========================= Monitoring OSDs and PGs ========================= High availability and high reliability require a fault-tolerant approach to managing hardware and software issues. Ceph has no single point of failure and it can service requests for data even when in a "degraded" mode. Ceph's `data placement`_ introduces a layer of indirection to ensure that data doesn't bind directly to specific OSDs. For this reason, tracking system faults requires finding the `placement group`_ (PG) and the underlying OSDs at the root of the problem. .. tip:: A fault in one part of the cluster might prevent you from accessing a particular object, but that doesn't mean that you are prevented from accessing other objects. When you run into a fault, don't panic. Just follow the steps for monitoring your OSDs and placement groups, and then begin troubleshooting. Ceph is self-repairing. However, when problems persist, monitoring OSDs and placement groups will help you identify the problem. Monitoring OSDs =============== An OSD is either *in* service (``in``) or *out* of service (``out``). An OSD is either running and reachable (``up``), or it is not running and not reachable (``down``). If an OSD is ``up``, it may be either ``in`` service (clients can read and write data) or it is ``out`` of service. If the OSD was ``in`` but then due to a failure or a manual action was set to the ``out`` state, Ceph will migrate placement groups to the other OSDs to maintin the configured redundancy. If an OSD is ``out`` of service, CRUSH will not assign placement groups to it. If an OSD is ``down``, it will also be ``out``. .. note:: If an OSD is ``down`` and ``in``, there is a problem and this indicates that the cluster is not in a healthy state. .. ditaa:: +----------------+ +----------------+ | | | | | OSD #n In | | OSD #n Up | | | | | +----------------+ +----------------+ ^ ^ | | | | v v +----------------+ +----------------+ | | | | | OSD #n Out | | OSD #n Down | | | | | +----------------+ +----------------+ If you run the commands ``ceph health``, ``ceph -s``, or ``ceph -w``, you might notice that the cluster does not always show ``HEALTH OK``. Don't panic. There are certain circumstances in which it is expected and normal that the cluster will **NOT** show ``HEALTH OK``: #. You haven't started the cluster yet. #. You have just started or restarted the cluster and it's not ready to show health statuses yet, because the PGs are in the process of being created and the OSDs are in the process of peering. #. You have just added or removed an OSD. #. You have just have modified your cluster map. Checking to see if OSDs are ``up`` and running is an important aspect of monitoring them: whenever the cluster is up and running, every OSD that is ``in`` the cluster should also be ``up`` and running. To see if all of the cluster's OSDs are running, run the following command: .. prompt:: bash $ ceph osd stat The output provides the following information: the total number of OSDs (x), how many OSDs are ``up`` (y), how many OSDs are ``in`` (z), and the map epoch (eNNNN). :: x osds: y up, z in; epoch: eNNNN If the number of OSDs that are ``in`` the cluster is greater than the number of OSDs that are ``up``, run the following command to identify the ``ceph-osd`` daemons that are not running: .. prompt:: bash $ ceph osd tree :: #ID CLASS WEIGHT TYPE NAME STATUS REWEIGHT PRI-AFF -1 2.00000 pool openstack -3 2.00000 rack dell-2950-rack-A -2 2.00000 host dell-2950-A1 0 ssd 1.00000 osd.0 up 1.00000 1.00000 1 ssd 1.00000 osd.1 down 1.00000 1.00000 .. tip:: Searching through a well-designed CRUSH hierarchy to identify the physical locations of particular OSDs might help you troubleshoot your cluster. If an OSD is ``down``, start it by running the following command: .. prompt:: bash $ sudo systemctl start ceph-osd@1 For problems associated with OSDs that have stopped or won't restart, see `OSD Not Running`_. PG Sets ======= When CRUSH assigns a PG to OSDs, it takes note of how many replicas of the PG are required by the pool and then assigns each replica to a different OSD. For example, if the pool requires three replicas of a PG, CRUSH might assign them individually to ``osd.1``, ``osd.2`` and ``osd.3``. CRUSH seeks a pseudo-random placement that takes into account the failure domains that you have set in your `CRUSH map`_; for this reason, PGs are rarely assigned to immediately adjacent OSDs in a large cluster. Ceph processes client requests with the **Acting Set** of OSDs: this is the set of OSDs that currently have a full and working version of a PG shard and that are therefore responsible for handling requests. By contrast, the **Up Set** is the set of OSDs that contain a shard of a specific PG. Data is moved or copied to the **Up Set**, or planned to be moved or copied, to the **Up Set**. See :ref:`Placement Group Concepts `. Sometimes an OSD in the Acting Set is ``down`` or otherwise unable to service requests for objects in the PG. When this kind of situation arises, don't panic. Common examples of such a situation include: - You added or removed an OSD, CRUSH reassigned the PG to other OSDs, and this reassignment changed the composition of