I am writing this article to understand the basic functionality of Multipath I/O. This article is targeted at Beginners of SAN technology.
Why we need multipath I/O?
In early days all storage solutions were SCSI based and directly attached to the host. When Fibre channel was introduced it enabled us to provide storage access to multiple hosts from a single centralized storage system. But those days all storage systems were Active/Passive arrays so communication happens in one path.
Modern day storage systems are Symmetric and Asymmetric active active arrays. Now we need to understand the difference between these two in order to understand MPIO
Lets consider a dual controller/node storage system and a LUN is provisioned to host. Lets assume the LUN is owned by controller A and there is two active paths between host and array. Lets assume host is accessing this LUN via controller B. Since the LUN is owned by controller A it alone will get exclusive access to write or read data from the LUN. Controller B will communicate with other controller via its internal link to get the IO served.
If its an Symmetric Active/Active array whoever is the owner of LUN any controller/nodes in the storage system will be able to read/write data on the LUN regardless of ownership.
Multipath IO is a feature will will effectively manage the active paths to storage array. There are various path selection polices(PSP) which can be set for individual LUN. Following are the commonly used PSPs
- Most Recently Used (MRU)
- Round Robin (RR)
Round Robin is the most commonly used path selection policy for various storage arrays due to its dynamic load sharing capability. In ESXi environments RR is preferred to achieve good performance.
Similarly the host software which is required to configure this has various names on different OS Platforms. Microsoft calls it as MPIO, in Linux world this host software is called as Device Mapper. Vmware goes by the name Native multipathing plug-in (NMP)
For optimum performance it is recommended to use device specific DSM files, here device meaning the storage array. Storage vendors will release DSM files for easier configuration and it will instruct the host is how to access the LUN using a set of predefined rules.
Apart from this, there is also wide range of software solutions to manage multipath for LUNs in a larger scale using softwares like EMC Powerpath, HP Secure Path, Hitachi HighCommand Dynamic Link Manager etc.
The best practice is to implement Multipath as recommended by the storage vendor. Selecting the appropriate storage specific PSP will lead to optimum performance of LUN.
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