introduction to vacuum freezing and xid

Copyright © 2016 NTT DATA Corporation

03/17/2016 NTT DATA Corporation Masahiko Sawada

Introduction to VACUUM, FREEZING and XID wraparound

2 Copyright © 2016NTT DATA Corporation

A little about me

Ø  Masahiko Sawada Ø  twitter : @sawada_masahiko

Ø  NTT DATA Corporation Ø  Database engineer

Ø  PostgreSQL Hacker Ø  Core feature Ø  pg_bigm (PostgreSQL full text search module for multi-byte language)

3 Copyright © 2016NTT DATA Corporation

Contents

•  VACUUM

•  Visibility Map

•  Freezing Tuple

•  XID wraparound

•  New VACUUM feature for 9.6

Copyright © 2016 NTT DATA Corporation 4

What is the VACUUM?

5 Copyright © 2016 NTT DATA Corporation

VACUUM

1 AAA

2 BBB

3 CCC

2 bbb

4 DDD Concurrently INSERT/DELETE/UPDATE

1 AAA

2 BBB

3 CCC

2 bbb

1 AAA

3 CCC

2 bbb

4 DDD

VACUUM Starts

VACUUM Done FSM

UPDATE : BBB->bbb

•  Postgres garbage collection feature

•  Acquire ShareUpdateExclusive Lock


Why do we need to VACUUM?

•  Recover or reuse disk space occupied

•  Update data statistics

•  Update visibility map to speed up Index-Only Scan.

•  Protect against loss of very old data due to XID wraparound


Evolution history of VACUUM

v8.1 (2005) v8.4 (2009)

autovacuum !?

Visibility Map Free Space Map

v9.5 (2016)

vacuumdb parallel option

v9.6


VACUUM Syntax

-- VACUUM whole database =# VACUUM;

-- Multiple option, analyzing only col1 column

=# VACUUM FREEZE VERBOSE ANALYZE hoge (col1);

-- Multiple option with parentheses

=# VACUUM (FULL, ANALYZE, VERBOSE) hoge;


Visibility Map


Visibility Map

•  Introduced at 8.4 •  A bit map for each table (1 bit per 1 page) •  A table relation can have a visibility map. •  keep track of which pages are all-visible page

•  keep track of which pages are having garbage. •  If 500GB table, Visibility Map is less than 10MB.

Table (base/XXX/1234)

Visibility Map (base/XXX/1234_vm) Block 0

Block 1 Block 2 Block 3 Block 4

11001…


State transition of Visibility Map bit

VACUUM

0 1

INSERT, UPDATE, DELETE

(NOT all-visible) (all-visible)


How does the VACUUM works actually?

•  VACUUM works with two phases;

1.  Scan table to collect TID

2.  Reclaim garbage (Table, Index)

maintenance_work_mem

Index

Table

Scan Table

Collect garbage TID

Reclaim garbages

1st Phase

2nd Phase


Performance improvement point of VACUUM

•  Scan table page one by one.

•  vacuum can skip, iff there are more than 32 consecutive all-visible pages

•  Store and remember garbage tuple ID to maintenance_work_mem.

VACUUM can skip to scan efficiency.

SLOW!! FAST!

VACUUM needs to scan all page.

: all-visible block

: Not all-visible block


XID wraparound and freezing tuple


What is the transaction ID (XID)?

•  Every tuple has two transaction IDs. •  xmin : Inserted XID •  xmax : Deleted/Updated XID

xmin | xmax | col -------+------+------ 1810 | 1820 | AAA 1812 | 0 | BBB 1814 | 1830 | CCC 1820 | 0 | XXX

In REPEATABLE READ transaction isolation level, •  Transaction 1815 can see ‘AAA’, ‘BBB’ and ‘CCC’. •  Transaction 1821 can see ‘BBB’, ‘CCC’ and ‘XXX’ •  Transaction 1831 can see ‘BBB’ and ‘XXX’.


What is the transaction ID (XID)?

•  Can represent up to 4 billion transactions (uint32).

•  XID space is circular with no endpoint.

•  There are 2 billion XIDs that are “older”, 2 billion XIDs that are “newer”.

0 232-1

Older (Not visible)

Newer (Visible)


What is the XID wraparound?

XID=100 XID=100

XID 100 become not visible

XID=100

Older (Visible)

Newer (Not visible)

XID 100 is visible

Older (Not visible) Older

(Not visible)

Newer (Visible)

Newer (Visible)

Still visible

•  Postgres could loss the very old data due to XID wraparound.

•  When tuple is more than 2 billion transaction old, it could be happen.

•  If 200 TPS system, it’s happen every 120 days.

•  Note that it could be happen on INSERT-only table.


Freezing tuple

•  Mark tuple as “FREEZE”

•  Marking “frozen” means that it will appear to be “in the past” to all transaction.

•  Must freeze old tuple *before* XID proceeds 2 billion.

XID=100 (FREEZE)

XID=100 (FREEZE)

Tuple is visible.

XID=100

Older (Visible)

Newer (Not visible)

XID 100 is visible

Older (Not visible) Older

(Not visible)

Newer (Visible)

Newer (Visible)

Still visible. Tuple is marked as ‘FREEZE’


To prevent old data loss due to XID wraparound

•  Emit WARNING log at 10 million transactions remaining.

•  Prohibit to generate new XID at 1 million transactions remaining.