the Acting Set and triggered the migration of data by means of a "backfill" process. - An OSD was ``down``, was restarted, and is now ``recovering``. - An OSD in the Acting Set is ``down`` or unable to service requests, and another OSD has temporarily assumed its duties. Typically, the Up Set and the Acting Set are identical. When they are not, it might indicate that Ceph is migrating the PG (in other words, that the PG has been remapped), that an OSD is recovering, or that there is a problem with the cluster (in such scenarios, Ceph usually shows a "HEALTH WARN" state with a "stuck stale" message). To retrieve a list of PGs, run the following command: .. prompt:: bash $ ceph pg dump To see which OSDs are within the Acting Set and the Up Set for a specific PG, run the following command: .. prompt:: bash $ ceph pg map {pg-num} The output provides the following information: the osdmap epoch (eNNN), the PG number ({pg-num}), the OSDs in the Up Set (up[]), and the OSDs in the Acting Set (acting[]):: osdmap eNNN pg {raw-pg-num} ({pg-num}) -> up [0,1,2] acting [0,1,2] .. note:: If the Up Set and the Acting Set do not match, this might indicate that the cluster is rebalancing itself or that there is a problem with the cluster. Peering ======= Before you can write data to a PG, it must be in an ``active`` state and it will preferably be in a ``clean`` state. For Ceph to determine the current state of a PG, peering must take place. That is, the primary OSD of the PG (that is, the first OSD in the Acting Set) must peer with the secondary and OSDs so that consensus on the current state of the PG can be established. In the following diagram, we assume a pool with three replicas of the PG: .. ditaa:: +---------+ +---------+ +-------+ | OSD 1 | | OSD 2 | | OSD 3 | +---------+ +---------+ +-------+ | | | | Request To | | | Peer | | |-------------->| | |<--------------| | | Peering | | | | Request To | | Peer | |----------------------------->| |<-----------------------------| | Peering | The OSDs also report their status to the monitor. For details, see `Configuring Monitor/OSD Interaction`_. To troubleshoot peering issues, see `Peering Failure`_. Monitoring PG States ==================== If you run the commands ``ceph health``, ``ceph -s``, or ``ceph -w``, you might notice that the cluster does not always show ``HEALTH OK``. After first checking to see if the OSDs are running, you should also check PG states. There are certain PG-peering-related circumstances in which it is expected and normal that the cluster will **NOT** show ``HEALTH OK``: #. You have just created a pool and the PGs haven't peered yet. #. The PGs are recovering. #. You have just added an OSD to or removed an OSD from the cluster. #. You have just modified your CRUSH map and your PGs are migrating. #. There is inconsistent data in different replicas of a PG. #. Ceph is scrubbing a PG's replicas. #. Ceph doesn't have enough storage capacity to complete backfilling operations. If one of these circumstances causes Ceph to show ``HEALTH WARN``, don't panic. In many cases, the cluster will recover on its own. In some cases, however, you might need to take action. An important aspect of monitoring PGs is to check their status as ``active`` and ``clean``: that is, it is important to ensure that, when the cluster is up and running, all PGs are ``active`` and (preferably) ``clean``. To see the status of every PG, run the following command: .. prompt:: bash $ ceph pg stat The output provides the following information: the total number of PGs (x), how many PGs are in a particular state such as ``active+clean`` (y), and the amount of data stored (z). :: x pgs: y active+clean; z bytes data, aa MB used, bb GB / cc GB avail .. note:: It is common for Ceph to report multiple states for PGs (for example, ``active+clean``, ``active+clean+remapped``, ``active+clean+scrubbing``. Here Ceph shows not only the PG states, but also storage capacity used (aa), the amount of storage capacity remaining (bb), and the total storage capacity of the PG. These values can be important in a few cases: - The cluster is reaching its ``near full ratio`` or ``full ratio``. - Data is not being distributed across the cluster due to an error in the CRUSH configuration. .. topic:: Placement Group IDs PG IDs consist of the pool number (not the pool name) followed by a period (.) and a hexadecimal number. You can view pool numbers and their names from in the output of ``ceph osd lspools``. For example, the first pool that was created corresponds to pool number ``1``. A fully qualified PG ID has the following form:: {pool-num}.{pg-id} It typically resembles the following:: 1.1701b To retrieve a list of PGs, run the following command: .. prompt:: bash $ ceph pg dump To format the output in JSON format and save it to a file, run the following command: .. prompt:: bash $ ceph pg dump -o {filename} --format=json To query a specific PG, run the following command: .. prompt:: bash $ ceph pg {poolnum}.