--- /dev/null
+Erasure Coded pool
+==================
+
+Purpose
+-------
+
+Erasure-coded pools require less storage space compared to replicated
+pools. The erasure-coding support has higher computational requirements and
+only supports a subset of the operations allowed on an object (for instance,
+partial write is not supported).
+
+Use cases
+---------
+
+Cold storage
+~~~~~~~~~~~~
+
+An erasure-coded pool is created to store a large number of 1GB
+objects (imaging, genomics, etc.) and 10% of them are read per
+month. New objects are added every day and the objects are not
+modified after being written. On average there is one write for 10,000
+reads.
+
+A replicated pool is created and set as a cache tier for the
+erasure coded pool. An agent demotes objects (i.e. moves them from the
+replicated pool to the erasure-coded pool) if they have not been
+accessed in a week.
+
+The erasure-coded pool crush ruleset targets hardware designed for
+cold storage with high latency and slow access time. The replicated
+pool crush ruleset targets faster hardware to provide better response
+times.
+
+Cheap multidatacenter storage
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Ten datacenters are connected with dedicated network links. Each
+datacenter contains the same amount of storage with no power-supply
+backup and no air-cooling system.
+
+An erasure-coded pool is created with a crush map ruleset that will
+ensure no data loss if at most three datacenters fail
+simultaneously. The overhead is 50% with erasure code configured to
+split data in six (k=6) and create three coding chunks (m=3). With
+replication the overhead would be 400% (four replicas).
+
+Interface
+---------
+
+Set up an erasure-coded pool::
+
+ $ ceph osd pool create ecpool 12 12 erasure
+
+Set up an erasure-coded pool and the associated crush ruleset::
+
+ $ ceph osd crush rule create-erasure ecruleset
+ $ ceph osd pool create ecpool 12 12 erasure \
+ default ecruleset
+
+Set the ruleset failure domain to osd (instead of the host which is the default)::
+
+ $ ceph osd erasure-code-profile set myprofile \
+ crush-failure-domain=osd
+ $ ceph osd erasure-code-profile get myprofile
+ k=2
+ m=1
+ plugin=jerasure
+ technique=reed_sol_van
+ crush-failure-domain=osd
+ $ ceph osd pool create ecpool 12 12 erasure myprofile
+
+Control the parameters of the erasure code plugin::
+
+ $ ceph osd erasure-code-profile set myprofile \
+ k=3 m=1
+ $ ceph osd erasure-code-profile get myprofile
+ k=3
+ m=1
+ plugin=jerasure
+ technique=reed_sol_van
+ $ ceph osd pool create ecpool 12 12 erasure \
+ myprofile
+
+Choose an alternate erasure code plugin::
+
+ $ ceph osd erasure-code-profile set myprofile \
+ plugin=example technique=xor
+ $ ceph osd erasure-code-profile get myprofile
+ k=2
+ m=1
+ plugin=example
+ technique=xor
+ $ ceph osd pool create ecpool 12 12 erasure \
+ myprofile
+
+Display the default erasure code profile::
+
+ $ ceph osd erasure-code-profile ls
+ default
+ $ ceph osd erasure-code-profile get default
+ k=2
+ m=1
+ plugin=jerasure
+ technique=reed_sol_van
+
+Create a profile to set the data to be distributed on six OSDs (k+m=6) and sustain the loss of three OSDs (m=3) without losing data::
+
+ $ ceph osd erasure-code-profile set myprofile k=3 m=3
+ $ ceph osd erasure-code-profile get myprofile
+ k=3
+ m=3
+ plugin=jerasure
+ technique=reed_sol_van
+ $ ceph osd erasure-code-profile ls
+ default
+ myprofile
+
+Remove a profile that is no longer in use (otherwise it will fail with EBUSY)::
+
+ $ ceph osd erasure-code-profile ls
+ default
+ myprofile
+ $ ceph osd erasure-code-profile rm myprofile
+ $ ceph osd erasure-code-profile ls
+ default
+
+Set the ruleset to take ssd (instead of default)::
+
+ $ ceph osd erasure-code-profile set myprofile \
+ crush-root=ssd
+ $ ceph osd erasure-code-profile get myprofile
+ k=2
+ m=1
+ plugin=jerasure
+ technique=reed_sol_van
+ crush-root=ssd
+
Code Review / stor4nfv.git / blobdiff