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437 lines
17 KiB
ReStructuredText
==================
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Placement Groups
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==================
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.. _preselection:
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A preselection of pg_num
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========================
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When creating a new pool with::
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ceph osd pool create {pool-name} pg_num
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it is mandatory to choose the value of ``pg_num`` because it cannot be
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calculated automatically. Here are a few values commonly used:
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- Less than 5 OSDs set ``pg_num`` to 128
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- Between 5 and 10 OSDs set ``pg_num`` to 512
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- Between 10 and 50 OSDs set ``pg_num`` to 1024
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- If you have more than 50 OSDs, you need to understand the tradeoffs
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and how to calculate the ``pg_num`` value by yourself
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- For calculating ``pg_num`` value by yourself please take help of `pgcalc`_ tool
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As the number of OSDs increases, chosing the right value for pg_num
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becomes more important because it has a significant influence on the
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behavior of the cluster as well as the durability of the data when
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something goes wrong (i.e. the probability that a catastrophic event
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leads to data loss).
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How are Placement Groups used ?
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===============================
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A placement group (PG) aggregates objects within a pool because
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tracking object placement and object metadata on a per-object basis is
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computationally expensive--i.e., a system with millions of objects
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cannot realistically track placement on a per-object basis.
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.. ditaa::
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/-----\ /-----\ /-----\ /-----\ /-----\
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| obj | | obj | | obj | | obj | | obj |
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\-----/ \-----/ \-----/ \-----/ \-----/
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| | | | |
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+--------+--------+ +---+----+
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v v
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+-----------------------+ +-----------------------+
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| Placement Group #1 | | Placement Group #2 |
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| | | |
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+-----------------------+ +-----------------------+
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+------------------------------+
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v
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+-----------------------+
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| Pool |
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+-----------------------+
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The Ceph client will calculate which placement group an object should
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be in. It does this by hashing the object ID and applying an operation
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based on the number of PGs in the defined pool and the ID of the pool.
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See `Mapping PGs to OSDs`_ for details.
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The object's contents within a placement group are stored in a set of
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OSDs. For instance, in a replicated pool of size two, each placement
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group will store objects on two OSDs, as shown below.
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.. ditaa::
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+-----------------------+ +-----------------------+
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| Placement Group #1 | | Placement Group #2 |
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| | | |
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+-----------------------+ +-----------------------+
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v v v v
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/----------\ /----------\ /----------\ /----------\
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| OSD #1 | | OSD #2 | | OSD #2 | | OSD #3 |
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| | | | | | | |
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\----------/ \----------/ \----------/ \----------/
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Should OSD #2 fail, another will be assigned to Placement Group #1 and
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will be filled with copies of all objects in OSD #1. If the pool size
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is changed from two to three, an additional OSD will be assigned to
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the placement group and will receive copies of all objects in the
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placement group.
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Placement groups do not own the OSD, they share it with other
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placement groups from the same pool or even other pools. If OSD #2
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fails, the Placement Group #2 will also have to restore copies of
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objects, using OSD #3.
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When the number of placement groups increases, the new placement
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groups will be assigned OSDs. The result of the CRUSH function will
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also change and some objects from the former placement groups will be
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copied over to the new Placement Groups and removed from the old ones.
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Placement Groups Tradeoffs
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==========================
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Data durability and even distribution among all OSDs call for more
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placement groups but their number should be reduced to the minimum to
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save CPU and memory.
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.. _data durability:
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Data durability
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---------------
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After an OSD fails, the risk of data loss increases until the data it
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contained is fully recovered. Let's imagine a scenario that causes
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permanent data loss in a single placement group:
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- The OSD fails and all copies of the object it contains are lost.
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For all objects within the placement group the number of replica
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suddently drops from three to two.
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- Ceph starts recovery for this placement group by chosing a new OSD
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to re-create the third copy of all objects.
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- Another OSD, within the same placement group, fails before the new
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OSD is fully populated with the third copy. Some objects will then
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only have one surviving copies.
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- Ceph picks yet another OSD and keeps copying objects to restore the
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desired number of copies.
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- A third OSD, within the same placement group, fails before recovery
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is complete. If this OSD contained the only remaining copy of an
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object, it is permanently lost.
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In a cluster containing 10 OSDs with 512 placement groups in a three
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replica pool, CRUSH will give each placement groups three OSDs. In the
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end, each OSDs will end up hosting (512 * 3) / 10 = ~150 Placement
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Groups. When the first OSD fails, the above scenario will therefore
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start recovery for all 150 placement groups at the same time.
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The 150 placement groups being recovered are likely to be
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homogeneously spread over the 9 remaining OSDs. Each remaining OSD is
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therefore likely to send copies of objects to all others and also
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receive some new objects to be stored because they became part of a
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new placement group.
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The amount of time it takes for this recovery to complete entirely
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depends on the architecture of the Ceph cluster. Let say each OSD is
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hosted by a 1TB SSD on a single machine and all of them are connected
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to a 10Gb/s switch and the recovery for a single OSD completes within
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M minutes. If there are two OSDs per machine using spinners with no
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SSD journal and a 1Gb/s switch, it will at least be an order of
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magnitude slower.
