I wanted to find out what effect changing the block size (bs=)
option of the dd command would have on partition copy speeds.
I also wanted to confirm that the default block size (if no bs
option was specified) was indeed 512 bytes as someone had said.

1.) I used dd to completely fill up a 75MB partition with data 
from a larger partition.

  /dev/hda1 = Debian 3.0 root, ext3
  /dev/hda3 = an empty, unformatted partition
  command used = "dd if=/dev/hda1 of=/dev/hda3"
  (Note: the above command stops copying and aborts once it has
  used up all the available space on /dev/hda3.)

2.) I then copied this 75MB partition to an identical partition 
on an identical drive using various block sizes and timing the

  time dd if=/dev/hda3 of=/dev/hdb3 bs=xx


  no bs=                78s     144584+0 records
  bs=512                78s     144584+0 records
  bs=1k         38s     72292+0 records
  bs=2k         38s     36146+0 records
  bs=4k         38s     18073+0 records
  bs=5k         39s     14458+1 records
  bs=50k                38s     1445+1 records
  bs=500k               39s     144+1 records
  bs=512k               39s     144+1 records
  bs=1M         39s     72+1 records
  bs=5M         39s     14+1 records
  bs=10M                39s     7+1 records


1. The default block size if no bs= option is specified is
512 bytes.

2. Any block size larger than the default (512 bytes) will 
double the copy speed, but using larger block sizes will not 
result in proportionately greater speed increases.

3. The output of dd shows the number of blocks (records) copied 
plus (+) the number of partial blocks copied. From the above
results I recalled that my hda1 partition was initialized using
a 4k block size (which was the default block size in cfdisk).

4. The dd man page should be updated to include this basic 
operational information (conclusions 1 thru 3) so that people
don't have to run their own tests to figure out how to use it.

5. The dd code should *probably* be updated so that 4k is the 
default block size rather than 512 bytes (since this seems to
be the default block size on modern hard disks and it results
in doubling of the copy speed.

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