YDB cluster configuration

The cluster configuration is specified in the YAML file passed in the --yaml-config parameter when the cluster nodes are run.

This article describes the main groups of configurable parameters in this file.

host_configs: Typical host configurations

A YDB cluster consists of multiple nodes, and one or more typical server configurations are usually used for their deployment. To avoid repeating its description for each node, there is a host_configs section in the configuration file that lists the used configurations and assigned IDs.

Syntax

host_configs:
- host_config_id: 1
  drive:
  - path: <path_to_device>
    type: <type>
  - path: ...
- host_config_id: 2
  ...  

The host_config_id attribute specifies a numeric configuration ID. The drive attribute contains a collection of descriptions of connected drives. Each description consists of two attributes:

• path: Path to the mounted block device, for example, /dev/disk/by-partlabel/ydb_disk_ssd_01
• type: Type of the device's physical media: ssd, nvme, or rot (rotational - HDD)

Examples

One configuration with ID 1 and one SSD disk accessible via /dev/disk/by-partlabel/ydb_disk_ssd_01:

host_configs:
- host_config_id: 1
  drive:
  - path: /dev/disk/by-partlabel/ydb_disk_ssd_01
    type: SSD

Two configurations with IDs 1 (two SSD disks) and 2 (three SSD disks):

host_configs:
- host_config_id: 1
  drive:
  - path: /dev/disk/by-partlabel/ydb_disk_ssd_01
    type: SSD
  - path: /dev/disk/by-partlabel/ydb_disk_ssd_02
    type: SSD
- host_config_id: 2
  drive:
  - path: /dev/disk/by-partlabel/ydb_disk_ssd_01
    type: SSD
  - path: /dev/disk/by-partlabel/ydb_disk_ssd_02
    type: SSD
  - path: /dev/disk/by-partlabel/ydb_disk_ssd_03
    type: SSD

Kubernetes features

The YDB Kubernetes operator mounts NBS disks for Storage nodes at the path /dev/kikimr_ssd_00. To use them, the following host_configs configuration must be specified:

host_configs:
- host_config_id: 1
  drive:
  - path: /dev/kikimr_ssd_00
    type: SSD

The example configuration files provided with the YDB Kubernetes operator contain this section, and it does not need to be changed.

hosts: Static cluster nodes

This group lists the static cluster nodes on which the Storage processes run and specifies their main characteristics:

• Numeric node ID
• DNS host name and port that can be used to connect to a node on the IP network
• ID of the standard host configuration
• Placement in a specific availability zone, rack
• Server inventory number (optional)

Syntax

hosts:
- host: <DNS host name>
  host_config_id: <numeric ID of the standard host configuration>
  port: <port> # 19001 by default
  location:
    unit: <string with the server serial number>
    data_center: <string with the availability zone ID>
    rack: <string with the rack ID>
- host: <DNS host name>
  ...

Examples

hosts:
- host: hostname1
  host_config_id: 1
  node_id: 1
  port: 19001
  location:
    unit: '1'
    data_center: '1'
    rack: '1'
- host: hostname2
  host_config_id: 1
  node_id: 2
  port: 19001
  location:
    unit: '1'
    data_center: '1'
    rack: '1'

Kubernetes features

When deploying YDB with a Kubernetes operator, the entire hosts section is generated automatically, replacing any user-specified content in the configuration passed to the operator. All Storage nodes use host_config_id = 1, for which the correct configuration must be specified.

domains_config: Cluster domain

This section contains the configuration of the YDB cluster root domain, including the Blob Storage (binary object storage), State Storage, and authentication configurations.

Syntax

domains_config:
  domain:
  - name: <root domain name>
    storage_pool_types: <Blob Storage configuration>
  state_storage: <State Storage configuration>
  security_config: <authentication configuration>

Blob Storage configuration

This section defines one or more types of storage pools available in the cluster for the data in the databases with the following configuration options:

• Storage pool name
• Device properties (for example, disk type)
• Data encryption (on/off)
• Fault tolerance mode

The following fault tolerance modes are available:

Syntax

  storage_pool_types:
  - kind: <storage pool name>
    pool_config:
      box_id: 1
      encryption: <optional, specify 1 to encrypt data on the disk>
      erasure_species: <fault tolerance mode name - none, block-4-2, or mirror-3-dc>
      kind: <storage pool name - specify the same value as above>
      pdisk_filter:
      - property:
        - type: <device type to be compared with the one specified in host_configs.drive.type>
      vdisk_kind: Default
  - kind: <storage pool name>
  ...

