Configuration
Create DiskPool(s)#
When a node allocates storage capacity for a replica of a persistent volume (PV) it does so from a DiskPool. Each node may create and manage zero, one, or more such pools. The ownership of a pool by a node is exclusive. A pool can manage only one block device, which constitutes the entire data persistence layer for that pool and thus defines its maximum storage capacity.
A pool is defined declaratively, through the creation of a corresponding DiskPool Custom Resource (CR) on the cluster. The DiskPool must be created in the same namespace where Replicated PV Mayastor has been deployed. User configurable parameters of this resource type include a unique name for the pool, the node name on which it will be hosted and a reference to a disk device which is accessible from that node. The pool definition requires the reference to its member block device to adhere to a discrete range of schemas, each associated with a specific access mechanism/transport/ or device type.
info
Replicated PV Mayastor versions before 2.0.1 had an upper limit on the number of retry attempts in the case of failure in create events in the DSP operator. With this release, the upper limit has been removed, which ensures that the DiskPool operator indefinitely reconciles with the CR.
Permissible Schemes for spec.disks under DiskPool Custom Resource
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It is highly recommended to specify the disk using a unique device link that remains unaltered across node reboots. Examples of such device links are: by-path or by-id.
Easy way to retrieve device link for a given node:
kubectl mayastor get block-devices worker
Usage of the device name (for example, /dev/sdx) is not advised, as it may change if the node reboots, which might cause data corruption.
| Type | Format | Example |
|---|---|---|
| Disk(non PCI) with disk-by-guid reference (Best Practice) | Device File | aio:///dev/disk/by-id/ |
| Asynchronous Disk(AIO) | Device File | /dev/sdx |
| Asynchronous Disk I/O (AIO) | Device File | aio:///dev/sdx |
| io_uring | Device File | uring:///dev/sdx |
Once a node has created a pool it is assumed that it henceforth has exclusive use of the associated block device; it should not be partitioned, formatted, or shared with another application or process. Any pre-existing data on the device will be destroyed.
warning
A RAM drive is not suitable for use in production as it uses volatile memory for backing the data. The memory for this disk emulation is allocated from the hugepages pool. Make sure to allocate sufficient additional hugepages resource on any storage nodes which will provide this type of storage.
Configure Pools#
To get started, it is necessary to create and host at least one pool on one of the nodes in the cluster. The number of pools available limits the extent to which the synchronous N-way mirroring (replication) of PVs can be configured; the number of pools configured should be equal to or greater than the desired maximum replication factor of the PVs to be created. Also, while placing data replicas ensure that appropriate redundancy is provided. Replicated PV Mayastor's control plane will avoid placing more than one replica of a volume on the same node. For example, the minimum viable configuration for a Replicated PV Mayastor deployment which is intended to implement 3-way mirrored PVs must have three nodes, each having one DiskPool, with each of those pools having one unique block device allocated to it.
Using one or more the following examples as templates, create the required type and number of pools.
Example DiskPool Definition
YAML
info
When using the examples given as guides to creating your own pools, remember to replace the values for the fields "metadata.name", "spec.node" and "spec.disks" as appropriate to your cluster's intended configuration. Note that whilst the "disks" parameter accepts an array of values, the current version of Replicated PV Mayastor supports only one disk device per pool.
note
Existing schemas in CR definitions (in older versions) will be updated from v1alpha1 to v1beta1 after upgrading to Replicated PV Mayastor 2.4 and above. To resolve errors encountered pertaining to the upgrade, click here.
Verify Pool Creation and Status#
The status of DiskPools may be determined by reference to their cluster CRs. Available, healthy pools should report their State as online. Verify that the expected number of pools have been created and that they are online.
Command
Example Output
Create Replicated PV Mayastor StorageClass(s)#
Replicated PV Mayastor dynamically provisions PersistentVolumes (PVs) based on StorageClass definitions created by the user. Parameters of the definition are used to set the characteristics and behaviour of its associated PVs. See storage class parameter description for a detailed description of these parameters. Most importantly StorageClass definition is used to control the level of data protection afforded to it (i.e.the number of synchronous data replicas which are maintained, for purposes of redundancy). It is possible to create any number of StorageClass definitions, spanning all permitted parameter permutations.
We illustrate this quickstart guide with two examples of possible use cases; one which offers no data redundancy (i.e. a single data replica), and another having three data replicas.
info
Both the example YAMLs given below have thin provisioning enabled. You can modify these as required to match your own desired test cases, within the limitations of the cluster under test.
Command (1 replica example)
Command (3 replicas example)
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The default installation of Replicated PV Mayastor includes the creation of a StorageClass with the name mayastor-single-replica. The replication factor is set to 1. Users may either use this StorageClass or create their own.
