@ -22,6 +22,8 @@ As with Minio in stand-alone mode, distributed Minio has a per tenant limit of m
Note that with distributed Minio you can play around with the number of nodes and drives as long as the limits are adhered to. For example, you can have 2 nodes with 4 drives each, 4 nodes with 4 drives each, 8 nodes with 2 drives each, and so on.
You can also use [storage classes](https://github.com/minio/minio/tree/master/docs/erasure/storage-class) to set custom data and parity distribution across total disks.
### Consistency Guarantees
Minio follows strict **read-after-write** consistency model for all i/o operations both in distributed and standalone modes.
Minio protects data against hardware failures and silent data corruption using erasure code and checksums. You may lose half the number (N/2) of drives and still be able to recover the data.
Minio protects data against hardware failures and silent data corruption using erasure code and checksums. With the highest level of redundancy, you may lose up to half (N/2) of the total drives and still be able to recover the data.
## What is Erasure Code?
Erasure code is a mathematical algorithm to reconstruct missing or corrupted data. Minio uses Reed-Solomon code to shard objects into N/2 data and N/2 parity blocks. This means that in a 12 drive setup, an object is sharded across as 6 data and 6 parity blocks. You can lose as many as 6 drives (be it parity or data) and still reconstruct the data reliably from the remaining drives.
Erasure code is a mathematical algorithm to reconstruct missing or corrupted data. Minio uses Reed-Solomon code to shard objects into variable data and parity blocks. For example, in a 12 drive setup, an object can be sharded to a variable number of data and parity blocks across all the drives - ranging from six data and six parity blocks to ten data and two parity blocks.
By default, Minio shards the objects across N/2 data and N/2 parity drives. Though, you can use [storage classes](https://github.com/minio/minio/tree/master/docs/erasure/storage-class) to use a custom configuration. We recommend N/2 data and parity blocks, as it ensures the best protection from drive failures.
In 12 drive example above, with Minio server running in the default configuration, you can lose any of the six drives and still reconstruct the data reliably from the remaining drives.
## Why is Erasure Code useful?
Erasure code protects data from multiple drives failure unlike RAID or replication. For eg RAID6 can protect against 2 drive failure whereas in Minio erasure code you can lose as many as half number of drives and still the data remains safe. Further Minio's erasure code is at object level and can heal one object at a time. For RAID, healing can only be performed at volume level which translates into huge down time. As Minio encodes each object individually with a high parity count. Storage servers once deployed should not require drive replacement or healing for the lifetime of the server. Minio's erasure coded backend is designed for operational efficiency and takes full advantage of hardware acceleration whenever available.
Erasure code protects data from multiple drives failure, unlike RAID or replication. For example, RAID6 can protect against two drive failure whereas in Minio erasure code you can lose as many as half of drives and still the data remains safe. Further, Minio's erasure code is at the object level and can heal one object at a time. For RAID, healing can be done only at the volume level which translates into high downtime. As Minio encodes each object individually, it can heal objects incrementally. Storage servers once deployed should not require drive replacement or healing for the lifetime of the server. Minio's erasure coded backend is designed for operational efficiency and takes full advantage of hardware acceleration whenever available.
Bit Rot also known as Data Rot or Silent Data Corruption is a serious data loss issue faced by disk drives today. Data on the drive may silently get corrupted without signalling an error has occurred. This makes Bit Rot more dangerous than permanent hard drive failure.
Bit Rot, also known as data rot or silent data corruption is a data loss issue faced by disk drives today. Data on the drive may silently get corrupted without signalling an error has occurred, making bit rot more dangerous than a permanent hard drive failure.
Minio's erasure coded backend uses high speed [BLAKE2](https://blog.minio.io/accelerating-blake2b-by-4x-using-simd-in-go-assembly-33ef16c8a56b#.jrp1fdwer) hash based checksums to protect against Bit Rot.
Nitish Tiwari
7 years ago
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