Postgresql on NFS - J.Battiato, pgday2016

Jonathan Battiato 2ndQuadrant Italia
@2ndquadrant_it @jonny_lee_84
PGDay.IT 2016 - 10th
edition
Prato Dec 13th
2016
PostgreSQL on Network File System?

edition
Prato Dec 13th
2016
© Lucasfilm™
PostgreSQL on Network File System?

edition
Prato Dec 13th
2016
What people say about NFS

edition
Prato Dec 13th
2016
What people say about NFS
IT’S FAST!
CC0 1.0

edition
Prato Dec 13th
2016
PostgreSQL & NFS
Myths & Truths
Jonathan Battiato
www.2ndquadrant.com

edition
Prato Dec 13th
2016
Jonathan Battiato
Linux SysAdmin & DBA
Jonathan Battiato
Linux SysAdmin & DBA
jonny_lee_84
jonathan.battiato@2ndquadrant.it
jonny-lee31
~$ whoami

edition
Prato Dec 13th
2016
Thanks to:
Giuseppe Broccolo, PhD
PostgreSQL & PostGIS consultant
@giubro gbroccolo7
giuseppe.broccolo@2ndquadrant.it
gbroccolo gemini__81

edition
Prato Dec 13th
2016
#PGNFS
#PostgreSQL
#NFS
TAGs
@jonny_lee_84
@2ndquadrant_it
@PGDayIT

edition
Prato Dec 13th
2016
Network File System
general characteristics
Part 1

edition
Prato Dec 13th
2016
Network File System (Sun Microsystem®
, 1984)
●
A protocol for distributed file system – v2, v3, v4
●
servers export the file system, clients mounts the export locally
●
Many clients, one server
●
High performances through fast network
LAN

edition
Prato Dec 13th
2016
NFS v3 vs. NFS v2
●
Strenght
– TCP (UDP is still a choice)
– Asynchronous communication (caching)
– A daemon manages file locking and connections
– Larger block size than 8KB (up to 64KB - depends on Kernel)
●
Weakness
– Stateless (NFS)
– Plain text data transmissions
– Host authentication only
– Dynamic port assignment

edition
Prato Dec 13th
2016
NFS v4 vs. NFS v3
●
Partial solutions
– Single port (2049)
easy to apply firewall rules to filter NFS traffic
– Authentication and locking deamons included in the protocol
– operations are always stateful
– UDP not allowed anymore
NFS v4 is the default on RHEL 6

edition
Prato Dec 13th
2016
Client-Server Optimization
●
Block size (wsize, rsize – nfs protocol level)
Requests are organised in data blocks exchanged between client & server
●
NFS relies on TCP or UDP protocols (transport level)
UDP is statless, TCP is stateful
TCP choose automatically the packet size
●
Packet size & MTU (network level)
Packets size can be optimised to transfer blocks within the network: jumbo frames
CHOOSE WISELY!

edition
Prato Dec 13th
2016
Main concerns about synchronisation
●
NO Time synchronisation
Use NTP!
●
File Locking
v2 does not support file locking
v3 uses daemon for locking, performances drop when in use
●
Delayed write cache
Caching writes could cause data loss
Without write cache performances drop
●
Read (metadata) cache
NFS server may show not-updated attributes, any program that relies on file attributes may not work

edition
Prato Dec 13th
2016
Part 2
NFS & PostgreSQL

edition
Prato Dec 13th
2016
NFS & PostgreSQL
www.postgresql.org/docs/9.6/static/creating-cluster.html#CREATING-CLUSTER-NFS
●
NFS disks are completely transparent for PostgreSQL
●
If NFS client/server implementation does not provide
standard file system semantics, this can cause
reliability problems
●
PostgreSQL advices
– NFS mount options
●
avoid soft-mounting: hard
– General mount options
●
avoid asynchronous writes: fsync
LAN

edition
Prato Dec 13th
2016
hard-mount option
NFS calls must be retried indefinitely
both data and WAL entries sequentiality may not be preserved
write cache
WALs have to be flushed as a database action is committed
many processes are involved during WALs flush on disk
→ several NFS clients
→ WAL entries sequentiality may not be preserved
attribute cache
many processes are involved during WALs flush on disk
→ several NFS clients
→ files attributes may not have consistent views
Sources of corruptions

edition
Prato Dec 13th
2016
Synchronous writes
sync vs. fsync=on:
– NFS v2:
●
if sync is specified, the server will not complete a request until both data/metadata are
written to the disk
– NFS v3 / v4:
●
if sync is specified, the server will complete a request returning the status of the write:
– NFS_FILE_SYNC, NFS_DATA_SYNC, NFS_UNSTABLE
●
data is effectively forced to be flushed on disk once a sync method system call is issued
– be careful to set fsync=on

edition
Prato Dec 13th
2016
Reliability vs. Performance
●
NFS exports must be mounted with safe options
●
Are these option fine for a database?
– is the performance deeply impacted?
– is data safely guaranteed in case of crash?