Each database in the cluster is assigned at least one of the available storage pools selected in the database creation operation. The names of storage pools among those assigned can be used in the DATA attribute when defining column groups in YQL operators CREATE TABLE/ALTER TABLE.

State Storage configuration

State Storage is an independent in-memory storage for variable data that supports internal YDB processes. It stores data replicas on multiple assigned nodes.

State Storage usually does not need scaling for better performance, so the number of nodes in it must be kept as small as possible taking into account the required level of fault tolerance.

State Storage availability is key for a YDB cluster because it affects all databases, regardless of which storage pools they use. To ensure fault tolerance of State Storage, its nodes must be selected to guarantee a working majority in case of expected failures.

The following guidelines can be used to select State Storage nodes:

When deploying State Storage on clusters that use multiple storage pools with a possible combination of fault tolerance modes, consider increasing the number of nodes and spreading them across different storage pools because unavailability of State Storage results in unavailability of the entire cluster.

Syntax

state_storage:
- ring:
    node: <StateStorage node array>
    nto_select: <number of data replicas in StateStorage>
  ssid: 1

Each State Storage client (for example, DataShard tablet) uses nto_select nodes to write copies of its data to State Storage. If State Storage consists of more than nto_select nodes, different nodes can be used for different clients, so you must ensure that any subset of nto_select nodes within State Storage meets the fault tolerance criteria.

Odd numbers must be used for nto_select because using even numbers does not improve fault tolerance in comparison to the nearest smaller odd number.

Authentication configuration

The authentication mode in the YDB cluster is created in the domains_config.security_config section.

Syntax

domains_config:
  ...
  security_config:
    enforce_user_token_requirement: Bool
  ...

Examples

domains_config:
  domain:
  - name: Root
    storage_pool_types:
    - kind: ssd
      pool_config:
        box_id: 1
        erasure_species: block-4-2
        kind: ssd
        pdisk_filter:
        - property:
          - type: SSD
        vdisk_kind: Default
  state_storage:
  - ring:
      node: [1, 2, 3, 4, 5, 6, 7, 8]
      nto_select: 5
    ssid: 1

domains_config:
  domain:
  - name: Root
    storage_pool_types:
    - kind: ssd
      pool_config:
        box_id: 1
        erasure_species: block-4-2
        kind: ssd
        pdisk_filter:
        - property:
          - type: SSD
        vdisk_kind: Default
  state_storage:
  - ring:
      node: [1, 2, 3, 4, 5, 6, 7, 8]
      nto_select: 5
    ssid: 1
  security_config:
    enforce_user_token_requirement: true

domains_config:
  domain:
  - name: global
    storage_pool_types:
    - kind: ssd
      pool_config:
        box_id: 1
        erasure_species: mirror-3-dc
        kind: ssd
        pdisk_filter:
        - property:
          - type: SSD
        vdisk_kind: Default
  state_storage:
  - ring:
      node: [1, 2, 3, 4, 5, 6, 7, 8, 9]
      nto_select: 9
    ssid: 1

domains_config:
  domain:
  - name: Root
    storage_pool_types:
    - kind: ssd
      pool_config:
        box_id: 1
        erasure_species: none
        kind: ssd
        pdisk_filter:
        - property:
          - type: SSD
        vdisk_kind: Default
  state_storage:
  - ring:
      node:
      - 1
      nto_select: 1
    ssid: 1

