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Documentation tweaks in Ceph section

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Joshua Boniface 2020-11-25 17:58:33 -05:00
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@ -70,29 +70,31 @@ Currently, only the `amd64` (Intel 64 or AMD64) architecture is officially suppo
## Storage Layout: Ceph and OSDs ## Storage Layout: Ceph and OSDs
The Ceph subsystem of PVC, if enabled, creates a "hyperconverged" cluster whereby storage and VM hypervisor functions are collocated onto the same physical servers. The performance of the storage must be taken into account when sizing the nodes as mentioned above. PVC makes use of Ceph, a distributed, replicated, self-healing, and self-managing storage system to provide shared VM storage. While a PVC administrator is not required to understand Ceph for day-to-day administraton, and PVC provides interfaces to most of the common storage functions required to operate a cluster, at least some knowledge of Ceph is advisable.
The Ceph system is laid out similar to the other daemons. The Ceph Monitor and Manager functions are delegated to the Coordinators over the storage network, with all nodes connecting to these hosts to obtain the CRUSH maps and select OSD disks. OSDs are then distributed on all hosts, including non-coordinator hypervisors, and communicate with clients and each other over the storage network. The Ceph subsystem of PVC creates a "hyperconverged" cluster whereby storage and VM hypervisor functions are collocated onto the same physical servers; PVC does not differentiate between "storage" and "compute" nodes, and while storage support can be disabled and an external Ceph cluster used, this is not recommended. The performance of the storage must be taken into account when sizing the nodes as mentioned above.
Disks must be balanced across all nodes. Therefore, adding 1 disk to 1 node is not sufficient; 1 disk must be added to all nodes at the same time for the available space to increase. Ideally, disk sizes should also be identical across all storage disks, though the weight of each disk can be configured when added to the cluster. Generally speaking, fewer larger disks are preferable to many smaller disks to minimize storage resource utilization, however slightly more storage performance can be gained from using many small disks; the administrator should therefore always aim to choose the biggest disks they can and grow by adding more identical disks as space or performance needs grow. Ceph on PVC is laid out similar to the other daemons. The Ceph Monitor and Manager functions are delegated to the Coordinators over the storage network, with all nodes connecting to these hosts to obtain the CRUSH maps and select OSD disks. OSDs are then distributed on all hosts, potentially including non-coordinator hypervisors if desired, and communicate with clients and each other over the storage network.
PVC Ceph pools make use of the replication mechanism of Ceph to store multiple copies of each object, thus ensuring that data is always available even when a host is unavailable. Only "replica"-based Ceph redundancy is supported by PVC; erasure coded pools are not supported due to major performance impacts related to rewrites and random I/O. Disks must be balanced across all storage-containing nodes. For instance, adding 1 disk to 1 node is not sufficient to increase storage space; 1 disk must be added to all storage-containing nodes, based on the configured replication scheme of the various pools (see below), at the same time for the available space to increase. Ideally, disk sizes should also be identical across all storage disks, though the weight of each disk can be configured when added to the cluster. Generally speaking, fewer larger disks are preferable to many smaller disks to minimize storage resource utilization, however slightly more storage performance can be gained from using many small disks, if the other cluster hardware, and specifically CPUs, are performant enough. The administrator should therefore always aim to choose the biggest disks they can and grow by adding more identical disks as space or performance needs grow.
The default replication level for a new pool is `copies=3, mincopies=2`. This will store 3 copies of each object, with a host-level failure domain, and will allow I/O as long as 2 copies are available. Thus, in a cluster of any size, all data is fully available even if a single host becomes unavailable. It will however use 3x the space for each piece of data stored, which must be considered when sizing the disk space for the cluster: a pool in this configuration, running on 3 nodes each with a single 400GB disk, will effectively have 400GB of total space available for use. As mentioned above, new disks must also be added in groups across nodes equal to the total number of `copies` to ensure new space is usable. PVC Ceph pools make use of the replication mechanism of Ceph to store multiple copies of each object, thus ensuring that data is always available even when a host is unavailable. Only "replica"-based Ceph redundancy is supported by PVC; erasure coded pools are not supported due to major performance impacts related to rewrites and random I/O as well as management overhead.
Non-default values can also be set at pool creation time. For instance, one could create a `copies=3, mincopies=1` pool, which would allow I/O with two hosts down but leaves the cluster susceptible to a write hole should a disk fail in this state. Alternatively, for more resilience, one could create a `copies=4, mincopies=3` pool, which will allow 2 hosts to fail without a write hole, but would consume 4x the space for each piece of data stored and require new disks to be added in groups of 4 instead. Practically any combination of values is possible, however these 3 are the most relevant for most use-cases, and for most, especially small, clusters, the default is sufficient to provide solid redundancy and guard against host failures until the administrator can respond. The default replication level for a new pool is `copies=3, mincopies=2`. This will store 3 copies of each object, with a host-level failure domain, and will allow I/O as long as 2 copies are available. Thus, in a cluster of any size, all data is fully available even if a single host becomes unavailable. It will however use 3x the space for each piece of data stored, which must be considered when sizing the disk space for the cluster: a pool in this configuration, running on 3 nodes each with a single 400GB disk, will effectively have 400GB of total space available for use. As mentioned above, new disks must also be added in groups across nodes equal to the total number of `copies` to ensure new space is usable; for instance in a `copies=3` scheme, at least 3 disks must thus be added to different hosts at the same time for the avilable space to grow.
