Question on DMA

[Sunil Patil]

Hi,

I am using RHEL 5.2.

In my kernel module, I am cloning a bio, hence now there are two bio's
both pointing to the same data pages (iovec's). I issue these bio's
(write operation) on two different devices. One bio goes to say device
1 and other bio goes to device 2.
1) In this case 2 DMA transfers will happen one to device 1 and other
to device 2, source of DMA being the same (same source memory
area/pages) in both cases.
2) Other approach would be to clone bio and in addition clone iovecs
in each bio i.e. copy data into different set of pages for the second
bio, so that each DMA operation can pick data from different memory
area/pages and send it to different devices.

My question is, will both the appraoches perform equally or (2) will
perform better than (1) or vice-versa. Assume that data involved in
the bio operation is not more than 2 MB.

Thanks,
Sunil