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138 lines
3.7 KiB
ReStructuredText
138 lines
3.7 KiB
ReStructuredText
Erasure Coded pool
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==================
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Purpose
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-------
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Erasure-coded pools require less storage space compared to replicated
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pools. The erasure-coding support has higher computational requirements and
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only supports a subset of the operations allowed on an object (for instance,
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partial write is not supported).
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Use cases
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---------
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Cold storage
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~~~~~~~~~~~~
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An erasure-coded pool is created to store a large number of 1GB
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objects (imaging, genomics, etc.) and 10% of them are read per
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month. New objects are added every day and the objects are not
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modified after being written. On average there is one write for 10,000
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reads.
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A replicated pool is created and set as a cache tier for the
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erasure coded pool. An agent demotes objects (i.e. moves them from the
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replicated pool to the erasure-coded pool) if they have not been
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accessed in a week.
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The erasure-coded pool CRUSH rule targets hardware designed for
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cold storage with high latency and slow access time. The replicated
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pool CRUSH rule targets faster hardware to provide better response
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times.
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Cheap multidatacenter storage
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Ten datacenters are connected with dedicated network links. Each
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datacenter contains the same amount of storage with no power-supply
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backup and no air-cooling system.
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An erasure-coded pool is created with a CRUSH rule that will
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ensure no data loss if at most three datacenters fail
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simultaneously. The overhead is 50% with erasure code configured to
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split data in six (k=6) and create three coding chunks (m=3). With
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replication the overhead would be 400% (four replicas).
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Interface
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---------
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Set up an erasure-coded pool::
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$ ceph osd pool create ecpool 12 12 erasure
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Set up an erasure-coded pool and the associated CRUSH rule ``ecrule``::
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$ ceph osd crush rule create-erasure ecrule
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$ ceph osd pool create ecpool 12 12 erasure \
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default ecrule
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Set the CRUSH failure domain to osd (instead of host, which is the default)::
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$ ceph osd erasure-code-profile set myprofile \
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crush-failure-domain=osd
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$ ceph osd erasure-code-profile get myprofile
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k=2
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m=2
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plugin=jerasure
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technique=reed_sol_van
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crush-failure-domain=osd
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$ ceph osd pool create ecpool 12 12 erasure myprofile
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Control the parameters of the erasure code plugin::
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$ ceph osd erasure-code-profile set myprofile \
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k=3 m=2
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$ ceph osd erasure-code-profile get myprofile
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k=3
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m=2
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plugin=jerasure
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technique=reed_sol_van
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$ ceph osd pool create ecpool 12 12 erasure \
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myprofile
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Choose an alternate erasure code plugin::
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$ ceph osd erasure-code-profile set myprofile \
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plugin=example technique=xor
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$ ceph osd erasure-code-profile get myprofile
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k=2
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m=2
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plugin=example
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technique=xor
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$ ceph osd pool create ecpool 12 12 erasure \
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myprofile
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Display the default erasure code profile::
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$ ceph osd erasure-code-profile ls
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default
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$ ceph osd erasure-code-profile get default
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k=2
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m=2
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plugin=jerasure
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technique=reed_sol_van
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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::
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$ ceph osd erasure-code-profile set myprofile k=3 m=3
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$ ceph osd erasure-code-profile get myprofile
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k=3
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m=3
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plugin=jerasure
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technique=reed_sol_van
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$ ceph osd erasure-code-profile ls
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default
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myprofile
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Remove a profile that is no longer in use (otherwise it will fail with EBUSY)::
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$ ceph osd erasure-code-profile ls
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default
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myprofile
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$ ceph osd erasure-code-profile rm myprofile
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$ ceph osd erasure-code-profile ls
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default
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Set the rule to ssd (instead of default)::
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$ ceph osd erasure-code-profile set myprofile \
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crush-root=ssd
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$ ceph osd erasure-code-profile get myprofile
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k=2
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m=2
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plugin=jerasure
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technique=reed_sol_van
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crush-root=ssd
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