the concept of active/active array

[yu xiao]

from VMware compatability guide, i see the some HP storage like EVA4400 is a active/active mode, and some is active/passive mode. what's the difference?

[Uwe Zessin]

An active/passive array is less complicated to implement. A 'passive' path presents SCSI LUNs, but read/write I/O is not possible through them. The storage array responds with a "NOT READY" message. Think of them as a 'standby' - if the active path fails, the server selects a standby and tells the storage array (e.g. an A/P EVA3000 or EVA5000 to transfer management ownership to the other controller).

[yu xiao]

but what's is active/active mode, how can HP ensure the data integrity if a LUN can be accessed from both controller simultaneously?

[Uwe Zessin]

One controller is the owner of a virtual disk at a time and manages all disk access. If the host sends an I/O request through the other controller it is re-routed to the owning controller.
If there are to many I/Os going through the non-owning controller, the EVA automatically does an ownership change to improve the performance.

[yu xiao]

you answer is very clear. but how about the driver on the host re-route I/O between HBAs, but i don't see any advantage of A/A mode over A/P mode. for a specific LUN, the controller can't do load-balancing, and the reroute will increase the processing time compare to the I/O go directly to the owner controller.

[Uwe Zessin · Accepted solution]

Historically, any active/passive implementation has been vendor-specific ('proprietary'). That means each vendor has to provides his own multipath solution: Compaq/HP: Secure Path, EMC: PowerPath, ?: ?) which needs to tap into the operating system and needs to be maintained.

For some years now, we have an industry standard for active/active multipath failover. This means that a lot of things needs to developed only once.

Microsoft, for example, provides an MPIO framework for Windows and the array vendors only supply some array-specific 'knowledge' in form of a DSM (Device Specific Module).

Some operating systems like NetWare, HP-UX, Sub Solaris(?) work 'out-of-the-box' with an active-active EVA because the operating system includes an MPIO. HP no longer needs to implement and maintain its own multipath solution - this cuts down costs(!), especially for platforms with a lower volume.

[yu xiao]

i got it. cost and standard is the key!