Multipathing is a host-based software that provides automated data path management and load-balancing capabilities for heterogeneous server, network, and storage deployed in physical and virtual environments.
- SUN Alternate Pathing
- HP PVlinks
- Native MPIO(IBM,AIX, Linux,Solaris).
- Windows MPIO
- Vmware Native Multipathing plugin
SUN Alternate Pathing:
Alternate pathing is a technology provided with redundant access to disk drives. It is used in SAN environments. It uses the ability of the SUN server in which disk devices with two controller connections, allow the redundant routes to the disk devices. If one controller fails the other controller continues without interruption to applications. This gives path failover feature.
It just gives a failover solution only. It doesn’t offer load balancing capabilities. A plink is simply a different path to the same disk that goes through a different controller and a different fiber. This provides redundancy in the case of HBA card failure.
Native MPIO (IBM, AIX, Linux, Solaris):
Each OS vendors developed its own multipathing software called native with path failover and load balancing capabilities. When it comes to specific storage vendors it may miss the features relevant to the storage vendor. As on DELL EMC developed powerpath different policies like Adaptive, CLAR_opt,sym_opt,rr … specific to their make of storage array. The advantage to native is it can upgrade automatically which is pushed through OS-level upgrade or patching.
Provides both path failover and load balancing capabilities but subjected to default policy called round-robin. A round-robin distributes I/O to all datapaths in sequence. failed paths are skipped but the clogged I/O are not skipped. It considers all paths have the same priority as long as there are active paths. Having load balancing capabilities improve performance but limited to only a round-robin algorithm.
Provides both path failover and load balancing capabilities to all storage vendors. The DPM policy is not specific to storage array its universal to all arrays.
Commands used to scan for the devices in the Host.
# vxdctl -f enable
# vxdisk -f scandisks
# vxdisk scandisks new
# vxdisk scandisks fabric
Vmware Native Multipathing plugin:
Vmware ESXi provides a multipathing module called Native Multipathing Plug-In (NMP).
VMware NMP supports all storage arrays listed on the VMware storage HCL and provides a default path selection algorithm based on the array type. The NMP associates a set of physical paths with a specific storage device or LUN.
For more specific multipathing operations, NMP uses submodules, called SATPs and PSPs.
- Manages to claiming and unclaiming the physical path.
- Logical registrations and de-registrations.
- Associates logical devices to physical paths.
- Facilitates the identification and remediation of pathway failures.
- Logical devices process I / O requests:
– Selects the optimum physical path for the request.
– Carries out necessary actions to handle path failures and retries of I / O commands.
- Supports administration tasks, such as resetting logical devices.
The key features of PowerPath,
- Multiple paths for higher availability and performance
- Automatic path failover
- Dynamic multipath load balancing
- Proactive path testing and automatic path recovery
- Online path configuration and management
- High-availability cluster support
OS vendors provide native multipathing options in the form of multipath I/O management offerings (MPIO) embedded in the OS. Because these are created by the OS vendor as distinct frameworks or products, each OS has its own particular version of MPIO. OS Native MPIO solutions may not offer such features as tunable performance, optimal data path utilization, or automated load balancing and failover
The VMware Native Multipathing Plugin provides the capability of Static Round-Robin and there are no dynamic network, I/O size, traffic and queue depth or storage array considerations used in determining storage paths use
- Powerpath Multipathing (Designed for Linux, AIX, HP-UX, Solaris, and Windows)
- Powerpath/VE (Designed for VMware vSphere and Windows Hyper-V)
Wide Range Powerpath Policies:
The current PowerPath load balancing and failover policies include ad (Adaptive), co (CLARiiON optimization), so (Symmetrix optimization), rr (Round Robin), li (Least I/Os), lb(Least Blocks), re (Request), nr (No Redirect), si (streamIO), and bf (Basic failover).
Adaptive: Adaptive (ad) policy is the default policy with a PowerPath license for Hitachi, HP, Invista, Netapp, VPLEX, Dell SC, and XtremIO storage systems.
CLARiiON optimization: CLAR_opt (co) is the default policy with a PowerPath license for Unity, VNX, and CLARiiON storage systems.
Symmetrix Optimization: Symm_opt (so) policy is available for the Symmetrix storage classes and is the default policy with a PowerPath license for Symmetrix storage systems.
Round Robin: In Round Robin (rr) policy, I/O requests are assigned to each available path in rotation. It can be used for all arrays that PowerPath supports. This policy is available for all the storage classes and is not a default policy on any storage system.
Least I/Os: Least I/Os (li) policy load balance is based on the number of pending I/Os. I/O requests are routed to the path with the fewest queued requests, regardless of total block volume. This policy is set for all the storage arrays supported by PowerPath and is not a default policy on any storage system.
Least Blocks: Least Blocks (lb) policy load balancing is based on the number of blocks in pending I/Os. I/O requests are routed to the path with the fewest queued blocks, regardless of the number of requests involved. This policy can be set on all storage arrays that PowerPath supports. This is not a default policy on any storage system.
Request: Request (re) uses the path that would have been used if PowerPath had not been installed. For pseudo devices, it uses one arbitrary path for all I/O. For all devices, path failover is in effect. However, load balancing is not in effect. This policy is not a default policy on any storage system.
No Redirect: No Redirect (nr) policy has neither load balancing nor failover in effect. This policy is available for ess, Hitachi, hpxp storage classes and this policy can be set only on the third-party arrays when PowerPath is unlicensed.
StreamIO: For each, I/O to a volume, streamed IO policy uses the same path which was used for the previous I/O to the volume. This policy is available for all the storage systems and is not a default policy on any storage system.
Basic Failover: Basic failover (bf) policy has no load balancing in effect. I/O routing on failure is limited to one host bus adapter and one port on each storage system interface. This policy is required for a non-disruptive upgrade. It protects against VNX and CLARiiON SP failures, Symmetrix FA port failures, and back-end failures, but does not protect against HBA failures. This policy is the default policy without a PowerPath license for EMC storage arrays.
The physical path between a server and the storage device, which supports it, can sometimes fail in storage networking. Other than being a useful tool of failover, multipathing can assist with load balancing, spreading I / O over several paths to reduce latency.