life on a_rollercoaster

Life on a rollercoasterScaling the PostgreSQL backup and recovery

Federico Campoli

Transferwise

2 November 2016

Federico Campoli (Transferwise) Life on a rollercoaster 2 November 2016 1 / 52

Table of contents

1 2012 - Who am I? What I’m doing?

2 2013 - On thin ice

3 2014 - The battle of five armies

4 2015 - THIS IS SPARTA!

5 2016 - Breathe

6 Wrap up


Warning!

The story you are about to hear is true.Only the names have been changed to protect the innocent.


Dramatis personae

A brilliant startup ACME


Dramatis personae

The clueless engineers CE


Dramatis personae

An elephant on steroids PG


Dramatis personae

The big cheese HW


Dramatis personae

The real hero DBA


In the beginning

Our story starts in the year 2012. The world was young and our DBA started anew brilliant career in ACME.

After the usual time required by the onboarding, to our DBA were handed theproduction’s servers.


Table of contents





5 2016 - Breathe

6 Wrap up


2012 - Who am I? What I’m doing?


Size does matter

PG, our powerful and friendly elephant was used for storing the data in a multishard configuration.Not really big actually but very troubled indeed!

A small logger database - 50 GB

A larger configuration and auth datababase - 200 GB

Two archive db - 4 TB each

One db for the business intelligence - 2 TB

Each db had an hot standby counterpart hosted on less powerful HW.

Our story tells the life of the BI database.


The carnival of monsters

In the early 2013 our brave DBA addressed the several problems found on thecurrent backup and recovery configuration.


Lagging standby

Suboptimal schema.Churn on large tables and high wal generation rateThe slave lagged just because there was autovacuum running

rsync used in archive command.The wal were archived over the network using rsync+sshThe *.ready files in the pg xlog increased the risk of the cluster’s crash.


Base backup

Rudimentary init standby script.Just a pg start backup call followed by a rsync between the master and theslave

The several tablespaces were synced using a single rsync process with the–delete option


Slow dump

Remote pg dump.Each cluster was dumped remotely on a separate server using the customformat.

The backup server had limited memory and cpu

Dump time between 3 hours and 2 days depending on the database size

The BI database was dumped on a daily basis, taking 14/18 hours.


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5 2016 - Breathe

6 Wrap up


2013 - On thin ice


Parallel rsync

Our DBA took the baby step approach. He started fixing one issue at a timewithout affecting ACME’s activity.

The first enhancement was the init backup script.

Two bash arrays listed the origin and destination’s tablespaces

An empty bash array stored the rsync pids

The script started the pg start backup

For each tablespace a rsync process were spawned and the pid was stored inthe third array

A loop checked that the pids were present in the process list

When all the rsync finished pg stop backup were executed

An email to DBA was sent to tell him to start the slave


Local archive

The high rate of wal generation required a different archive strategy.

The archive command changed to a local copy

A simple rsync script copied every minute the archives to the slave

The script queried remotely the slave for the last restartpoint

The restartpoint was used by pg archivecleanup on the master

Implementing this solution solved the *.ready files problem but the autovacuumstill caused high lag.


Autovacuum tune down

DBA investigated the autovacuum issue and finally addressed the cause.

The high lag on the slave was caused when autovacuum (or vacuum) hit a tableconcurrently updated. This behaviour is normal and is caused by the standbycode’s design.

With large denormalised tables which are updated constantly the only workaroundpossible was to increase autovacuum cost delay with a large value (1 second ormore).

When the autovacuum process reached an arbitrary cost during the executionthere was a 1 minute sleep before the activity resumed..The lag on the standbys disappeared at the cost of longer autovacuum runs.


In the meanwhile...

The CE decided to shard the business intelligence database using the hotstandby copy

The three new databases initially had the same amount of data which wasslowly cleaned up later

But even with one third of data on each shard, the daily dump was reallyslow at the point of overlapping over the 24 hours


A slowish dump

pg dump connects to the running cluster like any other backend; it pulls outdata using in the copy format

With the custom format the compression happens on the server wherepg dump runs

The backup server were hammered on the network and the cpu


You got speed!

Our DBA wrote a bash script doing the following steps

Dump the database in custom format locally

Generate the file’s md5 checksum

Ship the file on the backup via rsync

Check the remote file’s md5

Send a message to nagios for success or failure

The backup time per each cluster dropped dramatically to just 5 hours includingthe copy and the checksum verification.


Growing pains

Despite the business growth the CE ignored the problems with the poor schemadesign.

Speed was achieved by brute force using expensive SSD storage

The amount of data store in the BI db increased

The only accepted solution was to create new shards over and over again

By the end of 2013 the BI databases total size was 15 TB


In the meanwhile...

Our DBA upgraded all the PG to the version 9.2 with pg upgrade

THANKS BRUCE!!!!!


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5 2016 - Breathe

6 Wrap up


2014 - The battle of five armies


Bloating data

Q1 2014 opened with another backup performance issue

The dump size increased over the time

The database CPU usage increased constantly with no apparent reason

Most of the shards had the tablespace usage at 90%


Mostly harmless

Against all odds our DBA addressed the issue.

The table used by the BI database was causing the bloat

The table’s design was technically a materalised view

The table was partitioned in some way

The table had an harmless hstore field

Where everybody added new keys just changing the app code

And nobody did housekeeping of their data

The row length jumped from 200 bytes to 1200 bytes in few months

Each BI shard contained up to 2 billion rows...


