Online tech Talks (italian) Speaker: Antonio D'Ortenzio

1. © 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.© 2019, Amazon Web Services, Inc. or its Affiliates. All rights reserved.
AWS Online Tech Talks
La serie di webinar AWS in Italiano
Oracle DBMS vs Amazon RDS vs Amazon Aurora PostgreSQL:
principali similitudini e differenze architetturali e di gestione
Speaker
Antonio D’Ortenzio
Solutions Architect, AWS
Obiettivi
§ Confrontare la architettura high-level di Oracle RDBMS con
quella di PostgreSQL evidenziando principali similitudini e
differenze
§ Spiegare i miglioramenti introdotti da Amazon Aurora,
focalizzando il confronto sulla gestione della memoria e del
caching nei casi Oracle vs Amazon RDS PostgreSQL vs Amazon
Aurora PostgreSQL
§ Illustrare i vantaggi di Amazon Aurora, concludendo con un focus
sulla gestione, nei tre scenari, di Transazioni, Redo, Checkpoints,
Commits e Recovery

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Background
• Oracle to Amazon RDS PostgreSQL or Amazon Aurora PostgreSQL: are you
wondering what the differences between your source and your target will be?
• Will the way these engines administer your databases still be the same after
change?
• Application patterns à improvements after completing the platform change
• Strengths of new database technologies à improve your application
performance and reduce your database management burden
• A result of all this is … you surely have lots of comparative questions between
Oracle and PostgreSQL

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Objectives
What this session is
• A collection of most Frequently Asked Comparative Questions received from
customers over the last year
• A collection of the background concepts to the Answers we have been
giving to these questions
What this session is not
• A comprehensive list of ALL differences between Oracle and PostgreSQL
• A discussion around SQL-level and coding differences; for these, please
consider reading the Database Migration Playbook

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Agenda
• Software Architecture
• Memory Management
• Transactions and Recovery Management

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AWS Global Infrastructure
21 Regions
66 Availability Zones (AZ’s)
First 5 Years: 4 Regions
Next 5 Years: 7 Regions
2016-2019: 10 Regions
Coming Soon: 4 Regions
including …
Milan, Italy!

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AWS Regions and Availability Zones
US West (OR)
AZ A AZ B
AZ C
US East (OH)
AZ A AZ B
AZ C
US East (VA)
AZ A AZ B
AZ C AZ D
AZ E
EU (Ireland)
AZ A AZ B
AZ C
Asia Pacific
(Singapore)
AZ A AZ B
AZ C
EU (London)
AZ A AZ B
AWS Regions
China (Beijing)
AZ A AZ B
China (Bejing)
AZ A AZ B
Asia Pacific
(Seoul)
AZ A AZ B
AZ C
AZ A AZ B
AZ C
South America
(São Paulo)
Asia Pacific
(Sydney) Asia Pacific
(Mumbai)
AZ A AZ B
US West (CA)
AZ A AZ B
AZ C
Canada
AZ A AZ B
AZ F
GovCloud (US West)
AZ A AZ B
AZ C
AZ C
China (Ningxia)
AZ A AZ B
AZ C
EU (Paris)
AZ A AZ B
AZ C
EU (Frankfurt)
AZ A AZ B
AZ C
Asia Pacific
(Tokyo)
AZ A AZ B
AZ C AZ D

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Notes
By “Oracle” I will refer to the “On-Premises Oracle RDBMS Enterprise Edition 12c”
• Single-Node
• no ASM à topics in this presentation are not related to/affected by storage architecture details
• no RAC à topics in this presentation are not related to/affected by application profiles,
networking, high availability, etc
• no Exadata à topics in this presentation are not related to/affected by specific aspects of Oracle
Engineered Systems
By “RDS PostgreSQL” I will refer to “Amazon RDS PostgreSQL 11.2-R1”
• Default Configuration, Single-AZ
By “Aurora PostgreSQL” I will refer to “Amazon Aurora PostgreSQL 10.7”
• Default Configuration, Multi-AZ

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Compared Environments: A Bird’s Eye View
EBS EBS EBS
AZ 1 AZ 2 AZ 3
Amazon
Aurora
database
Instance
R/W Master
Node
Storage
Node
Storage
Node
Storage
Node
Storage
Node
Storage
Node
Storage
Node
Amazon
RDS
database
Instance
R/W Master
Node
AZ 1
On-
Premises
Server
R/W Active
Node
On-Premises Storage
ORACLE

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Software Architecture - Question
How is PostgreSQL software
architecture different than
Oracle?

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Architecture Comparison
Stats
Collector
Auto-
vacuum
BG
Writer
WAL
Writer
Check
pointer
Archiver
Database Cluster
OTHERS
…
PG Shared
Memory
Instance
pg_control
pg_xlog (WAL files)
postgresql.conf
pg_tblspc
Backend
“1”
Backend
“n”
PerBackEnd Memory
Postmaster
(Daemon)
…
.
Client
Application
PMON SMON DBWR LGWR CKPT ARCH
Database
OTHERS
…
Instance
Control
Files
Redo Log Files
Parameter
Files
Data Files
SGA
(System
Global Area)
Server
Process “1”
PGA (Program Global Area)
Listener …
.Server
Process “n”
Client
Application
ORACLE

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Software Architecture – Answer / Takeaway
How is PostgreSQL software
architecture different than
Oracle?
There are low-level differences, but at a
high-level background processes and
storage layer items can be conceptually
mapped.

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Memory Management – Questions
How is Memory handled in RDS PostgreSQL and Aurora
PostgreSQL? Are there differences between the two? And if
compared to Oracle?

