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Erasure Coding

minio

Table of Contents

MinIO Erasure Coding is a data redundancy and availability feature that allows MinIO deployments to automatically reconstruct objects on-the-fly despite the loss of multiple drives or nodes in the cluster. Erasure Coding provides object-level healing with less overhead than adjacent technologies such as RAID or replication.

MinIO splits each new object into data and parity blocks, where parity blocks support reconstruction of missing or corrupted data blocks. MinIO writes these blocks to a single erasure set <minio-ec-erasure-set> in the deployment. Since erasure set drives are striped across the deployment, a given node typically contains only a portion of data or parity blocks for each object. MinIO can therefore tolerate the loss of multiple drives or nodes in the deployment depending on the configured parity and deployment topology.

MinIO Erasure Coding example

At maximum parity, MinIO can tolerate the loss of up to half the drives per erasure set (N/2-1) and still perform read and write operations. MinIO defaults to 4 parity blocks per object with tolerance for the loss of 4 drives per erasure set. For more complete information on selecting erasure code parity, see minio-ec-parity.

Erasure coding requires a minimum of 4 drives is only available with distributed <minio-installation-comparison> MinIO deployments. Erasure coding is a core requirement for the following MinIO features:

  • Object Versioning <minio-bucket-versioning>
  • Server-Side Replication <minio-bucket-replication>
  • Write-Once Read-Many Locking <minio-bucket-locking>

Use the MinIO Erasure Code Calculator when planning and designing your MinIO deployment to explore the effect of erasure code settings on your intended topology.

Erasure Sets

An Erasure Set is a set of drives in a MinIO deployment that support Erasure Coding. MinIO evenly distributes object data and parity blocks among the drives in the Erasure Set. MinIO randomly and uniformly distributes the data and parity blocks across drives in the erasure set with no overlap. Each unique object has no more than one data or parity block per drive in the set.

MinIO calculates the number and size of Erasure Sets by dividing the total number of drives in the Server Pool <minio-intro-server-pool> into sets consisting of between 4 and 16 drives each.

Use the MinIO Erasure Coding Calculator to determine the optimal erasure set size for your preferred MinIO topology.

Erasure Code Parity (EC:N)

MinIO uses a Reed-Solomon algorithm to split objects into data and parity blocks based on the Erasure Set <minio-ec-erasure-set> size in the deployment. For a given erasure set of size M, MinIO splits objects into N parity blocks and M-N data blocks.

MinIO uses the EC:N notation to refer to the number of parity blocks (N) in the deployment. MinIO defaults to EC:4 or 4 parity blocks per object. MinIO uses the same EC:N value for all erasure sets and server pools <minio-intro-server-pool> in the deployment.

MinIO can tolerate the loss of up to N drives per erasure set and continue performing read and write operations ("quorum"). If N is equal to exactly 1/2 the drives in the erasure set, MinIO write quorum requires N+1 drives to avoid data inconsistency ("split-brain").

Setting the parity for a deployment is a balance between availability and total usable storage. Higher parity values increase resiliency to drive or node failure at the cost of usable storage, while lower parity provides maximum storage with reduced tolerance for drive/node failures. Use the MinIO Erasure Code Calculator to explore the effect of parity on your planned cluster deployment.

The following table lists the outcome of varying erasure code parity levels on a MinIO deployment consisting of 1 node and 16 1TB drives:

Outcome of Parity Settings on a 16 Drive MinIO Cluster
Parity Total Storage Storage Ratio Minimum Drives for Read Operations Minimum Drives for Write Operations
EC: 4 (Default) 12 Tebibytes 0.750 12 12
EC: 6 10 Tebibytes 0.625 10 10
EC: 8 8 Tebibytes 0.500 8 9

Storage Classes

MinIO supports storage classes with Erasure Coding to allow applications to specify per-object parity <minio-ec-parity>. Each storage class specifies a EC:N parity setting to apply to objects created with that class.

MinIO storage classes are distinct from Amazon Web Services storage classes <storage-class-intro.html>. MinIO storage classes define parity settings per object, while AWS storage classes define storage tiers per object.

MinIO provides the following two storage classes:

STANDARD

The STANDARD storage class is the default class for all objects. MinIO sets the STANDARD parity based on the number of volumes in the Erasure Set:

Erasure Set Size Default Parity (EC:N)
5 or Fewer EC:2
6 - 7 EC:3
8 or more EC:4

You can override the default STANDARD parity using either:

  • The MINIO_STORAGE_CLASS_STANDARD environment variable, or
  • The mc admin config command to modify the storage_class.standard configuration setting.

The maximum value is half of the total drives in the Erasure Set <minio-ec-erasure-set>. The minimum value is 2.

STANDARD parity must be greater than or equal to REDUCED_REDUNDANCY. If REDUCED_REDUNDANCY is unset, STANDARD parity must be greater than 2.

REDUCED_REDUNDANCY

The REDUCED_REDUNDANCY storage class allows creating objects with lower parity than STANDARD. REDUCED_REDUNDANCY requires at least 5 drives in the MinIO deployment.

MinIO sets the REDUCED_REDUNDANCY parity to EC:2 by default. You can override REDUCED_REDUNDANCY storage class parity using either:

  • The MINIO_STORAGE_CLASS_RRS environment variable, or
  • The mc admin config command to modify the storage_class.rrs configuration setting.

REDUCED_REDUNDANCY parity must be less than or equal to STANDARD.

MinIO references the x-amz-storage-class header in request metadata for determining which storage class to assign an object. The specific syntax or method for setting headers depends on your preferred method for interfacing with the MinIO server.

  • For the mc command line tool, certain commands include a specific option for setting the storage class. For example, the mc cp command has the ~mc cp storage-class option for specifying the storage class to assign to the object being copied.
  • For MinIO SDKs, the S3Client object has specific methods for setting request headers. For example, the minio-go SDK S3Client.PutObject method takes a PutObjectOptions data structure as a parameter. The PutObjectOptions data structure includes the StorageClass option for specifying the storage class to assign to the object being created.

BitRot Protection

Silent data corruption or bitrot is a serious problem faced by disk drives resulting in data getting corrupted without the user’s knowledge. The reasons are manifold (ageing drives, current spikes, bugs in disk firmware, phantom writes, misdirected reads/writes, driver errors, accidental overwrites) but the result is the same - compromised data.

MinIO’s optimized implementation of the HighwayHash algorithm ensures that it will never read corrupted data - it captures and heals corrupted objects on the fly. Integrity is ensured from end to end by computing a hash on READ and verifying it on WRITE from the application, across the network and to the memory/drive. The implementation is designed for speed and can achieve hashing speeds over 10 GB/sec on a single core on Intel CPUs.