Linux DM Multipath
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Wikipedia – Linux DM-Multipath
Wikipedia – Linux DM-Multipath 1. Definition DM-Multipathing(DM-MPIO) provides I/O failover and load-balancing within Linux for block devices(ref). By utilizing device-mapper(ref), multipathd provides the host-side logic to use multiple paths of a redundant network to provide continuous availability and higher bandwidth connectivity between the host server and the block-level device. DM-Multipath handles the rerouting of block I/O to an alternate path in the event of a path failure. DM-Multipath can also balance the I/O load across all of the available paths which are typically Fibre Channel (FC) or iSCSI SAN environments. DM MPIO is based on the Device-Mapper which provides the basic framework that maps one block device onto another.(ref)
Components DM-MPIO in Linux consists of kernel components and user space components. Kernel – device-mapper – block subsystem which provides layering mechanism for block devices. User-space – multipath-tools – provides the tools to manage multipathed devices by instructing the device-mapper multipath module what to do. The tools are: Multipath: scans the system for multipathed devices, assembles them, updates the device-mapper’s map. Multipathd: daemon which waits for maps events then executes multipath and monitors the paths. Marks a path as failed when the path becomes faulty. Depending on the failback policy, it can reactive the path. Devmap-name: provides a meaningful device name to udev for devmaps Kpartx: maps linear devmaps to device partitions to make multipath maps partionable (novell module ref) Multipath.conf: configuration file for the multipath daemon. Used to overwrite the built-in configuration table of multipathd.
Output of multipath command
Wikipedia – Linux DM-Multipath 1. Definition DM-Multipathing(DM-MPIO) provides I/O failover and load-balancing within Linux for block devices(ref). By utilizing device-mapper(ref), multipathd provides the host-side logic to use multiple paths of a redundant network to provide continuous availability and higher bandwidth connectivity between the host server and the block-level device. DM-Multipath handles the rerouting of block I/O to an alternate path in the event of a path failure. DM-Multipath can also balance the I/O load across all of the available paths which are typically Fibre Channel (FC) or iSCSI SAN environments. DM MPIO is based on the Device-Mapper which provides the basic framework that maps one block device onto another.(ref)
Components DM-MPIO in Linux consists of kernel components and user space components. Kernel – device-mapper – block subsystem which provides layering mechanism for block devices. User-space – multipath-tools – provides the tools to manage multipathed devices by instructing the device-mapper multipath module what to do. The tools are: Multipath: scans the system for multipathed devices, assembles them, updates the device-mapper’s map. Multipathd: daemon which waits for maps events then executes multipath and monitors the paths. Marks a path as failed when the path becomes faulty. Depending on the failback policy, it can reactive the path. Devmap-name: provides a meaningful device name to udev for devmaps Kpartx: maps linear devmaps to device partitions to make multipath maps partionable (novell module ref) Multipath.conf: configuration file for the multipath daemon. Used to overwrite the built-in configuration table of multipathd.
Output of multipath command
Figure 1: multipath -ll output
Annotated output of multipath command
1.2. Considerations 2. Device Mapper 3. Device Mapper Mulitpathing 3.1. Multipath 3.1.1. Configuration file 3.2. devmap-name 4. Terminology 5. Linux distributions supported 6. References
References Michael, T., Kabir, R., Giles, J. & Hull, J. (2006.) Configuring Linux to Enable Multipath I/O. Retrieved from http://www.dell.com/downloads/global/power/ps3q06-20060189-Michael.pdf Goggin, E., Kergon, A., Varoqui, C., & Olien, D. (2005) Proceedings of the Linux Symposium – Linux Multipathing. Retrieved from http://www.linuxinsight.com/files/ols2005/goggin-reprint.pdf Red Hat Documentation. (n.d.) Red Hat Enterprise Linux 5, DM Multipath. Retrieved from http://linux.web.cern.ch/linux/scientific5/docs/rhel/DM_Multipath/index.html Varoqui, C. (2010.) The Linux multipath implementation. Retrieved from http://christophe.varoqui.free.fr/refbook.html
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