Mirroring Overview

Creating a mirror volume is similar to creating a normal read/write volume. However, when you create a mirror volume, you must specify a source volume that the mirror retrieves content from. This retrieval is called the mirroring operation. Like a normal volume, a mirror volume has a configurable replication factor. Only one copy of the data is transmitted from the source volume to the mirror volume; the source and mirror volumes handle their own replication independently.

The MapR system creates a temporary snapshot of the source volume at the start of a mirroring operation. The mirroring process reads content from the snapshot into the mirror volume. The source volume remains available for read and write operations during the mirroring process. If the mirroring operation is schedule-based, the snapshot expires according to the value of the schedule's Retain For parameter. Snapshots created during manual mirroring persist until they are deleted manually.

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.

Mirroring is extremely resilient. In the case of a network partition, where some or all of the machines that host the source volume cannot communicate with the machines that host the mirror volume, the mirroring operation periodically retries the connection. Once the network is restored, the mirroring operation resumes.

When the root volume on a cluster is mirrored, the source root volume contains a writable volume link, .rw that points to the read/write copies of all local volumes. In that case, the mount path / refers to one of the root volume's mirrors, and is read-only. The mount path /.rw refers to the source volume, and is read/write.

A mount path that consists entirely of mirrored volumes refers to a mirrored copy of the specified volume. When a mount path contains volumes that are not mirrored, the path refers to the target volume directly. In cases where a path refers to a mirrored copy, the .rw link is useful for navigating to the read/write source volume. The table below provides examples.

The following example shows a volume topology with mirrors: