Purpose: 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.
Procedure: 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 process. time dd if=/dev/hda3 of=/dev/hdb3 bs=xx Results: 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 Conclusions: 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. Dexter Graphic _______________________________________________ Eug-LUG mailing list [EMAIL PROTECTED] http://mailman.efn.org/cgi-bin/listinfo/eug-lug
