Live migration of virtual machines is now commonplace but the issue of synchronising storage remains an obstacle to wide-area migration between datacentres. We discuss the range of possible designs for synchronising disk images and argue that we have to be adaptive at the sector level to efficiently transfer disk-state to the destination. We show, using a number of production workloads, that delaying the transmission of "hot"sectors can achieve considerable bandwidth reduction over a naive eager strategy. We introduce the Activity Based Sector Synchronisation (ABSS) algorithm and show by trace-driven simulations that ABSS is able to deliver a useful trade-off with a data transmission overhead between 0.3% and 2.4% of a gigabit link whilst incurring in the majority of cases a migration latency of less than 1.1 s.
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Akoush et al. (2011) studied this question.
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