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519 lines
22 KiB
ReStructuredText
=========================
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Monitoring OSDs and PGs
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=========================
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High availability and high reliability require a fault-tolerant approach to
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managing hardware and software issues. Ceph has no single point-of-failure, and
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can service requests for data in a "degraded" mode. Ceph's `data placement`_
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introduces a layer of indirection to ensure that data doesn't bind directly to
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particular OSD addresses. This means that tracking down system faults requires
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finding the `placement group`_ and the underlying OSDs at root of the problem.
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.. tip:: A fault in one part of the cluster may prevent you from accessing a
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particular object, but that doesn't mean that you cannot access other objects.
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When you run into a fault, don't panic. Just follow the steps for monitoring
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your OSDs and placement groups. Then, begin troubleshooting.
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Ceph is generally self-repairing. However, when problems persist, monitoring
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OSDs and placement groups will help you identify the problem.
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Monitoring OSDs
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===============
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An OSD's status is either in the cluster (``in``) or out of the cluster
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(``out``); and, it is either up and running (``up``), or it is down and not
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running (``down``). If an OSD is ``up``, it may be either ``in`` the cluster
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(you can read and write data) or it is ``out`` of the cluster. If it was
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``in`` the cluster and recently moved ``out`` of the cluster, Ceph will migrate
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placement groups to other OSDs. If an OSD is ``out`` of the cluster, CRUSH will
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not assign placement groups to the OSD. If an OSD is ``down``, it should also be
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``out``.
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.. note:: If an OSD is ``down`` and ``in``, there is a problem and the cluster
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will not be in a healthy state.
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.. ditaa::
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+----------------+ +----------------+
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| | | |
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| OSD #n In | | OSD #n Up |
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+----------------+ +----------------+
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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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| OSD #n Out | | OSD #n Down |
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+----------------+ +----------------+
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If you execute a command such as ``ceph health``, ``ceph -s`` or ``ceph -w``,
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you may notice that the cluster does not always echo back ``HEALTH OK``. Don't
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panic. With respect to OSDs, you should expect that the cluster will **NOT**
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echo ``HEALTH OK`` in a few expected circumstances:
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#. You haven't started the cluster yet (it won't respond).
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#. You have just started or restarted the cluster and it's not ready yet,
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because the placement groups are getting created and the OSDs are in
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the process of peering.
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#. You just added or removed an OSD.
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#. You just have modified your cluster map.
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An important aspect of monitoring OSDs is to ensure that when the cluster
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is up and running that all OSDs that are ``in`` the cluster are ``up`` and
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running, too. To see if all OSDs are running, execute::
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ceph osd stat
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The result should tell you the total number of OSDs (x),
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how many are ``up`` (y), how many are ``in`` (z) and the map epoch (eNNNN). ::
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x osds: y up, z in; epoch: eNNNN
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If the number of OSDs that are ``in`` the cluster is more than the number of
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OSDs that are ``up``, execute the following command to identify the ``ceph-osd``
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daemons that are not running::
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ceph osd tree
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::
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#ID CLASS WEIGHT TYPE NAME STATUS REWEIGHT PRI-AFF
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-1 2.00000 pool openstack
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-3 2.00000 rack dell-2950-rack-A
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-2 2.00000 host dell-2950-A1
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0 ssd 1.00000 osd.0 up 1.00000 1.00000
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1 ssd 1.00000 osd.1 down 1.00000 1.00000
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.. tip:: The ability to search through a well-designed CRUSH hierarchy may help
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you troubleshoot your cluster by identifying the physical locations faster.
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If an OSD is ``down``, start it::
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sudo systemctl start ceph-osd@1
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See `OSD Not Running`_ for problems associated with OSDs that stopped, or won't
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restart.
