This is where Multipathing or Path failover solutions can address High Availability and Business Continuity because not only do they eliminate single points of failure between the server and the storage but also help in achieving better performance by balancing the load (I/O load or LUN load) across multiple paths.
Most new servers bought today by customers connect into SANs. Furthermore, most of these servers have high availability and redundancy requirements thus are connecting to highly available, redundant fabrics and disk arrays. When any component in the data path fails, failover to the surviving data path occurs non-disruptively and automatically.
So the premise of Multipathing or path failover is to provide redundant server connections to storage and:
- Provide path failover in the event of a path failure
- Monitor I/O paths and provide alerts on critical events
Over the years, administrators have recognized this need and so after the purchase of a server, they would also purchase a 3rd party multipathing solution, typically from their storage vendor. Apart from the fact that these 3rd party solutions were not designed as part of Operating System and some did not integrate particularly well, in addition, they did not interoperate well with multipathing solutions from other storage vendors that needed to installed on the same server. In essence, storage vendor specific multipathing solutions solved one problem while creating another one. This problem has lasted for years and was addressed only recently.
Over the past 2-3 years a flurry of OS native multipathing solutions have emerged. Thus, today’s multipathing solution distribution model has changed drastically. Multipathing solutions can be distributed either as:
- 3rd party software (Symantec/Veritas DMP, PowerPath, HDLM, SDD, RDAC, SANPath etc).
- Embedded in the Operating System (Solaris MPxIO, AIX MPIO, Windows MPIO, Linux Device Mapper-Multipath, HP-UX PVLinks, VMware ESX Server, Netware via NSS).
- As an HBA vendor device driver that works with most, if not all, storage arrays (i.e Qlogic’s Linux/Netware failover driver, Windows QLDirect)
- As an HBA vendor device driver (Emulex MultiPulse) available to OEMs only who in turn incorporate the technology into their own products via calls made to the HBA APIs provided by the HBA vendor.
Increasingly, the trend is toward the deployment of OS native multipathing solutions. In fact, with the exception of one Operating System, a substantial server/storage vendor has all but abandoned support of their traditional Multipathing solution for their newer storage arrays, in favor of the OS native ones.
There are two drivers behind this trend. Cost is one reason customers elect to deploy OS native multipathing solutions. After all, you can’t beat “free”. A secondary, but equally important, driver is to achieve better interoperability among various vendors’ storage devices that happen to provision the same server(s). One driver stack and one set of HBAs talks to everybody.
From a Windows standpoint, it is important to note that Microsoft is strongly encouraging all storage vendors to support its MPIO specification. Network Appliance supports this specification with a Device Specific Module (DSM) for our disk subsystems. It’s equally important to note that Windows MPIO enables the co-existence of multiple storage vendor DSMs within the same server. In fact, the current approach is similar to what Symantec/Veritas has done over the years with the Array Support Library (ASL) that provides vendor disk subsystem attributes and multipathing information to the Symantec/Veritas Device Discovery Layer (DDL) and Dynamic Multipathing (DMP) components.
Early last year Microsoft indicated they were considering the development of a Generic DSM for Fibre Channel (Generic DSM for iSCSI already exists) that will support all storage vendors as long as they (storage vendors) comply with the SCSI Primary Commands revision 3 (SCP-3). Furthermore, Microsoft, at the time, indicated that a Generic DSM would be incorporated into the Windows Vista release.
Network Appliance’s primary multipathing approach is to support all OS native multipathing solutions, as well as, support some popular 3rd party (i.e Symantec/VxDMP) solutions across all supported Operating Systems. Depending on customer demand, certification with disk array vendor specific multipathing solutions is always a possibility, assuming the necessary Customer Support Agreements are in place.