domains_config:
  domain:
  - name: Root
    storage_pool_types:
    - kind: ssd
      pool_config:
        box_id: '1'
        erasure_species: block-4-2
        kind: ssd
        pdisk_filter:
        - property:
          - {type: SSD}
        vdisk_kind: Default
    - kind: rot
      pool_config:
        box_id: '1'
        erasure_species: block-4-2
        kind: rot
        pdisk_filter:
        - property:
          - {type: ROT}
        vdisk_kind: Default
    - kind: rotencrypted
      pool_config:
        box_id: '1'
        encryption_mode: 1
        erasure_species: block-4-2
        kind: rotencrypted
        pdisk_filter:
        - property:
          - {type: ROT}
        vdisk_kind: Default
    - kind: ssdencrypted
      pool_config:
        box_id: '1'
        encryption_mode: 1
        erasure_species: block-4-2
        kind: ssdencrypted
        pdisk_filter:
        - property:
          - {type: SSD}
        vdisk_kind: Default
  state_storage:
  - ring:
      node: [1, 16, 31, 46, 61, 76, 91, 106]
      nto_select: 5
    ssid: 1

Actor system

The CPU resources are mainly used by the actor system. Depending on the type, all actors run in one of the pools (the name parameter). Configuring is allocating a node's CPU cores across the actor system pools. When allocating them, please keep in mind that PDisks and the gRPC API run outside the actor system and require separate resources.

You can set up your actor system either automatically or manually. In the actor_system_config section, specify:

• Node type and the number of CPU cores allocated to the ydbd process by automatic configuring.
• Number of CPU cores for each YDB cluster subsystem in the case of manual configuring.

Automatic configuring adapts to the current system workload. It is recommended in most cases.

You might opt for manual configuring when a certain pool in your actor system is overwhelmed and undermines the overall database performance. You can track the workload on your pools on the Embedded UI monitoring page.

Automatic configuring

Example of the actor_system_config section for automatic configuring of the actor system:

actor_system_config:
  use_auto_config: true
  node_type: STORAGE
  cpu_count: 10

Manual configuring

Example of the actor_system_config section for manual configuring of the actor system:

actor_system_config:
  executor:
  - name: System
    spin_threshold: 0
    threads: 2
    type: BASIC
  - name: User
    spin_threshold: 0
    threads: 3
    type: BASIC
  - name: Batch
    spin_threshold: 0
    threads: 2
    type: BASIC
  - name: IO
    threads: 1
    time_per_mailbox_micro_secs: 100
    type: IO
  - name: IC
    spin_threshold: 10
    threads: 1
    time_per_mailbox_micro_secs: 100
    type: BASIC
  scheduler:
    progress_threshold: 10000
    resolution: 256
    spin_threshold: 0

blob_storage_config: Static cluster group

Specify a static cluster group's configuration. A static group is necessary for the operation of the basic cluster tablets, including Hive, SchemeShard, and BlobstorageContoller.
As a rule, these tablets do not store a lot of data, so we don't recommend creating more than one static group.

For a static group, specify the disks and nodes that the static group will be placed on. For example, a configuration for the erasure: none model can be as follows:

blob_storage_config:
  service_set:
    groups:
    - erasure_species: none
      rings:
      - fail_domains:
        - vdisk_locations:
          - node_id: 1
            path: /dev/disk/by-partlabel/ydb_disk_ssd_02
            pdisk_category: SSD
....

For a configuration located in 3 availability zones, specify 3 rings. For a configuration within a single availability zone, specify exactly one ring.

BlobStorage production configurations

To ensure the required fault tolerance of YDB, configure the cluster disk subsystem properly: select the appropriate fault tolerance mode and hardware configuration for your cluster.

Fault tolerance modes

We recommend using the following fault tolerance modes for YDB production installations:

• block-4-2: For a cluster hosted in a single availability zone.
• mirror-3-dc: For a cluster hosted in three availability zones.

A fail model of YDB is based on concepts such as fail domain and fail realm.

Fail domain

A set of hardware that may fail concurrently.

For example, a fail domain includes disks of the same server (as all server disks may be unavailable if the server PSU or network controller is down). A fail domain also includes servers located in the same server rack (as the entire hardware in the rack may be unavailable if there is a power outage or some issue with the network hardware in the same rack).

Any domain fail is handled automatically, without shutting down the system.

Fail realm

A set of fail domains that may fail concurrently.

An example of a fail realm is hardware located in the same data center that may fail as a result of a natural disaster.

Usually a fail domain is a server rack, while a fail realm is a data center.