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PG Sets
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=======
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When CRUSH assigns placement groups to OSDs, it looks at the number of replicas
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for the pool and assigns the placement group to OSDs such that each replica of
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the placement group gets assigned to a different OSD. For example, if the pool
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requires three replicas of a placement group, CRUSH may assign them to
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``osd.1``, ``osd.2`` and ``osd.3`` respectively. CRUSH actually seeks a
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pseudo-random placement that will take into account failure domains you set in
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your `CRUSH map`_, so you will rarely see placement groups assigned to nearest
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neighbor OSDs in a large cluster. We refer to the set of OSDs that should
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contain the replicas of a particular placement group as the **Acting Set**. In
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some cases, an OSD in the Acting Set is ``down`` or otherwise not able to
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service requests for objects in the placement group. When these situations
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arise, don't panic. Common examples include:
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- You added or removed an OSD. Then, CRUSH reassigned the placement group to
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other OSDs--thereby changing the composition of the Acting Set and spawning
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the migration of data with a "backfill" process.
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- An OSD was ``down``, was restarted, and is now ``recovering``.
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- An OSD in the Acting Set is ``down`` or unable to service requests,
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and another OSD has temporarily assumed its duties.
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Ceph processes a client request using the **Up Set**, which is the set of OSDs
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that will actually handle the requests. In most cases, the Up Set and the Acting
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Set are virtually identical. When they are not, it may indicate that Ceph is
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migrating data, an OSD is recovering, or that there is a problem (i.e., Ceph
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usually echoes a "HEALTH WARN" state with a "stuck stale" message in such
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scenarios).
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To retrieve a list of placement groups, execute::
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ceph pg dump
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To view which OSDs are within the Acting Set or the Up Set for a given placement
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group, execute::
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ceph pg map {pg-num}
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The result should tell you the osdmap epoch (eNNN), the placement group number
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({pg-num}), the OSDs in the Up Set (up[]), and the OSDs in the acting set
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(acting[]). ::
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osdmap eNNN pg {raw-pg-num} ({pg-num}) -> up [0,1,2] acting [0,1,2]
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.. note:: If the Up Set and Acting Set do not match, this may be an indicator
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that the cluster rebalancing itself or of a potential problem with
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the cluster.
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Peering
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=======
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Before you can write data to a placement group, it must be in an ``active``
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state, and it **should** be in a ``clean`` state. For Ceph to determine the
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current state of a placement group, the primary OSD of the placement group
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(i.e., the first OSD in the acting set), peers with the secondary and tertiary
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OSDs to establish agreement on the current state of the placement group
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(assuming a pool with 3 replicas of the PG).
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.. ditaa::
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+---------+ +---------+ +-------+
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| OSD 1 | | OSD 2 | | OSD 3 |
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+---------+ +---------+ +-------+
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| | |
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| Request To | |
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| Peer | |
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|-------------->| |
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|<--------------| |
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| Peering |
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| |
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| Request To |
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| Peer |
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|----------------------------->|
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|<-----------------------------|
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| Peering |
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The OSDs also report their status to the monitor. See `Configuring Monitor/OSD
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Interaction`_ for details. To troubleshoot peering issues, see `Peering
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Failure`_.
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Monitoring Placement Group States
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=================================
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If you execute a command such as ``ceph health``, ``ceph -s`` or ``ceph -w``,
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you may notice that the cluster does not always echo back ``HEALTH OK``. After
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you check to see if the OSDs are running, you should also check placement group
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states. You should expect that the cluster will **NOT** echo ``HEALTH OK`` in a
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number of placement group peering-related circumstances:
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#. You have just created a pool and placement groups haven't peered yet.
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#. The placement groups are recovering.
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#. You have just added an OSD to or removed an OSD from the cluster.
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#. You have just modified your CRUSH map and your placement groups are migrating.
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#. There is inconsistent data in different replicas of a placement group.
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#. Ceph is scrubbing a placement group's replicas.
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#. Ceph doesn't have enough storage capacity to complete backfilling operations.
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If one of the foregoing circumstances causes Ceph to echo ``HEALTH WARN``, don't
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panic. In many cases, the cluster will recover on its own. In some cases, you
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may need to take action. An important aspect of monitoring placement groups is
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to ensure that when the cluster is up and running that all placement groups are
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``active``, and preferably in the ``clean`` state. To see the status of all
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placement groups, execute::
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ceph pg stat
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The result should tell you the total number of placement groups (x), how many
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placement groups are in a particular state such as ``active+clean`` (y) and the
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amount of data stored (z). ::
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x pgs: y active+clean; z bytes data, aa MB used, bb GB / cc GB avail
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.. note:: It is common for Ceph to report multiple states for placement groups.