Storage Class Parameters#
Storage Class resource in Kubernetes is used to supply parameters to volumes when they are created. It is a convenient way of grouping volumes with common characteristics. All parameters take a string value. Brief explanation of each parameter is as follows.
info
The Storage Class parameter local has been deprecated and is a breaking change in Replicated PV Mayastor version 2.0. Ensure that this parameter is not used.
"fsType"#
File system that will be used when mounting the volume. The supported file systems are ext4, xfs and btrfs and the default file system when not specified is ext4. We recommend to use xfs that is considered to be more advanced and performant. Please ensure the requested filesystem driver is installed on all worker nodes in the cluster before using it.
"protocol"#
The parameter 'protocol' takes the value nvmf(NVMe over TCP protocol). It is used to mount the volume (target) on the application node.
"repl"#
The string value should be a number and the number should be greater than zero. Replicated PV Mayastor control plane will try to keep always this many copies of the data if possible. If set to one then the volume does not tolerate any node failure. If set to two, then it tolerates one node failure. If set to three, then two node failures, etc.
"thin"#
The volumes can either be thick or thin provisioned. Adding the thin parameter to the StorageClass YAML allows the volume to be thinly provisioned. To do so, add thin: true under the parameters spec, in the StorageClass YAML. Sample YAML
When the volumes are thinly provisioned, the user needs to monitor the pools, and if these pools start to run out of space, then either new pools must be added or volumes deleted to prevent thinly provisioned volumes from getting degraded or faulted. This is because when a pool with more than one replica runs out of space, Replicated PV Mayastor moves the largest out-of-space replica to another pool and then executes a rebuild. It then checks if all the replicas have sufficient space; if not, it moves the next largest replica to another pool, and this process continues till all the replicas have sufficient space.
info
The capacity usage on a pool can be monitored using exporter metrics.
The agents.core.capacity.thin spec present in the Replicated PV Mayastor helm chart consists of the following configurable parameters that can be used to control the scheduling of thinly provisioned replicas:
- poolCommitment parameter specifies the maximum allowed pool commitment limit (in percent).
- volumeCommitment parameter specifies the minimum amount of free space that must be present in each replica pool in order to create new replicas for an existing volume. This value is specified as a percentage of the volume size.
- volumeCommitmentInitial minimum amount of free space that must be present in each replica pool in order to create new replicas for a new volume. This value is specified as a percentage of the volume size.
note
- By default, the volumes are provisioned as
thick. - For a pool of a particular size, say 10 Gigabytes, a volume > 10 Gigabytes cannot be created, as Replicated PV Mayastor currently does not support pool expansion.
- The replicas for a given volume can be either all thick or all thin. Same volume cannot have a combination of thick and thin replicas.
"stsAffinityGroup"#
stsAffinityGroup represents a collection of volumes that belong to instances of Kubernetes StatefulSet. When a StatefulSet is deployed, each instance within the StatefulSet creates its own individual volume, which collectively forms the stsAffinityGroup. Each volume within the stsAffinityGroup corresponds to a pod of the StatefulSet.
This feature enforces the following rules to ensure the proper placement and distribution of replicas and targets so that there is not any single point of failure affecting multiple instances of StatefulSet.
Anti-Affinity among single-replica volumes: This rule ensures that replicas of different volumes are distributed in such a way that there is no single point of failure. By avoiding the colocation of replicas from different volumes on the same node.
Anti-Affinity among multi-replica volumes:
If the affinity group volumes have multiple replicas, they already have some level of redundancy. This feature ensures that in such cases, the replicas are distributed optimally for the stsAffinityGroup volumes.
- Anti-affinity among targets:
The High Availability feature ensures that there is no single point of failure for the targets.
The stsAffinityGroup ensures that in such cases, the targets are distributed optimally for the stsAffinityGroup volumes.
By default, the stsAffinityGroup feature is disabled. To enable it, modify the storage class YAML by setting the parameters.stsAffinityGroup parameter to true.
"cloneFsIdAsVolumeId"#
cloneFsIdAsVolumeId is a setting for volume clones/restores with two options: true and false. By default, it is set to false.
- When set to
true, the created clone/restore's filesystemuuidwill be set to the restore volume'suuid. This is important because some file systems, like XFS, do not allow duplicate filesystemuuidon the same machine by default. - When set to
false, the created clone/restore's filesystemuuidwill be same as the orignal volumeuuid, but it will be mounted using thenouuidflag to bypass duplicateuuidvalidation.
note
This option needs to be set to true when using a btrfs filesystem, if the application using the restored volume is scheduled on the same node where the original volume is mounted, concurrently.