You have to be careful with the maximum number of disks per virtual adapter on VIO clients.
Use the following formula to evaluate this maximum number of disks per adapter.

 

Maximum disks per adapter = ( 512 – 2 ) / ( 3 + queue depth ).

 

Because virtual SCSI connections operate at memory speed, there is generally no performance gain from adding multiple adapters between a Virtual I/O Server and client.

For AIX virtual I/O client partitions, each adapter pair can handle up to 85 virtual devices with the default queue depth of 3.

Where the number of virtual devices per adapter are expected to exceed that number, or where the queue depth on some devices might be increased above the default, reserve additional adapter slots for the virtual I/O server and client partitions.

When tuning queue depths, the vSCSI adapters have a fixed queue depth of 512 command elements per adapter. The adapter itself uses 2 of these, 3 are reserved for each vSCSI LUN for error recovery and the rest are used for I/O requests.

So, with the default queue depth of 3 for vSCSI LUNs, that allows for up to 85 LUNs to use an adapter: (512 - 2) / (3 + 3) = 85 rounding down. If you need higher queue depths for the devices, then the number of LUNs per adapter is reduced.

Some examples:

A queue depth of 8 allows 510 / 11 = 46 LUNs per adapter.
A queue depth of 16 allows 510 / 19 = 26 LUNs per adapter.
A queue depth of 25 allows 510 / 28 = 18 LUNs per adapter.
A queue depth of 32 allows 510 / 35 = 14 LUNs per adapter.
A queue depth of 64 allows 510 / 67 = 7 LUNs per adapter.