This is documentation for MapR Version 5.0. You can also refer to MapR documentation for the latest release.

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The mirroring process transmits only the differences between the source volume and the mirror. The initial mirroring operation copies the entire source volume, but subsequent mirroring operations can be extremely fast. The mirroring operation never consumes all available network bandwidth, and throttles back when other processes need more network bandwidth. The server sending mirror data continuously monitors the total round-trip time between the data transmission and arrival, and uses this information to restrict itself to 70% of the available bandwidth (continuously calculated). If the network or servers anywhere along the entire path need more bandwidth, the sending server throttles back automatically. If more bandwidth opens up, the sender automatically increases how fast it sends data. Mirror throttling can be disabled so that all available bandwidth is devoted to mirror operations. See Disabling Mirror Throttling for details.

During the copy process, the mirror is a fully-consistent image of the source volume. Mirrors are atomically updated at the mirror destination. The mirror does not change until all bits are transferred, at which point all the new files, directories, blocks, etc., are atomically moved into their new positions in the mirror-volume. The previous mirror is left behind as a snapshot, which can be accessed from the .snapshot directory. These old snapshots can be deleted on a schedule.

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Backup mirrors for disaster recovery can be located on physical media outside the cluster or in a remote cluster. In the event of a disaster affecting the source cluster, you can check the time of last successful synchronization to determine how current the backup is (see Mirror Status below).

Creating Remote Mirrors

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By default, mirror throttling is enabled, which means that the server that sends mirror data restricts itself to 70% 30% (by default) of the available bandwidth. Mirror throttling is based on the number of outstanding requests on the networks and outstanding I/O requests on disk. It can be tuned using the parameters mfs.disk.iothrottle.count, mfs.disk.resynciothrottle.factor, and mfs.network.resynciothrottle.factor in mfs.conf file. When other processes need more network bandwidth, the server throttles back to slow down the rate of data transfer.

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