Pg_Migrator allows for rapid upgrades between major PostgreSQL releases without requiring a data dump and restore. It works by installing new system tables in the new cluster while reusing the data files from the old cluster. This is done by freezing rows in the new cluster, transferring transaction logs and IDs, restoring the schema, and then copying over user data files. Pg_Migrator upgrades have been shown to take only minutes, much faster than a traditional dump and restore between major versions.

1. Rapid Upgrades With Pg_Migrator
BRUCE MOMJIAN,
ENTERPRISEDB
March, 2010
Abstract
Pg_Migrator allows migration between major releases of Postgres
without a data dump/reload. This presentation explains how
pg_migrator works.
Creative Commons Attribution License http://momjian.us/presentations

2. Traditional Postgres Upgrade Options
pg_dump (logical dump)/restore
Slony
Rapid Upgrades With Pg-Migrator 1

3. Why Upgrading Postgres Is Complex
New features often require system table changes
However, the internal data format rarely changes
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4. Why Pg_Migrator
Very fast upgrades
Optionally no additional disk space
pg_migrator installs new system tables while using data files from the
previous Postgres version.
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5. How It Works: Initial Setup
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22
11 17 23
12 18 24
13 19 25
14 20 26
15 21 27
clog clog
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6. Decouple New Clog Via Freezing
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2
3
5
6
8
9
2
3
Freeze 5
6
8
9
pg_class pg_class
User Tables User Tables
10 16 22
11 17 23
12 18 24
13 19 25
14 20 26
15 21 27
X X
clog clog
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7. Transfer Clog and XID
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22
11 17 23
12 18 24
13 19 25
14 20 26
15 21 27
clog clog
controldata xid controldata
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8. Get Schema Dump
pg_dumpall - -schema
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22
11 17 23
12 18 24
13 19 25
14 20 26
15 21 27
clog clog
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9. Reserve TOAST OIDs Using Relfilenodes (pre-9.0)
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22
11 17 23
12 18 24 18
13 19 25
14 20 26 26
15 21 27
clog clog
This is necessary because heap references to TOAST tables contain the
TOAST oids for easy lookup.
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10. Restore Schema In New Cluster
pg_dumpall - -schema
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22 30 36 42
11 17 23 31 37 43
12 18 24 32 18 44
13 19 25 33 39 45
14 20 26 34 40 26
15 21 27 35 41 47
clog clog
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11. Connect TOAST Placeholders To the Proper Relations
(pre-9.0)
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22 30 36 42
11 17 23 31 37 43
12 18 24 32 18 44
13 19 25 33 39 45
14 20 26 34 40 26
15 21 27 35 X
41 47
clog clog
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12. Copy User Heap/Index Files
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22 30 36 42
11 17 23 31 37 43
12 18 24 32 18 44
13 19 25 33 39 45
14 20 26 34 40 26
15 21 27 35 41 47
clog clog
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13. Complete
Old Cluster New Cluster
System Tables System Tables
1 4 7 1 4 7
2 5 8 2 5 8
3 6 9 3 6 9
pg_class pg_class
User Tables User Tables
10 16 22 30 36 42
11 17 23 31 37 43
12 18 24 32 18 44
13 19 25 33 39 45
14 20 26 34 40 26
15 21 27 35 41 47
clog clog
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14. How It Works: In Detail
Check for cluster compatability
– locale
– encoding
– integer datetimes (default changed from 8.3 -> 8.4)
Use pg_dumpall to dump old cluster schema (no data)
Freeze all new cluster rows (remove reference to clog entries)
Rename tablespaces to *_old (pre-9.0)
New cluster uses old xid counter value (see freeze above)
– Set system table frozen xids to match the current xid
Create new users/databases
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15. Collect cluster information
Install support functions that call internal backend functions
Create placeholder files to reserve relfilenode file names (pre-9.0)
Create schema in new cluster
Adjust new cluster to use reserved relfilenode names (pre-9.0)
– Delete placeholder toast relfilenode files
– Remove new cluster toast tables
– Create new cluster toast table using reserved relfilenode
– Assign new toast tables with proper relfilenodes to relations
Copy or link files from old cluster to new cluster
– Toast tables have the same relfilenodes as in the old cluster
Warn about any remaining issues, like REINDEX requirements
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16. Sample Run: Performing Consistency Checks
Performing Consistency Checks
-----------------------------
Checking old data directory (/u/pgsql.old/data) ok
Checking new data directory (/u/pgsql/data) ok
Checking for columns with user-defined composite types ok
Checking for columns with user-defined array types ok
Checking for columns with user-defined enum types ok
Checking for /contrib/isn with bigint-passing mismatch ok
Checking for invalid ’name’ user columns ok
Checking for tsquery user columns ok
Creating script to adjust sequences ok
Creating catalog dump ok
Checking for presence of required libraries ok
| If pg_migrator fails after this point, you must
| re-initdb the new cluster before continuing.
| You will also need to remove the ".old" suffix
| from /u/pgsql.old/data/global/pg_control.old and old
| tablespace directory names.
Many of these checks are unnecessary when migrating to Postgres 9.0.
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17. Sample Run: Performing Migration
Performing Migration
--------------------
Adding ".old" suffix to tablespace directory names ok
Adding ".old" suffix to old global/pg_control ok
Analyzing all rows in the new cluster ok
Freezing all rows on the new cluster ok
Deleting new commit clogs ok
Copying old commit clogs to new server ok
Setting next transaction id for new cluster ok
Resetting WAL archives ok
Setting frozenxid counters in new cluster ok
Creating databases in the new cluster ok
Creating placeholder relfiles for toast relations
/u/pgsql/data/base/16397/16586
Adding support functions to new cluster ok
Restoring database schema to new cluster ok
Restoring relations to use fixed toast file names
/u/pgsql/data/base/16397/16586
Removing support functions from new cluster ok
Restoring user relation files
/u/pgsql.old/data/base/11511/2613
Setting next oid for new cluster ok
Creating script to delete old cluster ok
Adjusting sequences ok
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18. Sample Run: Completion
Upgrade complete
----------------
| Optimizer statistics and free space information
| are not transferred by pg_migrator so consider
| running:
| vacuumdb --all --analyze
| on the newly-upgraded cluster.
| Running this script will delete the old cluster’s data files:
| /u/postgres/pg_migrator_output/delete_old_cluster.sh
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19. Possible Data
Format Changes
Change Conversion Method
clog none
heap page header, including bitmask convert to new page format on read
tuple header, including bitmask convert to new page format on read
data value format create old data type in new cluster
index page format reindex, or recreate index methods
TOAST page format convert to new page format on read
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20. Migration Timings
Migration Method Minutes
dump/restore 300.0
dump with parallel restore 180.0
pg_migrator in copy mode 44.0
pg_migrator in link mode 0.7
Database size: 150GB, 850 tables
The last duration is 44 seconds.
Timings courtesy of
Stefan Kaltenbrunner
(mastermind on IRC)
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21. Conclusion
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