{pg-id} query Ceph will output the query in JSON format. The following subsections describe the most common PG states in detail. Creating -------- PGs are created when you create a pool: the command that creates a pool specifies the total number of PGs for that pool, and when the pool is created all of those PGs are created as well. Ceph will echo ``creating`` while it is creating PGs. After the PG(s) are created, the OSDs that are part of a PG's Acting Set will peer. Once peering is complete, the PG status should be ``active+clean``. This status means that Ceph clients begin writing to the PG. .. ditaa:: /-----------\ /-----------\ /-----------\ | Creating |------>| Peering |------>| Active | \-----------/ \-----------/ \-----------/ Peering ------- When a PG peers, the OSDs that store the replicas of its data converge on an agreed state of the data and metadata within that PG. When peering is complete, those OSDs agree about the state of that PG. However, completion of the peering process does **NOT** mean that each replica has the latest contents. .. topic:: Authoritative History Ceph will **NOT** acknowledge a write operation to a client until that write operation is persisted by every OSD in the Acting Set. This practice ensures that at least one member of the Acting Set will have a record of every acknowledged write operation since the last successful peering operation. Given an accurate record of each acknowledged write operation, Ceph can construct a new authoritative history of the PG--that is, a complete and fully ordered set of operations that, if performed, would bring an OSD’s copy of the PG up to date. Active ------ After Ceph has completed the peering process, a PG should become ``active``. The ``active`` state means that the data in the PG is generally available for read and write operations in the primary and replica OSDs. Clean ----- When a PG is in the ``clean`` state, all OSDs holding its data and metadata have successfully peered and there are no stray replicas. Ceph has replicated all objects in the PG the correct number of times. Degraded -------- When a client writes an object to the primary OSD, the primary OSD is responsible for writing the replicas to the replica OSDs. After the primary OSD writes the object to storage, the PG will remain in a ``degraded`` state until the primary OSD has received an acknowledgement from the replica OSDs that Ceph created the replica objects successfully. The reason that a PG can be ``active+degraded`` is that an OSD can be ``active`` even if it doesn't yet hold all of the PG's objects. If an OSD goes ``down``, Ceph marks each PG assigned to the OSD as ``degraded``. The PGs must peer again when the OSD comes back online. However, a client can still write a new object to a ``degraded`` PG if it is ``active``. If an OSD is ``down`` and the ``degraded`` condition persists, Ceph might mark the ``down`` OSD as ``out`` of the cluster and remap the data from the ``down`` OSD to another OSD. The time between being marked ``down`` and being marked ``out`` is determined by ``mon_osd_down_out_interval``, which is set to ``600`` seconds by default. A PG can also be in the ``degraded`` state because there are one or more objects that Ceph expects to find in the PG but that Ceph cannot find. Although you cannot read or write to unfound objects, you can still access all of the other objects in the ``degraded`` PG. Recovering ---------- Ceph was designed for fault-tolerance, because hardware and other server problems are expected or even routine. When an OSD goes ``down``, its contents might fall behind the current state of other replicas in the PGs. When the OSD has returned to the ``up`` state, the contents of the PGs must be updated to reflect that current state. During that time period, the OSD might be in a ``recovering`` state. Recovery is not always trivial, because a hardware failure might cause a cascading failure of multiple OSDs. For example, a network switch for a rack or cabinet might fail, which can cause the OSDs of a number of host machines to fall behind the current state of the cluster. In such a scenario, general recovery is possible only if each of the OSDs recovers after the fault has been resolved.] Ceph provides a number of settings that determine how the cluster balances the resource contention between the need to process new service requests and the need to recover data objects and restore the PGs to the current state. The ``osd_recovery_delay_start`` setting allows an OSD to restart, re-peer, and even process some replay requests before starting the recovery process. The ``osd_recovery_thread_timeout`` setting determines the duration of a thread timeout, because multiple OSDs might fail, restart, and re-peer at staggered rates. The ``osd_recovery_max_active`` setting limits the number of recovery requests an OSD can entertain simultaneously, in order to prevent the OSD from failing to serve. The ``osd_recovery_max_chunk`` setting limits the size of the recovered data chunks, in order to prevent network congestion. Back Filling ------------ When a new OSD joins the cluster, CRUSH will reassign PGs from OSDs that are already in the cluster to the newly added OSD. It can put excessive load on the new OSD to force it to immediately accept the reassigned PGs. Back filling the OSD with the PGs allows this process to begin in the background. After