Replication levels cannot be changed within PVC once a pool is created, however they can be changed via manual Ceph commands on a coordinator should the administrator require this. In any case, the administrator should carefully consider sizing, failure domains, and performance when selecting storage devices to ensure the right level of resiliency versus data usage for their use-case and cluster size. Non-default values can also be set at pool creation time. For instance, one could create a `copies=3, mincopies=1` pool, which would allow I/O with two hosts down, but leaves the cluster susceptible to a write hole should a disk fail in this state; this configuration is not recommended in most situations. Alternatively, for additional resilience, one could create a `copies=4, mincopies=2` pool, which would also allow 2 hosts to fail, without a write hole, but would consume 4x the space for each piece of data stored and require new disks to be added in groups of 4 instead. Practically any combination of values is possible, however these 3 are the most relevant for most use-cases, and for most, especially small, clusters, the default is sufficient to provide solid redundancy and guard against host failures until the administrator can respond.
Replication levels cannot be changed within PVC once a pool is created, however they can be changed via manual Ceph commands on a coordinator should the administrator require this, though discussion of this process is outside of the scope of this documentation. The administrator should carefully consider sizing, failure domains, and performance when first selecting storage devices and creating pools, to ensure the right level of resiliency versus data usage for their use-case and planned cluster size.
## Physical network considerations ## Physical network considerations
At a minimum, a production PVC cluster should use at least two 1Gbps Ethernet interfaces, connected in an LACP or active-backup bond on one or more switches. On top of this bond, the various cluster networks are configured as 802.3q vLANs. PVC is be able to support configurations without 802.1q vLAN support using multiple physical interfaces and no bridged client networks, but this is strongly discouraged due to the added complexity this introduces; the switches chosen for the cluster should include these requirements as a minimum. At a minimum, a production PVC cluster should use at least two 1Gbps Ethernet interfaces, connected in an LACP or active-backup bond on one or more switches. On top of this bond, the various cluster networks should be configured as 802.3q vLANs. PVC is be able to support configurations without bonding or 802.1q vLAN support, using multiple physical interfaces and no bridged client networks, but this is strongly discouraged due to the added complexity this introduces; the switches chosen for the cluster should include these requirements as a minimum.
More advanced physical network layouts are also possible. For instance, one could have two isolated networks. On the first network, each node has two 10Gbps Ethernet interfaces, which are combined in a bond across two redundant switch fabrics and that handle the upstream and cluster networks. On the second network, each node has an additional two 10Gbps, which are also combined in a bond across the redundant switch fabrics and handle the storage network. This configuration could support up to 10Gbps of aggregate client traffic while also supporting 10Gbps of aggregate storage traffic. Even more complex network configurations are possible if the cluster requires such performance. See the [Example Configurations](#example-configurations) section for some examples. More advanced physical network layouts are also possible. For instance, one could have two isolated networks. On the first network, each node has two 10Gbps Ethernet interfaces, which are combined in a bond across two redundant switch fabrics and that handle the upstream and cluster networks. On the second network, each node has an additional two 10Gbps, which are also combined in a bond across the redundant switch fabrics and handle the storage network. This configuration could support up to 10Gbps of aggregate client traffic while also supporting 10Gbps of aggregate storage traffic. Even more complex network configurations are possible if the cluster requires such performance. See the [Example Configurations](#example-configurations) section for some basic topology examples.
Only Ethernet networks are supported by PVC. More exotic interconnects such as Infiniband are not supported by default, and must be manually set up with Ethernet (e.g. EoIB) layers on top to be usable with PVC. Only Ethernet networks are supported by PVC. More exotic interconnects such as Infiniband are not supported by default, and must be manually set up with Ethernet (e.g. EoIB) layers on top to be usable with PVC.
PVC manages the IP addressing of all nodes itself and creates the required addresses during node daemon startup; thus, the on-boot network configuration of each interface should be set to "manual" with no IP addresses configured. PVC manages the IP addressing of all nodes itself and creates the required addresses during node daemon startup; thus, the on-boot network configuration of each interface should be set to "manual" with no IP addresses configured. This can be ignored safely, however, and the addresses specified manually in the networking configurations. PVC nodes use a split (`/etc/network/interfaces.d/<iface>`) network configuration model.
## Network Layout: Considering the required networks ## Network Layout: Considering the required networks