# 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 amount 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.

## 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:

Fault tolerance
mode
Fail
domain
Fail
realm
Number of
data centers
Number of
server racks
Fault tolerance
level
block-4-2 Rack Data center 1 9 or more Can stand a failure of 2 racks
block-4-2 ½ a rack Data center 1 5 or more Can stand a failure of 1 rack
block-4-2 Server Data center 1 Doesn't matter Can stand a failure of 2 servers
mirror-3-dc Rack Data center 3 4 in each data center Can stand a failure of a data center and 1 rack in one of the two other data centers
mirror-3-dc Server Data center 3 Doesn't matter Can stand a failure of a data center and 1 server in one of the two other data centers