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In a cluster of this size, the number of placement groups has almost
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no influence on data durability. It could be 128 or 8192 and the
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recovery would not be slower or faster.
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However, growing the same Ceph cluster to 20 OSDs instead of 10 OSDs
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is likely to speed up recovery and therefore improve data durability
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significantly. Each OSD now participates in only ~75 placement groups
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instead of ~150 when there were only 10 OSDs and it will still require
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all 19 remaining OSDs to perform the same amount of object copies in
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order to recover. But where 10 OSDs had to copy approximately 100GB
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each, they now have to copy 50GB each instead. If the network was the
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bottleneck, recovery will happen twice as fast. In other words,
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recovery goes faster when the number of OSDs increases.
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If this cluster grows to 40 OSDs, each of them will only host ~35
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placement groups. If an OSD dies, recovery will keep going faster
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unless it is blocked by another bottleneck. However, if this cluster
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grows to 200 OSDs, each of them will only host ~7 placement groups. If
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an OSD dies, recovery will happen between at most of ~21 (7 * 3) OSDs
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in these placement groups: recovery will take longer than when there
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were 40 OSDs, meaning the number of placement groups should be
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increased.
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No matter how short the recovery time is, there is a chance for a
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second OSD to fail while it is in progress. In the 10 OSDs cluster
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described above, if any of them fail, then ~17 placement groups
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(i.e. ~150 / 9 placement groups being recovered) will only have one
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surviving copy. And if any of the 8 remaining OSD fail, the last
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objects of two placement groups are likely to be lost (i.e. ~17 / 8
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placement groups with only one remaining copy being recovered).
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When the size of the cluster grows to 20 OSDs, the number of Placement
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Groups damaged by the loss of three OSDs drops. The second OSD lost
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will degrade ~4 (i.e. ~75 / 19 placement groups being recovered)
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instead of ~17 and the third OSD lost will only lose data if it is one
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of the four OSDs containing the surviving copy. In other words, if the
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probability of losing one OSD is 0.0001% during the recovery time
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frame, it goes from 17 * 10 * 0.0001% in the cluster with 10 OSDs to 4 * 20 *
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0.0001% in the cluster with 20 OSDs.
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In a nutshell, more OSDs mean faster recovery and a lower risk of
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cascading failures leading to the permanent loss of a Placement
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Group. Having 512 or 4096 Placement Groups is roughly equivalent in a
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cluster with less than 50 OSDs as far as data durability is concerned.
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Note: It may take a long time for a new OSD added to the cluster to be
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populated with placement groups that were assigned to it. However
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there is no degradation of any object and it has no impact on the
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durability of the data contained in the Cluster.
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.. _object distribution:
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Object distribution within a pool
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---------------------------------
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Ideally objects are evenly distributed in each placement group. Since
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CRUSH computes the placement group for each object, but does not
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actually know how much data is stored in each OSD within this
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placement group, the ratio between the number of placement groups and
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the number of OSDs may influence the distribution of the data
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significantly.
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For instance, if there was single a placement group for ten OSDs in a
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three replica pool, only three OSD would be used because CRUSH would
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have no other choice. When more placement groups are available,
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objects are more likely to be evenly spread among them. CRUSH also
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makes every effort to evenly spread OSDs among all existing Placement
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Groups.
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As long as there are one or two orders of magnitude more Placement
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Groups than OSDs, the distribution should be even. For instance, 300
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placement groups for 3 OSDs, 1000 placement groups for 10 OSDs etc.
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Uneven data distribution can be caused by factors other than the ratio
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between OSDs and placement groups. Since CRUSH does not take into
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account the size of the objects, a few very large objects may create
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an imbalance. Let say one million 4K objects totaling 4GB are evenly
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spread among 1000 placement groups on 10 OSDs. They will use 4GB / 10
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= 400MB on each OSD. If one 400MB object is added to the pool, the
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three OSDs supporting the placement group in which the object has been
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placed will be filled with 400MB + 400MB = 800MB while the seven
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others will remain occupied with only 400MB.
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.. _resource usage:
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Memory, CPU and network usage
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-----------------------------
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For each placement group, OSDs and MONs need memory, network and CPU
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at all times and even more during recovery. Sharing this overhead by
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clustering objects within a placement group is one of the main reasons
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they exist.
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Minimizing the number of placement groups saves significant amounts of
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resources.
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Choosing the number of Placement Groups
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=======================================
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If you have more than 50 OSDs, we recommend approximately 50-100
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placement groups per OSD to balance out resource usage, data
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durability and distribution. If you have less than 50 OSDs, chosing
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among the `preselection`_ above is best. For a single pool of objects,
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you can use the following formula to get a baseline::
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(OSDs * 100)
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Total PGs = ------------
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pool size
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Where **pool size** is either the number of replicas for replicated
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pools or the K+M sum for erasure coded pools (as returned by **ceph
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osd erasure-code-profile get**).
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You should then check if the result makes sense with the way you
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designed your Ceph cluster to maximize `data durability`_,
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`object distribution`_ and minimize `resource usage`_.
