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Since kraken, Ceph enforces a 1:1 correspondence between CRUSH ruleset and CRUSH rule, so effectively ruleset and rule are the same thing, although the term "ruleset" still survives - notably in the CRUSH rule itself, where it effectively denotes the number of the rule. This commit updates the documentation to more faithfully reflect the current state of the code. Fixes: http://tracker.ceph.com/issues/20559 Signed-off-by: Nathan Cutler <ncutler@suse.com>
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=1
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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=1
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$ ceph osd erasure-code-profile get myprofile
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k=3
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m=1
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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=1
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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=1
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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=1
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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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