•  Run anti-wraparound VACUUM automatically.


anti-wraparound VACUUM


Anti-wraparound VACUUM

•  All table has pg_class.relfrozenxid value. •  All tuples which had been inserted by XID older than relfrozenxid have been

marked as “FREEZE”. •  Same as forcibly executed VACUUM *FREEZE*.

Current XID pg_class. relfrozenxid

anti-wraparound VACUUM is

launched forcibly

VACUUM could do a whole table scan

autovacuum_max_freeze_age (default 200 million)

+ 2 billion

vacuum_freeze_table_age (default 150 million)

XID wraparound



At this XID, lazy VACUUM is executed.



launched forcibly



+ 2 billion


XID wraparound

VACUUM




If you execute VACUUM at this XID, anti-wraparound VACUUM will be

executed.

If you do VACUUM at this XID, anti-wraparound VACUUM is executed.

pg_class. relfrozenxid


launched forcibly


+ 2 billion


XID wraparound

anti-wraparound VACUUM

Current XID



After current XID is exceeded, anti-wraparound VACUUM is launched forcibly by autovacuum.

pg_class. relfrozenxid


launched forcibly


+ 2 billion


XID wraparound

anti-wraparound auto VACUUM

Current XID




After anti-wraparound VACUUM, relrozenxid value is updated.


vacuum_freeze_min_age (default 50 million)


anti-wraparound VACUUM is too slow

•  Scanning whole table is always needed.

•  Because lazy vacuum could skip page having the visible but not frozen tuple.

Visibility Map

Block # xmin

0 0 FREEZE FREEZE

1 1 FREEZE FREEZE

1 2 101

102

103

0 3 Garbage

104

Lazy VACUUM



How can we improve anti-wraparound VACUUM?


Approaches

•  Freeze Map

•  Track pages which are necessary to be frozen.

•  64bit XID

•  Change size of XID from 32bit to 64bit.

•  LSN to XID map

•  Mapping XID to LSN.


Freeze Map

•  New feature for 9.6.

•  Improve VACUUM FREEZE, anti-wraparound VACUUM performance.

•  Bring us to functionality for VLDB.


Idea - Add an additional bit

•  Not adding new map.

•  Add a additional bit to Visibility Map.

•  The additional bits tracks which pages are all-frozen.

•  All-frozen page should be all-visible as well.

10110010 all-visible all-frozen


State transition of two bits

00

10 11

all-visible all-frozen

VACUUM UPDATE/ DELETE/ INSERT

UPDATE/ DELETE/ INSERT

VACUUM FREEZE

VACUUM FREEZE


Idea - Improve anti-wraparound VACUUM performance

•  VACUUM can skip all-frozen page even if anti-wraparound VACUUM is

required.

Lazy VACUUM


Visiblity Map Block # xmin

visible frozen

1 0 0 FREEZE FREEZE

1 1 1 FREEZE FREEZE

1 0 2 101

102

103

0 0 3 Garbage

104


Pros/Cons

•  Pros

•  Dramatically performance improvement for VACUUM FREEZE.

•  Read only table. (future)

•  Cons

•  Bloat Visibility Map size as twice.


No More Full-Table Vacuums

http://rhaas.blogspot.jp/2016/03/no-more-full-table-vacuums.html#comment-form


Another work


Vacuum Progress Checker

•  New feature for 9.6. (under reviewing)

•  Report progress information of VACUUM via system view.


Future works

•  Read Only Table

•  Report progress information of other maintenance command.



PostgreSQL git repository

git://git.postgresql.org/git/postgresql.git


VERBOSE option

=# VACUUM VERBOSE hoge; INFO: vacuuming "public.hoge"

INFO: scanned index "hoge_idx1" to remove 1000 row versions

DETAIL: CPU 0.00s/0.01u sec elapsed 0.01 sec.

INFO: "hoge": removed 1000 row versions in 443 pages

DETAIL: CPU 0.00s/0.00u sec elapsed 0.00 sec.

INFO: index "hoge_idx1" now contains 100000 row versions in 276 pages DETAIL: 1000 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: "hoge": found 1000 removable, 100000 nonremovable row versions in 447 out of 447 pages DETAIL: 0 dead row versions cannot be removed yet.

There were 0 unused item pointers.

Skipped 0 pages due to buffer pins.

0 pages are entirely empty.

CPU 0.00s/0.05u sec elapsed 0.05 sec.

VACUUM


FREEZE option

•  Aggressive freezing of tuples

•  Same as running normal VACUUM with vacuum_freeze_min_age = 0 and

vacuum_freeze_table_age = 0

•  Always scan whole table


ANALYZE option

•  Do ANALYZE after VACUUM •  Update data statistics used by planner

-- VACUUM and analyze with VERBOSE option =# VACUUM ANALYZE VERBOSE hoge;

INFO: vacuuming "public.hoge"

:

INFO: analyzing "public.hoge"

INFO: "hoge": scanned 452 of 452 pages, containing 100000 live rows and 0 dead rows; 30000 rows in sample, 100000 estimated total rows

VACUUM


FULL option

•  Completely different from lazy VACUUM

•  Similar to CLUSTER

•  Acquire AccessExclusiveLock

•  Take much longer than lazy VACUUM

•  Need more space at most twice as table size.

•  Rebuild table and indexes

•  Freeze tuple while VACUUM FULL (9.3~)

introduction to vacuum freezing and xid

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