Request-based Device-mapper multipath and Dynamic load balancing

Multipath I/O is the ability to provide increased performance and fault tolerant access to a device by addressing it through more than one path. For storage devices, Linux has seen several solutions that were of two types: high-level approaches that live above the I/O scheduler (BIO mappers), and low-level subsystem specific approaches. Each type of implementation has its advantage because of the position in the storage stack in which it has been implemented. The authors focus on a solution that attempts to reap the benefits of each type of solution by moving the kernel's current multipath layer, dm-multipath, below the I/O scheduler and above the hardware-specific subsystem. This paper describes the block, Device-mapper, and SCSI layer changes for the solution and its effect on performance.


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