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Remaining System Memory
Flash Cache
OS Page Cache
Memory Architectures and Cache Management
Amazon EBS
SGA
Shared Pool
Library Cache
Data Dictionary Cache
Redo Log Buffer
PGAs
----------------------
Database Buffer Cache
Shared Memory
WAL Buffer
Backend Processes
----------------------
Shared Buffer Pool
Duplicate OS Buffers
Shared Memory
WAL Buffer
Backend Processes
---------- ----------- -
Shared Buffer Pool
Aurora StorageOn-Premises Storage
ORACLE

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Memory Management – Answers / Takeaways
How is Memory handled in RDS PostgreSQL and Aurora
PostgreSQL? Are there differences between the two? And if
compared to Oracle?
PostgreSQL doesn't maintain a Shared Pool; executable
versions of SQL statements can be cached at the session-level
rather than shared. This approach is against "one-fits-all" but
rather favors a more granular tailoring of SQL executions.
RDS PostgreSQL leverages OS buffer caching in addition to
Shared Memory. Aurora doesn't do that, hence allowing for
larger room for Buffers.

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TXNs and Recovery Mgmt – Questions
Are UNDO tablespaces going to be available with PostgreSQL?
How are Recovery, Checkpoints and Commits going to be
handled by RDS PostgreSQL and Aurora PostgreSQL?

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MVCC (Multi-Version Concurrency Control): Differences
ORACLE: Undo Tablespaces <> Redo Logs
Undo Tablespaces contain before images of data blocks that have been read into the
SGA and have been amended. They are used for:
a. Read Consistency
b. Process-Level Transaction Rollback
PostgreSQL: No Undo Tablespaces
• Before images are maintained inside the data files and marked as “expired” when no
longer current
• PostgreSQL does not rollback changes to database rows when a transaction exits
abnormally
• A PostgreSQL table may contain data from transactions exited abnormally
à until the VACUUM process cleans them up

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Redo, Checkpoints, Commits
ORACLE / PostgreSQL
REDO LOGS / WAL
1. No big differences in the way Redo Logs / WAL are used and what their role is
2. REDO logs/WAL are used for DB integrity.
3. Once a change gets written on REDO logs/WAL it’s permanent.
4. Unlike Undo Tablespaces (which are used for individual process-level transaction rollback), REDO logs/WAL are used
for Instance-level recovery.
5. Rolling forward those changes that were written on REDO logs/WAL but not yet on data files
6. Rolling back those changes that were potentially written on data files but not yet on REDO logs/WAL
CHECKPOINT
• Writes all modified Buffers from Memory to Disk
• Trade-Off = Recovery Time vs Performance
COMMIT
• Per Single Transaction
• Guarantees Permanency of a Single Transaction

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Putting It All Together
Stats
Collector
Auto-
vacuum
BG
Writer
WAL
Writer
Check
pointer
Archiver
Database Cluster
OTHERS
…
PG Shared
Memory
Instance
pg_control
pg_xlog (WAL files)
postgresql.conf
pg_tblspc
Backend
“1”
Backend
“n”
PerBackEnd Memory
Postmaster
(Daemon)
…
.
Client
Application
PMON SMON DBWR LGWR CKPT ARCH
Database
OTHERS
…
Instance
Control
Files
Redo Log Files
Parameter
Files
Data Files
SGA
(System
Global Area)
Server
Process “1”
PGA (Program Global Area)
Listener …
.Server
Process “n”
Client
Application
Redo
Log
Buffer
WAL
Buffer
update update
Undo
commit commit
checkpoint checkpoint
ORACLE

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Aurora: No Bottleneck due to Redo/WAL Log Buffer
Amazon Elastic Block Store
(EBS)
/
Redo Log/WAL work items
Redo Log/WAL Buffer
Redo Log/WAL
AZ1
Shared Storage Volume – Transaction Aware
AZ2 AZ3
A B C
D E F
A B C
1234 12344 12344
A AA BB
CCC
B
A B C
A B C
ORACLE

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Amazon Aurora Reduced Recovery Time Up to 97%
Transaction-aware storage system recovers almost instantly
3 GiB redo
recovered in 19 seconds
10 GiB redo
recovered in 50 seconds
30 GiB redo
recovered in 123 seconds
Amazon Aurora has no redo.
Recovered in 3 seconds while maintaining
signficantly greater throughput.
0
20
40
60
80
100
120
140
160
0 20000 40000 60000 80000 100000 120000 140000
Recoverytimeinseconds(lessisbetter)
writes / second (more is better)
Recovery time from crash under load
Bubble size represents redo log which must be recovered
As PostgreSQL throughput
goes up, so does log size
and crash recovery time.

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TXNs and Recovery Mgmt – Answers / Takeaways
Are UNDO tablespaces going to be available with PostgreSQL?
How are Recovery, Checkpoints and Commits going to be
handled by RDS PostgreSQL and Aurora PostgreSQL?
No, Undo information is contained in the Data Files.
Oracle and RDS PostgreSQL handle Recovery, Checkpoints, and
Commits in a very similar way. Aurora is very different in that it
handles Commits via Write Quorums, and Recovery time is near-0
thanks to instantaneous lookup of transactions with a valid
Quorum.

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Conclusions
1. There are differences between Oracle and PostgreSQL, but also many similar
aspects (Software Architecture, Recovery Management)
2. Dive deep in your use cases and application patterns à there is a large
number of use cases, currently covered by Oracle, where PostgreSQL
represents an ideal alternative at a significantly lower cost
3. Evaluate covering these use cases with PostgreSQL and allow them to
benefit from the enrichments coming from Amazon Aurora’s specific
implementation and optimizations
4. Finally, consider that getting to know Amazon RDS PostgreSQL or Amazon
Aurora PostgreSQL is going to be an easy learning process as there are many
points of contact between your background and the architectural aspects of
these technologies

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Thank you!
email: antodort@amazon.it
twitter: @adortenz