the backfill operations have completed, the new OSD will begin serving requests as soon as it is ready. During the backfill operations, you might see one of several states: ``backfill_wait`` indicates that a backfill operation is pending, but is not yet underway; ``backfilling`` indicates that a backfill operation is currently underway; and ``backfill_toofull`` indicates that a backfill operation was requested but couldn't be completed due to insufficient storage capacity. When a PG cannot be backfilled, it might be considered ``incomplete``. The ``backfill_toofull`` state might be transient. It might happen that, as PGs are moved around, space becomes available. The ``backfill_toofull`` state is similar to ``backfill_wait`` in that backfill operations can proceed as soon as conditions change. Ceph provides a number of settings to manage the load spike associated with the reassignment of PGs to an OSD (especially a new OSD). The ``osd_max_backfills`` setting specifies the maximum number of concurrent backfills to and from an OSD (default: 1). The ``backfill_full_ratio`` setting allows an OSD to refuse a backfill request if the OSD is approaching its full ratio (default: 90%). This setting can be changed with the ``ceph osd set-backfillfull-ratio`` command. If an OSD refuses a backfill request, the ``osd_backfill_retry_interval`` setting allows an OSD to retry the request after a certain interval (default: 30 seconds). OSDs can also set ``osd_backfill_scan_min`` and ``osd_backfill_scan_max`` in order to manage scan intervals (default: 64 and 512, respectively). Remapped -------- When the Acting Set that services a PG changes, the data migrates from the old Acting Set to the new Acting Set. Because it might take time for the new primary OSD to begin servicing requests, the old primary OSD might be required to continue servicing requests until the PG data migration is complete. After data migration has completed, the mapping uses the primary OSD of the new Acting Set. Stale ----- Although Ceph uses heartbeats in order to ensure that hosts and daemons are running, the ``ceph-osd`` daemons might enter a ``stuck`` state where they are not reporting statistics in a timely manner (for example, there might be a temporary network fault). By default, OSD daemons report their PG, up through, boot, and failure statistics every half second (that is, in accordance with a value of ``0.5``), which is more frequent than the reports defined by the heartbeat thresholds. If the primary OSD of a PG's Acting Set fails to report to the monitor or if other OSDs have reported the primary OSD ``down``, the monitors will mark the PG ``stale``. When you start your cluster, it is common to see the ``stale`` state until the peering process completes. After your cluster has been running for a while, however, seeing PGs in the ``stale`` state indicates that the primary OSD for those PGs is ``down`` or not reporting PG statistics to the monitor. Identifying Troubled PGs ======================== As previously noted, a PG is not necessarily having problems just because its state is not ``active+clean``. When PGs are stuck, this might indicate that Ceph cannot perform self-repairs. The stuck states include: - **Unclean**: PGs contain objects that have not been replicated the desired number of times. Under normal conditions, it can be assumed that these PGs are recovering. - **Inactive**: PGs cannot process reads or writes because they are waiting for an OSD that has the most up-to-date data to come back ``up``. - **Stale**: PG are in an unknown state, because the OSDs that host them have not reported to the monitor cluster for a certain period of time (determined by ``mon_osd_report_timeout``). To identify stuck PGs, run the following command: .. prompt:: bash $ ceph pg dump_stuck [unclean|inactive|stale|undersized|degraded] For more detail, see `Placement Group Subsystem`_. To troubleshoot stuck PGs, see `Troubleshooting PG Errors`_. Finding an Object Location ========================== 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, the CRUSH algorithm calculates how to map the object to a PG, and then the algorithm calculates how to dynamically assign the PG to an OSD. To find the object location given only the object name and the pool name, run a command of the following form: .. prompt:: bash $ ceph osd map {poolname} {object-name} [namespace] .. topic:: Exercise: Locate an Object As an exercise, let's create an object. We can specify an object name, a path to a test file that contains some object data, and a pool name by using the ``rados put`` command on the command line. For example: .. prompt:: bash $ 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, run the following command: .. prompt:: bash $ rados -p data ls To identify the object location, run the following commands: .. prompt:: bash $ 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 by running the ``rados rm`` command. For example: .. prompt:: bash $ 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 spares you the burden of manually performing the migration. For details, see the `Architecture`_ section. .. _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