Just a little trick

Despite the impending doom and the CE resistance DBA succeeded in convertingthe hstore field to a conventional columnar store (SORRY OLEG!).

The storage usage dropped by 30%

The CPU usage dropped by 60%

The speed of ACME’s product boosted

ACME saved $BIG BUNCH OF MONEY in new HW otherwise required toshard again the dying databases


In the meanwhile...

DBA knew the fix was just a workaround

He asked the CE to help him in the schema redesign

He told them things would be problematic again in just one year

Nobody listened


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5 2016 - Breathe

6 Wrap up


2015 - THIS IS SPARTA!


I hate to say that, but I told you so

As predicted by our DBA the time required for backing up the BI databasesincreased again, approaching dangerously the 24 hours.


Parallel is da way!

Back in the 2013 PG 9.3 added the parallel export. But, to DBA greatdisappointment, the version 9.3 was initially cursed by some bugs causing datacorruption. DBA could not use the parallel dump.However...

The parallel backup takes advantage of the snapshot export introduced in thePG 9.2

The debian packaging allows different PG’s major versions on the samemachine

DBA installed the client 9.3 and used its pg dump to dump the 9.2 in parallel


It worked very well...

The wrapper script required some adjustments

Accept the -j parameter

Check if the 9.3+ client is installed

Override the format to directory if the parallel backup is possible

Adapt the checksum procedure to check the files in the dump directory


...with just a little infinitesimal catch

All fine right?Not exactly

The restore test complained about the unknown parameter lock timeout

The backup hit the speed record since 2013

The schema was still the same of 2013

The databases performance were massively affected with 6 parallel jobs

DBA found that with just 4 parallel jobs the databases worked with minimaldisruption


In the meanwhile...

Our DBA upgraded PG to the latest version 9.4.

THANK YOU AGAIN BRUCE!!!!!

No more errors for the restore test.


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5 2016 - Breathe

6 Wrap up


2016 - Breathe


A new hope

The upgrade to PG 9.4 improved the performance issues and DBA had some timeto breath.

The script to ship the archived wal was improved to support multiple slavesin cascading replica

Each slave had a dedicated rsync process configurable with compression andprotocol (rsync or rsync +ssh)

The script determined automatically the farthest slave querying the remotecontrolfiles and cleaned the local archive accordingly


A new hope

The init standby script switched to the rsync protocol

The automated restore script used the ALTER SYSTEM added to the PG 9.4to switch between the restore and production configuration

Therefore the restore time improved to at most 9 hours for the largest BIdatabase (4.5 TB)

Working with BOFH JR, DBA wrapped the backup script in the$BACKUP MANAGER pre and post execution hooks


The rise of the machines

In the 2016 Q2, finally, DBA completed the configuration for $DEVOP TOOL anddeployed the several scripts to the 17 BI databases with minimal effort.


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5 2016 - Breathe

6 Wrap up


BI database at a glance

Year N. Databases Average size Total size Version2012 1 2 TB 2 TB 9.12013 5 3 TB 15 TB 9.22014 9 2.2 TB 19 TB 9.22015 13 2.7 TB 32 TB 9.42016 16 2.5 TB 40 TB 9.4


Few words of wisdom

Reading the products source code is always a good practice.

Bad design can lead to disasters, in particular if the business is successful.It’s never too early to book the CE to a SQL training course.

“One bad programmer can easily create two new jobs a year.” – David Parnas

If in doubt ask your DBA for advice.If you don’t have a DBA, get one hired ASAP!


Did you say hire?

WE ARE HIRING!https://transferwise.com/jobs/


https://transferwise.com/jobs/

That’s all folks!

QUESTIONS?


Boring legal stuff

LAPD badge - source wikicommons

Montparnasse derailment - source wikipedia

Base jumper - copyright Chris McNaught

Disaster girl - source memegenerator

Blue elephant - source memecenter

Commodore 64 - source memecenter

Deadpool- source memegenerator

Thin ice - source Boating on Lake Winnebago

Boromir - source memegenerator

Sparta birds - source memestorage

Dart Vader - source memegenerator

Angry old man - source memegenerator


https://commons.wikimedia.org/wiki/File:Badge_of_the_Los_Angeles_Police_Department.png

https://en.wikipedia.org/wiki/Montparnasse_derailment

https://commons.wikimedia.org/wiki/File:BASE_jumping_-_Perrine_Bridge.jpg

https://memegenerator.net/instance/64551469

http://www.memecenter.com/fun/845966/i-posted-this-because-of-the-cute-elephant

http://www.nostalgianerd.com/the-commodore-64


http://www.boatingwinnebago.com/news/three-days-three-drownings-has-wdnr-warning-about-thin-ice/


http://memestorage.com/news/this_is_sparta_birds/2014-02-17-7390



Contacts and license

Twitter: 4thdoctor scarf

Blog:http://www.pgdba.co.uk

Brighton PostgreSQL Meetup:http://www.meetup.com/Brighton-PostgreSQL-Meetup/

This document is distributed under the terms of the Creative Commons


http://www.pgdba.co.uk

http://www.meetup.com/Brighton-PostgreSQL-Meetup/

Life on a rollercoasterScaling the PostgreSQL backup and recovery

Federico Campoli

Transferwise

2 November 2016


life on a_rollercoaster

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