When a disk to disk transfer occurs and the disks reside on different host bus adapters (HBA) does the data get DMAed to from one HBA to the other HBA or does one HBA DMA data to ram while the other DMAs from ram? For performance the HBA to HBA DMA would be the fastest.

As far as my hardward knowledge goes, DMA is short for Direct Memory Access. It is used by devices like hard disks, to temporarily gain control over the system bus (normally, the CPU has control over the bus), so that they can access the memory directly for read/write operations.
In other words, DMA simply omits the CPU as "middle-man" in hard disk <-> memory transactions. The device will notify the CPU afterwards so that it can regain control over the bus.

In theory, the disk could use it's control over the bus to "contact" another device (ie another hard disk), but
this seems unlikely, for many reasons: ie different hard disk speeds, different lengths of data bursts, disk caching, 2 disks with alternating control over the bus, etc).

I'm guessing that a HBA can only DMA to system memory (ram) but that a smart OS could simultaneously setup a read from one HBA and a write to another HBA. It could be done in blocks of sectors (128?). The write would occur after the first block was read. So the transfer would be limited to the system speed unless the system speed was at least twice as fast as the slowest HBA/drive. Probably the slowest HBA/drive is the limiting factor.

thanx for the reply