When creating a storage group, YDB groups VDisks that are located on PDisks from different fail domains. For block-4-2 mode, a PDisk should be distributed across at least 8 fail domains, and for mirror-3-dc mode, across 3 fail realms with at least 3 fail domains in each of them.

Hardware configuration

If a disk fails, YDB may automatically reconfigure a storage group so that, instead of the VDisk located on the failed hardware, a new VDisk is used that the system tries to place on hardware that is running while the group is being reconfigured. In this case, the same rule applies as when creating a group: a VDisk is created in a fail domain that is different from the fail domains of any other VDisk in this group (and in the same fail realm as that of the failed VDisk for mirror-3-dc).

This causes some issues when a cluster's hardware is distributed across the minimum required amount of fail domains:

• If the entire fail domain is down, reconfiguration no longer makes sense, since a new VDisk can only be located in the fail domain that is down.
• If part of a fail domain is down, reconfiguration is possible, but the load that was previously handled by the failed hardware will only be redistributed across hardware in the same fail domain.

If the number of fail domains in a cluster exceeds the minimum amount required for creating storage groups at least by one (that is, 9 domains for block-4-2 and 4 domains in each fail realm for mirror-3-dc), in case some hardware fails, the load can be redistributed across all the hardware that is still running.

The system can work with fail domains of any size. However, if there are few domains and a different number of disks in different domains, the number of storage groups that you can create will be limited. In this case, some hardware in fail domains that are too large may be underutilized. If the hardware is used in full, significant distortions in domain sizes may make reconfiguration impossible.

For example, there are 15 racks in a cluster with block-4-2 fault tolerance mode. The first of the 15 racks hosts 20 servers and the other 14 racks host 10 servers each. To fully utilize all the 20 servers from the first rack, YDB will create groups so that 1 disk from this largest fail domain is used in each group. As a result, if any other fail domain's hardware is down, the load can't be distributed to the hardware in the first rack.

YDB can group disks of different vendors, capacity, and speed. The resulting characteristics of a group depend on a set of the worst characteristics of the hardware that is serving the group. Usually the best results can be achieved if you use same-type hardware. When creating large clusters, keep in mind that hardware from the same batch is more likely to have the same defect and fail simultaneously.

Therefore, we recommend the following optimal hardware configurations for production installations:

• A cluster hosted in 1 availability zone: It uses block4-2 fault tolerance mode and consists of 9 or more racks with the same amount of identical servers in each rack.
• A cluster hosted in 3 availability zones: It uses mirror3-dc fault tolerance mode and is distributed across 3 data centers with 4 or more racks in each of them, the racks being equipped with the same amount of identical servers.

See also Simplified hardware configurations.

Redundancy recovery

Auto reconfiguration of storage groups reduces the risk of data loss in the event of multiple failures that occur within intervals sufficient to recover the redundancy. By default, reconfiguration is done one hour after YDB detects a failure.

Once a group is reconfigured, a new VDisk is automatically populated with data to restore the required storage redundancy in the group. This increases the load on other VDisks in the group and the network. To reduce the impact of redundancy recovery on the system performance, the total data replication speed is limited both on the source and target VDisks.

The time it takes to restore the redundancy depends on the amount of data and hardware performance. For example, replication on fast NVMe SSDs may take an hour, while on large HDDs more than 24 hours. To make reconfiguration possible in general, a cluster should have free slots for creating VDisks in different fail domains. When determining the number of slots to be kept free, factor in the risk of hardware failure, the time it takes to replicate data and replace the failed hardware.

Simplified hardware configurations

If it's not possible to use the recommended amount of hardware, you can divide servers within a single rack into two dummy fail domains. In this configuration, a failure of 1 rack means a failure of 2 domains and not a single one. In both fault tolerance modes, YDB will keep running if 2 domains fail. If you use the configuration with dummy fail domains, the minimum number of racks in a cluster is 5 for block-4-2 mode and 2 in each data center for mirror-3-dc mode.

Fault tolerance level

The table below describes fault tolerance levels for different fault tolerance modes and hardware configurations of a YDB cluster:

Sample cluster configurations

You can find model cluster configurations for deployment in the repository. Check them out before deploying a cluster.