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The result should be **rounded up to the nearest power of two.**
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Rounding up is optional, but recommended for CRUSH to evenly balance
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the number of objects among placement groups.
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As an example, for a cluster with 200 OSDs and a pool size of 3
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replicas, you would estimate your number of PGs as follows::
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(200 * 100)
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----------- = 6667. Nearest power of 2: 8192
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3
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When using multiple data pools for storing objects, you need to ensure
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that you balance the number of placement groups per pool with the
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number of placement groups per OSD so that you arrive at a reasonable
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total number of placement groups that provides reasonably low variance
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per OSD without taxing system resources or making the peering process
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too slow.
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For instance a cluster of 10 pools each with 512 placement groups on
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ten OSDs is a total of 5,120 placement groups spread over ten OSDs,
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that is 512 placement groups per OSD. That does not use too many
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resources. However, if 1,000 pools were created with 512 placement
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groups each, the OSDs will handle ~50,000 placement groups each and it
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would require significantly more resources and time for peering.
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.. _setting the number of placement groups:
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Set the Number of Placement Groups
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==================================
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To set the number of placement groups in a pool, you must specify the
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number of placement groups at the time you create the pool.
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See `Create a Pool`_ for details. Once you've set placement groups for a
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pool, you may increase the number of placement groups (but you cannot
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decrease the number of placement groups). To increase the number of
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placement groups, execute the following::
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ceph osd pool set {pool-name} pg_num {pg_num}
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Once you increase the number of placement groups, you must also
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increase the number of placement groups for placement (``pgp_num``)
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before your cluster will rebalance. The ``pgp_num`` will be the number of
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placement groups that will be considered for placement by the CRUSH
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algorithm. Increasing ``pg_num`` splits the placement groups but data
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will not be migrated to the newer placement groups until placement
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groups for placement, ie. ``pgp_num`` is increased. The ``pgp_num``
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should be equal to the ``pg_num``. To increase the number of
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placement groups for placement, execute the following::
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ceph osd pool set {pool-name} pgp_num {pgp_num}
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Get the Number of Placement Groups
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==================================
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To get the number of placement groups in a pool, execute the following::
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ceph osd pool get {pool-name} pg_num
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Get a Cluster's PG Statistics
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=============================
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To get the statistics for the placement groups in your cluster, execute the following::
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ceph pg dump [--format {format}]
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Valid formats are ``plain`` (default) and ``json``.
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Get Statistics for Stuck PGs
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============================
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To get the statistics for all placement groups stuck in a specified state,
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execute the following::
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ceph pg dump_stuck inactive|unclean|stale|undersized|degraded [--format <format>] [-t|--threshold <seconds>]
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**Inactive** Placement groups cannot process reads or writes because they are waiting for an OSD
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with the most up-to-date data to come up and in.
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**Unclean** Placement groups contain objects that are not replicated the desired number
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of times. They should be recovering.
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**Stale** Placement groups are in an unknown state - the OSDs that host them have not
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reported to the monitor cluster in a while (configured by ``mon_osd_report_timeout``).
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Valid formats are ``plain`` (default) and ``json``. The threshold defines the minimum number
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of seconds the placement group is stuck before including it in the returned statistics
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(default 300 seconds).
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Get a PG Map
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============
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To get the placement group map for a particular placement group, execute the following::
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ceph pg map {pg-id}
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For example::
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ceph pg map 1.6c
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Ceph will return the placement group map, the placement group, and the OSD status::
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osdmap e13 pg 1.6c (1.6c) -> up [1,0] acting [1,0]
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Get a PGs Statistics
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====================
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To retrieve statistics for a particular placement group, execute the following::
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ceph pg {pg-id} query
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Scrub a Placement Group
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=======================
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To scrub a placement group, execute the following::
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ceph pg scrub {pg-id}
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Ceph checks the primary and any replica nodes, generates a catalog of all objects
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in the placement group and compares them to ensure that no objects are missing
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or mismatched, and their contents are consistent. Assuming the replicas all
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match, a final semantic sweep ensures that all of the snapshot-related object
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metadata is consistent. Errors are reported via logs.
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Revert Lost
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===========
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If the cluster has lost one or more objects, and you have decided to
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abandon the search for the lost data, you must mark the unfound objects
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as ``lost``.
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If all possible locations have been queried and objects are still
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lost, you may have to give up on the lost objects. This is
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possible given unusual combinations of failures that allow the cluster
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to learn about writes that were performed before the writes themselves
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are recovered.
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Currently the only supported option is "revert", which will either roll back to
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a previous version of the object or (if it was a new object) forget about it
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entirely. To mark the "unfound" objects as "lost", execute the following::
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ceph pg {pg-id} mark_unfound_lost revert|delete
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.. important:: Use this feature with caution, because it may confuse
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applications that expect the object(s) to exist.
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.. toctree::
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:hidden:
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pg-states
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pg-concepts
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.. _Create a Pool: ../pools#createpool
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.. _Mapping PGs to OSDs: ../../../architecture#mapping-pgs-to-osds
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.. _pgcalc: http://ceph.com/pgcalc/
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