See Immediate Write Mirroring Design for current design

Introduction

Currently the only way to get a redundancy for a file (or have it mirrored) is to run lfs utility. The file is supposed to be closed at that time or the utility interrupts and must be restarted from the beginning. If any application opens a file being replicated then the replication interrupts as well.
Another problem with such offline mirroring is resource consumption as the replication needs to read data from OSTs (disk IOs, then network transfers), then send it back to another OSTs (network transfers, then disk IOs). For any 1MB of data to be replicated we need: 1MB disk/network transfer to write original data, 1MB disk/network transfer to read original data and 1MB disk/network transfer to write a replica - 3 x disk/network bandwidth.
In contrast immediate mirroring should let clients maintain replica(s) while file is being used by regular apps and save 1 disk/network transfer - 2x disk/network bandwidth, not 3x.

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