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In addition to the placement group states, Ceph will also echo back the amount of
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storage capacity used (aa), the amount of storage capacity remaining (bb), and the total
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storage capacity for the placement group. These numbers can be important in a
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few cases:
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- You are reaching your ``near full ratio`` or ``full ratio``.
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- Your data is not getting distributed across the cluster due to an
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error in your CRUSH configuration.
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.. topic:: Placement Group IDs
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Placement group IDs consist of the pool number (not pool name) followed
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by a period (.) and the placement group ID--a hexadecimal number. You
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can view pool numbers and their names from the output of ``ceph osd
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lspools``. For example, the first pool created corresponds to
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pool number ``1``. A fully qualified placement group ID has the
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following form::
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{pool-num}.{pg-id}
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And it typically looks like this::
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1.1f
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To retrieve a list of placement groups, execute the following::
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ceph pg dump
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You can also format the output in JSON format and save it to a file::
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ceph pg dump -o {filename} --format=json
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To query a particular placement group, execute the following::
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ceph pg {poolnum}.{pg-id} query
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Ceph will output the query in JSON format.
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The following subsections describe the common pg states in detail.
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Creating
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--------
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When you create a pool, it will create the number of placement groups you
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specified. Ceph will echo ``creating`` when it is creating one or more
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placement groups. Once they are created, the OSDs that are part of a placement
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group's Acting Set will peer. Once peering is complete, the placement group
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status should be ``active+clean``, which means a Ceph client can begin writing
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to the placement group.
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.. ditaa::
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/-----------\ /-----------\ /-----------\
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| Creating |------>| Peering |------>| Active |
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\-----------/ \-----------/ \-----------/
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Peering
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-------
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When Ceph is Peering a placement group, Ceph is bringing the OSDs that
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store the replicas of the placement group into **agreement about the state**
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of the objects and metadata in the placement group. When Ceph completes peering,
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this means that the OSDs that store the placement group agree about the current
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state of the placement group. However, completion of the peering process does
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**NOT** mean that each replica has the latest contents.
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.. topic:: Authoritative History
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Ceph will **NOT** acknowledge a write operation to a client, until
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all OSDs of the acting set persist the write operation. This practice
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ensures that at least one member of the acting set will have a record
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of every acknowledged write operation since the last successful
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peering operation.
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With an accurate record of each acknowledged write operation, Ceph can
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construct and disseminate a new authoritative history of the placement
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group--a complete, and fully ordered set of operations that, if performed,
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would bring an OSD’s copy of a placement group up to date.
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Active
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------
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Once Ceph completes the peering process, a placement group may become
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``active``. The ``active`` state means that the data in the placement group is
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generally available in the primary placement group and the replicas for read
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and write operations.
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Clean
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-----
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When a placement group is in the ``clean`` state, the primary OSD and the
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replica OSDs have successfully peered and there are no stray replicas for the
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placement group. Ceph replicated all objects in the placement group the correct
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number of times.
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Degraded
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--------
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When a client writes an object to the primary OSD, the primary OSD is
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responsible for writing the replicas to the replica OSDs. After the primary OSD
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writes the object to storage, the placement group will remain in a ``degraded``
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state until the primary OSD has received an acknowledgement from the replica
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OSDs that Ceph created the replica objects successfully.
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The reason a placement group can be ``active+degraded`` is that an OSD may be
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``active`` even though it doesn't hold all of the objects yet. If an OSD goes
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``down``, Ceph marks each placement group assigned to the OSD as ``degraded``.
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The OSDs must peer again when the OSD comes back online. However, a client can
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still write a new object to a ``degraded`` placement group if it is ``active``.
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If an OSD is ``down`` and the ``degraded`` condition persists, Ceph may mark the
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``down`` OSD as ``out`` of the cluster and remap the data from the ``down`` OSD
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to another OSD. The time between being marked ``down`` and being marked ``out``
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is controlled by ``mon osd down out interval``, which is set to ``600`` seconds
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by default.
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A placement group can also be ``degraded``, because Ceph cannot find one or more
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objects that Ceph thinks should be in the placement group. While you cannot
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read or write to unfound objects, you can still access all of the other objects
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in the ``degraded`` placement group.
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Recovering
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----------
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Ceph was designed for fault-tolerance at a scale where hardware and software
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problems are ongoing. When an OSD goes ``down``, its contents may fall behind
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the current state of other replicas in the placement groups. When the OSD is
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back ``up``, the contents of the placement groups must be updated to reflect the
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current state. During that time period, the OSD may reflect a ``recovering``
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state.
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Recovery is not always trivial, because a hardware failure might cause a
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cascading failure of multiple OSDs. For example, a network switch for a rack or
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cabinet may fail, which can cause the OSDs of a number of host machines to fall
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behind the current state of the cluster. Each one of the OSDs must recover once
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the fault is resolved.
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Ceph provides a number of settings to balance the resource contention between
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new service requests and the need to recover data objects and restore the
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placement groups to the current state. The ``osd recovery delay start`` setting
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allows an OSD to restart, re-peer and even process some replay requests before
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starting the recovery process. The ``osd
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recovery thread timeout`` sets a thread timeout, because multiple OSDs may fail,
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restart and re-peer at staggered rates. The ``osd recovery max active`` setting
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limits the number of recovery requests an OSD will entertain simultaneously to
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prevent the OSD from failing to serve . The ``osd recovery max chunk`` setting
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limits the size of the recovered data chunks to prevent network congestion.
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Back Filling
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------------
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When a new OSD joins the cluster, CRUSH will reassign placement groups from OSDs
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in the cluster to the newly added OSD. Forcing the new OSD to accept the
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reassigned placement groups immediately can put excessive load on the new OSD.
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Back filling the OSD with the placement groups allows this process to begin in
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the background. Once backfilling is complete, the new OSD will begin serving
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requests when it is ready.
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During the backfill operations, you may see one of several states:
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``backfill_wait`` indicates that a backfill operation is pending, but is not
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underway yet; ``backfilling`` indicates that a backfill operation is underway;
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and, ``backfill_toofull`` indicates that a backfill operation was requested,
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but couldn't be completed due to insufficient storage capacity. When a
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placement group cannot be backfilled, it may be considered ``incomplete``.
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The ``backfill_toofull`` state may be transient. It is possible that as PGs
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are moved around, space may become available. The ``backfill_toofull`` is
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similar to ``backfill_wait`` in that as soon as conditions change
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backfill can proceed.
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Ceph provides a number of settings to manage the load spike associated with
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reassigning placement groups to an OSD (especially a new OSD). By default,
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``osd_max_backfills`` sets the maximum number of concurrent backfills to and from
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an OSD to 1. The ``backfill full ratio`` enables an OSD to refuse a
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backfill request if the OSD is approaching its full ratio (90%, by default) and
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change with ``ceph osd set-backfillfull-ratio`` command.
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If an OSD refuses a backfill request, the ``osd backfill retry interval``
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enables an OSD to retry the request (after 30 seconds, by default). OSDs can
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also set ``osd backfill scan min`` and ``osd backfill scan max`` to manage scan
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intervals (64 and 512, by default).
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Remapped
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--------
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When the Acting Set that services a placement group changes, the data migrates
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from the old acting set to the new acting set. It may take some time for a new
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primary OSD to service requests. So it may ask the old primary to continue to
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service requests until the placement group migration is complete. Once data
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migration completes, the mapping uses the primary OSD of the new acting set.
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Stale
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-----
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While Ceph uses heartbeats to ensure that hosts and daemons are running, the
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``ceph-osd`` daemons may also get into a ``stuck`` state where they are not
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reporting statistics in a timely manner (e.g., a temporary network fault). By
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default, OSD daemons report their placement group, up through, boot and failure
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statistics every half second (i.e., ``0.5``), which is more frequent than the
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heartbeat thresholds. If the **Primary OSD** of a placement group's acting set
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fails to report to the monitor or if other OSDs have reported the primary OSD
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``down``, the monitors will mark the placement group ``stale``.
