1. Section 7 Erasure Coding Overview

2. Objectives What is Erasure Coding? Erasure Coding in Ceph
Configure Erasure Coding

3. What is Erasure Coding?
Objective Notes:

4. What is Erasure Code?
In information theory : an erasure code is a forward error correction (FEC) code for the binary erasure channel, which transforms a message of k symbols into a longer message (code word) with n symbols such that the original message can be recovered from a subset of the n symbols.
The fraction r = k/n is called the code rate, the fraction k’/k, where k’ denotes the number of symbols required for recovery, is called reception efficiency.
Thanks Wikipedia – that really helps!

5. Why do we have Erasure Coding?
The default replication strategy in SES is simple replication
Defined by the size parameter of a pool
Each object is replicated a number of times to provide resilience
This approach is simple and effective but comes at a price
For replication size of n the raw storage requirement is n times the amount of data being stored
Data replication overhead is high, especially as replication size increases
Erasure coding provides an alternative
Trading off some resilience and performance to lower the raw disk requirements for storage

6. Replication vs Erasure Coding
Replication (default)
Use for active data
Simple and fast
Uses more disk space
Erasure coding
Use for archive data
Calculates recovery data
Definable redundancy level
Needs a cache layer for use with rbd
Data is accessed via a replicated pool and then migrated to the Erasure Coded pool
Can use both at same time
But in different pools

7. Erasure Coding in Ceph
Objective Notes:

8. A quick video overview...

9. Normal Ceph Read/Write

10. Erasure Coded Write

11. Erasure Coded Write Encode takes place on Primary OSD host node
Example is k=3,m=2 so 5 OSDs required

12. Erasure Coded Write
With k=3, data is split into 3 shards, each written
to a different OSD (via CRUSH calculation)

13. Erasure Coded Write With m=2, 2 recovery shards are calculated and
written to different OSDs

14. Erasure Coding (just the basics)
Calculates parity blocks to recover data
Configurable K+M parameters (example at 10+6)
All data now stored on 16 disks
Requires 10 disks to recover
Data safe with 6 failures
Only requires 60% extra raw capacity
Performance
All disks need to acknowledge writes
Slower recovery (think RAID 6)
More chance of failure during rebuild
Do not use K+M of 10+1 – you need to have sufficient failure cover

15. Erasure Coding overview
Makes storage much cheaper
With no reduction in reliability (if properly configured)
Great for archival storage
Power consumption advantages
Trades disk running for CPU load for writing and recovery
Makes reads, writes and recovery slower
You will probably want to add a cache tier
To maximize the performance
Can access via RADOS
RADOS is Ceph native API
Requires a cache tier for RBD access
But you probably want one anyway

16. Erasure Coding Plugins
The EC algorithm and implementation are pluggable
jerasure/gf-complete (free, open, and very fast) (
ISA-L (Intel library; optimized for modern Intel processors)
LRC (local recovery code – layers over existing plugins)
SHEC (trades extra storage for recovery efficiency – new from Fujitsu)
Parameterized
Pick “k” and “m”, stripe size
OSD handles data path, placement, rollback, etc.
Erasure plugin handles
Encode and decode maths
Given these available shards, which ones should I fetch to satisfy a read?
Given these available shards and these missing shards, which ones should I fetch to recover?

17. Erasure Coding Parameters
Two key parameters in Erasure Coding configuration
K : Erasure coding works by spitting data into shards which are then written to separate OSDs. K determines the number of shards into which data is split. The default is k=2
M : Erasure coding calculates additional data which is used to reconstruct missing shards (for example caused by OSD failure). M determines how many additional shards are calculated, and this is also the number of OSD failures which the an erasure coded pool can withstand. The default is m=1
The default values stripe data on two osds, and calculated data on a third. The loss of any one OSD is tolerable, similar to a replication size of 2
For 1GB of storage, replication size of 2 needs 2GB, erasure coded data with k=2/m=1 only requires 1.5GB

18. Configure Erasure Coding

19. Erasure Code Profiles
Profiles define the parameters for erasure coding
Profile contains
k and m values
CRUSH ruleset
Plugin
Default jerasure
Technique
Default reed_sol_van
When a pool is created as an EC pool the profile determines the setup for Erasure Coding
This cannot be changed later

20. Command: ceph osd erasure-code-profile
Syntax: ceph osd erasure-code-profile OPTIONS
Option Description
get view details of an existing EC profile
set set a profile, requires k and m values, with optional values such as ruleset, plugin
ls list profiles
Notes:

21. Default EC Profile
The default profile provides a basic erasure coding configuration which will function in almost any cluster
Uses minimum practical values
k=2, m=1
Is written over 3 OSDs
Two data shards
One recovery shard

22. Setting a custom EC Profile
Use the ceph osd erasure-code-profile set command with the following options:
Profile name
K : number of stripes required
M : number of failed units
ruleset-failure-domain = crush bucket level for failure
OSD, Host, rack etc as defined in CRUSH map
Example with k=8,m=2 and failure at host level
ceph osd erasure-code-profile set example-profile k=8 m=2 ruleset-failure-domain=host

23. Command: ceph osd pool create
Syntax: ceph osd pool create <name> <pg> erasure <profile>
Same command used to create standard replication pools but with the addition of the erasure option and a profile name (or the default profile is used)
Notes:

24. Section 7 Exercises
Objective Notes: