It should be remembered that in configurations where erasure coding is used, parity sums are always calculated by Chunkservers. Hardware configurationĪ minimal number of Chunkservers for EC configuration is 8+2n, where n is the desired number of parity stripes to be calculated by a cluster. All stripes (both original and parity) are always of the same size. It is an 8+n type of erasure coding algorithm. One may define up to 9 parity stripes to be calculated for every 8 stripes of original data. The extra parity stripes allow the cluster to recover missing parts of the original data when necessary. There are special, additional parts called “parity stripes” or “erasure codes” which are calculated with a special algorithm from the original data. Instead of keeping several copies of each file, which is disk space inefficient, each chunk of data is divided into parts. Cluster with Erasure CodingĮrasure Coding is another way of ensuring data in a cluster. It is unavailable for MooseFS 1.x, 2.x and 3.x Community Edition. HA configurations are available for MooseFS 3.x Pro and MooseFS 4.x versions. LACP should be used for network redundancy (as mentioned above) otherwise network becomes a single point of failure. MooseFS protocol supports HA for all client-cluster communication. However, it may not be possible due to a protocol or implementation limitations. Using external gateways (SMB, NFS, etc.) in front of the MooseFS HA cluster requires configuring HA for these gateways independently. It is important to notice that with HA configuration all pending client-side I/O operations are not interrupted, they may be just sustained for a short period of time (usually less than a few seconds). The HA-aware (client side) mfsmount daemon assures constant file system access for client applications – even during automatic Leader Master Server election. A user may install more than one Follower Master Server to get a higher degree of cluster availability. Installing Metaloggers for HA configuration is not necessary as Follower Master Servers take care of metadata backups. Please refer to Erasure Coding configuration described below. Such a case requires more Chunkservers as each chunk is divided into many “parts”. One may use Erasure Coding with HA configuration. In the case when only 1 Chunkserver is available, MooseFS cluster waits for another one (in order to elect a new Leader Master Server) and is not able to perform any operations: data may be safe but is inaccessible. With 3 Chunkservers, when one of them goes down, data is still accessible and it may be replicated to the 2nd available Chunkserver. As the minimal odd number greater than 2 (required for redundancy) is 3 – so this is the minimal number of Chunkservers required.Īnother reason for using at least 3 Chunkservers is to keep replication goal at the safe level (at least 2) even in case of failure of one Chunkserver. The election process is designed in such a way that it prevents a possible cluster split-brain scenario.
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