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When you start your cluster, it is common to see the ``stale`` state until
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the peering process completes. After your cluster has been running for awhile,
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seeing placement groups in the ``stale`` state indicates that the primary OSD
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for those placement groups is ``down`` or not reporting placement group statistics
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to the monitor.
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Identifying Troubled PGs
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========================
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As previously noted, a placement group is not necessarily problematic just
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because its state is not ``active+clean``. Generally, Ceph's ability to self
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repair may not be working when placement groups get stuck. The stuck states
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include:
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- **Unclean**: Placement groups contain objects that are not replicated the
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desired number of times. They should be recovering.
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- **Inactive**: Placement groups cannot process reads or writes because they
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are waiting for an OSD with the most up-to-date data to come back ``up``.
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- **Stale**: Placement groups are in an unknown state, because the OSDs that
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host them have not reported to the monitor cluster in a while (configured
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by ``mon osd report timeout``).
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To identify stuck placement groups, execute the following::
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ceph pg dump_stuck [unclean|inactive|stale|undersized|degraded]
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See `Placement Group Subsystem`_ for additional details. To troubleshoot
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stuck placement groups, see `Troubleshooting PG Errors`_.
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Finding an Object Location
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==========================
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|
||
To store object data in the Ceph Object Store, a Ceph client must:
|
||
|
||
#. Set an object name
|
||
#. Specify a `pool`_
|
||
|
||
The Ceph client retrieves the latest cluster map and the CRUSH algorithm
|
||
calculates how to map the object to a `placement group`_, and then calculates
|
||
how to assign the placement group to an OSD dynamically. To find the object
|
||
location, all you need is the object name and the pool name. For example::
|
||
|
||
ceph osd map {poolname} {object-name} [namespace]
|
||
|
||
.. topic:: Exercise: Locate an Object
|
||
|
||
As an exercise, lets create an object. Specify an object name, a path to a
|
||
test file containing some object data and a pool name using the
|
||
``rados put`` command on the command line. For example::
|
||
|
||
rados put {object-name} {file-path} --pool=data
|
||
rados put test-object-1 testfile.txt --pool=data
|
||
|
||
To verify that the Ceph Object Store stored the object, execute the following::
|
||
|
||
rados -p data ls
|
||
|
||
Now, identify the object location::
|
||
|
||
ceph osd map {pool-name} {object-name}
|
||
ceph osd map data test-object-1
|
||
|
||
Ceph should output the object's location. For example::
|
||
|
||
osdmap e537 pool 'data' (1) object 'test-object-1' -> pg 1.d1743484 (1.4) -> up ([0,1], p0) acting ([0,1], p0)
|
||
|
||
To remove the test object, simply delete it using the ``rados rm`` command.
|
||
For example::
|
||
|
||
rados rm test-object-1 --pool=data
|
||
|
||
|
||
As the cluster evolves, the object location may change dynamically. One benefit
|
||
of Ceph's dynamic rebalancing is that Ceph relieves you from having to perform
|
||
the migration manually. See the `Architecture`_ section for details.
|
||
|
||
.. _data placement: ../data-placement
|
||
.. _pool: ../pools
|
||
.. _placement group: ../placement-groups
|
||
.. _Architecture: ../../../architecture
|
||
.. _OSD Not Running: ../../troubleshooting/troubleshooting-osd#osd-not-running
|
||
.. _Troubleshooting PG Errors: ../../troubleshooting/troubleshooting-pg#troubleshooting-pg-errors
|
||
.. _Peering Failure: ../../troubleshooting/troubleshooting-pg#failures-osd-peering
|
||
.. _CRUSH map: ../crush-map
|
||
.. _Configuring Monitor/OSD Interaction: ../../configuration/mon-osd-interaction/
|
||
.. _Placement Group Subsystem: ../control#placement-group-subsystem
|