Chapter 32. Erasure code profiles

The default erasure code profile sustains the loss of a single OSD. It is equivalent to a replicated pool of size two but requires 1.5TB instead of 2TB to store 1TB of data. The default profile can be displayed with:

ceph osd erasure-code-profile get default
directory=.libs
k=2
m=1
plugin=jerasure
ruleset-failure-domain=host
technique=reed_sol_van

$ ceph osd erasure-code-profile get default
directory=.libs
k=2
m=1
plugin=jerasure
ruleset-failure-domain=host
technique=reed_sol_van

The most important parameters of the profile are K, M and ruleset-failure-domain because they define the storage overhead and the data durability. For instance, if the desired architecture must sustain the loss of two racks with a storage overhead of 40% overhead, the following profile can be defined:

ceph osd erasure-code-profile set myprofile \
   k=3 \
   m=2 \
   ruleset-failure-domain=rack
ceph osd pool create ecpool 12 12 erasure *myprofile*
echo ABCDEFGHI | rados --pool ecpool put NYAN -
rados --pool ecpool get NYAN -
ABCDEFGHI

$ ceph osd erasure-code-profile set myprofile \
   k=3 \
   m=2 \
   ruleset-failure-domain=rack
$ ceph osd pool create ecpool 12 12 erasure *myprofile*
$ echo ABCDEFGHI | rados --pool ecpool put NYAN -
$ rados --pool ecpool get NYAN -
ABCDEFGHI

The primary OSD will divide the NYAN object in three (K=3) and create two additional chunks (M=2). The value of M defines how many OSDs can be lost simultaneously without losing any data. The ruleset-failure-domain=rack will create a CRUSH ruleset that ensures no two chunks are stored in the same rack.