mirror of
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dc7a2aaf7a
1) ruleset is an obsolete term, and
2) crush-{rule,failure-domain,...} is more descriptive.
Note that we are changing the names of the erasure code profile keys
from ruleset-* to crush-*. We will update this on upgrade when the
luminous flag is set, but that means that during mon upgrade you cannot
create EC pools that use these fields.
When the upgrade completes (users sets require_osd_release = luminous)
existing ec profiles are updated automatically.
Signed-off-by: Sage Weil <sage@redhat.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 ruleset 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 ruleset 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 map ruleset 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 ruleset::
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$ ceph osd crush rule create-erasure ecruleset
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$ ceph osd pool create ecpool 12 12 erasure \
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default ecruleset
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Set the ruleset failure domain to osd (instead of the 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 ruleset to take 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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