edition
Prato Dec 13th
2016
Part 3
NFS & PostgreSQL
performance
&
reliability benchmarks

edition
Prato Dec 13th
2016
The customer
NetApp
FAS 8080 Full Flash
Clustered Mode
physical host
storage
2 CPUs x 8GB RAM
RHEL 6
Kernel v. 2.6.32
PostgreSQL 9.5.3
Ethernet
10Gb/s
No routers

edition
Prato Dec 13th
2016
Reliability tests
●
try different crash scenarios under high concurrency load, check if
the instance recovers properly, then execute a VACUUM FULL:
– kill -9 to postmaster
– forced reboot through kernel SysReq:
●
echo 1 > /proc/sys/kernel/sysrq ; echo b > /proc/sysrq-trigger
– power off of the VM from the VMWare vSphere® remote panel
– kill the TCP/IP connections through tcpkill
[root@pgsql] ~# tcpkill host X.X.X.X
tcpkill: listening on eth0 [host X.X.X.X]

edition
Prato Dec 13th
2016
●
try different crash scenarios under high concurrency load, check if
the instance recovers properly, then execute a VACUUM FULL:
– kill -9 to postmaster
– forced reboot through kernel SysReq:
●
echo 1 > /proc/sys/kernel/sysrq ; echo b > /proc/sysrq-trigger
– power off of the VM from the VMWare vSphere® remote panel
– kill the TCP/IP connections through tcpkill
[root@pgsql] ~# tcpkill host X.X.X.X
tcpkill: listening on eth0 [host X.X.X.X]
Reliability tests

edition
Prato Dec 13th
2016
T. Vondra – test the persistence of recycled WALs in ext4
– update attributes of recycled WALs flush them on disk through→ fdatasync
– fdatasync does not force the flush of metadata the update may get lost after a crash→
– logged changes are contained in file “in the future” data loss!→
github.com/2ndQuadrant/ext4-data-loss
●
INSERT/UPDATE new records in parallel and synchronously on the db and on a file
●
simulate a crash compare db & file contents after the crash recovery→
Reliability tests

edition
Prato Dec 13th
2016
Check NFS behaviour for recycled WALs (file attributes caching)
– noac
●
– ac
●
Reliability tests

edition
Prato Dec 13th
2016
– noac
●
several tests, no data loss!
– ac
●
Reliability tests

edition
Prato Dec 13th
2016
– noac
●
– ac
●
Reliability tests

edition
Prato Dec 13th
2016
All tests passed, but this does not ensure that it is totally safe!
Reliability tests

edition
Prato Dec 13th
2016
Enhance reliability for PostgreSQL DBs
●
allow page checksums (9.3+): initdb –data-checksums ...
– execute CRC32 calculation for each 8KB data block
●
a checksum failure means that the data block is corrupted
– force wal_log_hint=true
●
write the entire 8KB page to the WAL, even for hint bits modification
– take into account the impact to the performance:
●
R: checksum extra-calculation every 8KB
●
W: increase the amount of information logged into WALs

edition
Prato Dec 13th
2016
Enhance performance for PostgreSQL DBs
●
sync’ed writes are slow, even if NFS caching is enabled
●
if possible, consider asynchronous commit:
– let the server return success as soon as the transaction is logically completed
●
synchronous_commit=off – it can be set per user/session
●
WAL entries will be flushed in a second moment,
but not later than 3x wal_writer_delay
●
in case of crashes, the DB can be recovered in a consistent state,
but there could be a window of data loss

edition
Prato Dec 13th
2016
file system I/O speed:
– reads: 700MB/s
– writes: 700MB/s
synchronous_commit=on:
– INSERT:
●
noac ~ ac (single & multi client)
– SELECT:
●
noac ~ 16x ac
DB commit rate and SELECT rate:
synchronous_commit=off:
– INSERT:
●
noac~ 8x ac (single & multi client)
– SELECT:
●
noac ~ 16x ac

edition
Prato Dec 13th
2016
Conclusions
●
NFS is not natively thought for reliability purposes
– the protocol is thought to enhance the performance
– NFS v4 is preferable
●
PostgreSQL allows to adopt many countermeasures
– it is at least able to promptly detect data corruptions
●
PostgreSQL can be used with NFS
– ready to accept minimal data loss

11/06/2010 ITPUG - ConfSL 2010 37Jonathan Battiato 2ndQuadrant Italia
edition
Prato Dec 13th
2016
Creative Commons license
This work is licensed under a Creative Commons
Attribution-ShareAlike 4.0 International License
https://creativecommons.org/licenses/by-nc-sa/4.0/
© 2016 2ndQuadrant Italia – http://www.2ndquadrant.it

Postgresql on NFS - J.Battiato, pgday2016

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

En vedette

En vedette (20)

Similaire à Postgresql on NFS - J.Battiato, pgday2016

Similaire à Postgresql on NFS - J.Battiato, pgday2016 (20)

Dernier

Dernier (20)

Postgresql on NFS - J.Battiato, pgday2016