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)
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