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HP P6000 Enterprise Virtual Array Systems - Basic Explanation of Proxy Read I/O and Host LUN Latency Performance Issues
The HP P6000 Enterprise Virtual Array models are classified as active-active SAN Storage Array types. This does not include the EVA3000/EVA5000 with VCS FW version 3.110 or earlier which are active-passive SAN Array types.
Active/active storage arrays can also be sub-divided into Symmetrical or Asymmetric types:
The P9000/XP SAN Storage Array is a Symmetrical Active-Active type Storage Array. The I/O request can be issued over all paths and every controller in the array can accept and send I/O to the LUN.
The P6000/EVA is a dual controller Asymmetric Active-Active Storage Array and is Asymmetric Logical Unit Access (ALUA) compliant for LUN access/failover and I/O processing.
In an Asymmetric Active-Active Array both controllers are online and both can accept I/O, but only one controller is assigned as the owning controller of the LUN. The owning controller can issue I/O commands directly to the LUN, which is called the "optimized" path. The non-owning controller or proxy controller can accept I/O commands, but cannot communicate with the LUN. This is called a "non-optimized" path. If a read request reaches the array through the proxy controller, it will be forwarded to the LUN's owning controller via the mirror port. This behavior is called a Proxy Read.
Read I/O requests received by the proxy controller (1) are sent to the owning controller (2), which retrieves the data from disk (3), caches the read data (4) and mirrors the data to the cache of the proxy controller (5). The proxy controller satisfies the host read request (6), making this process transparent to the host. Proxy reads add latency to the I/O request and increases the mirror port utilization.
Write I/O commands to the proxy controller have less performance impact because all writes are mirrored in both controllers (mirrored write cache), but the owning controller is still responsible for flushing the data to disk.
The controller mirror ports are used for cache writes and proxy reads. If the host’s multipathing configuration is set up correctly (ALUA compliant or non-compliant), then the mirror ports will only have to handle the write mirroring.
NOTE: When using ALUA and ALB (Windows Adaptive Load Balance), it is important to balance LUN ownership between controllers. It is best practice on most Operating Systems to set the preferred path on each Vdisks to failover/failback. This information is referenced in the HP P6300/P6500 Enterprise Virtual Array User Guide.
Other ALUA and ALB compliant Multipath I/O reference documents:
Windows 2008: HP MPIO Full Featured DSM for EVA4x00/ 6x00/8x00 family of Disk Arrays 4.01.00 installation and reference guide.
Native Linux (RHEL and SUSE) Device-Mapper (DM) Multipath: Native Linux Multipath Disk Arrays Device-Mapper for HP StorageWorks reference guide.
VMWare (ESX 4.x and 5) Native MultiPathing (NMP): HP Enterprise Virtual Array Family with VMware Vsphere 4.0, 4.1 and 5.0 Configuration Best Practices guide.