Using DMA in read/write, setting block size for I/O

[Karthik Sarangan <karthiks@cdac.in>]

In my application, I have the following code.

int main(void)
{
    char *pcBuffer;

    posix_memalign((void **) pcBuffer, 512, 262144);
    int ifd = open("/dev/sdb", O_DIRECT | O_RDWR);
    long lLen;

    lLen = read(ifd, pcBuffer, 262144);
   
    close(ifd);
    return 0;
}

Will the underlying block device read a single 256KB block from the hdd 
into pcBuffer
or will it read 256KB as a set of smaller blocks?

Since the buffer is memory aligned will it enable DMA?

scsi disk driver is adaptec aic79xx.o
distro is RedHat Enterprise Linux WS 4 (kernel-2.6.9-11)