpages表浪费了多少个数据块, wastedipages索引浪费了多少个数据块;
wastedbytes表浪费了多少字节, wastedibytes索引浪费了多少字节;
六. 检查垃圾数据TOP5
select current_database(),schemaname,relname,n_dead_tup from pg_stat_all_tables where n_live_tup>0 and n_dead_tup/n_live_tup>0.2 and schemaname not in ($$pg_toast$$,$$pg_catalog$$) order by n_dead_tup desc limit 5'
建议
通常垃圾过多, 可能是因为无法回收垃圾, 或者回收垃圾的进程繁忙或没有及时唤醒, 或者没有开启autovacuum, 或在短时间内产生了大量的垃圾 .
可以等待autovacuum进行处理, 或者手工执行vacuum table .
参考
1. 《PostgreSQL Free Space Map Principle》
2. 《PostgreSQL 9.3 improve pgstattuple》
3. http://www.postgresql.org/docs/9.3/static/pgstattuple.html
4. http://www.postgresql.org/docs/9.3/static/storage-page-layout.html
5. 《PostgreSQL Improve fsm & vm output》
6. http://www.postgresql.org/docs/devel/static/pgfreespacemap.html
7. https://wiki.postgresql.org/wiki/Show_database_bloat