[CentOS] imaging a drive with dd

[Fred Smith]

On Thu, Mar 02, 2017 at 10:57:51PM -0500, Robert Moskowitz wrote:
> 
> 
> On 03/02/2017 10:02 PM, Fred Smith wrote:
> >On Thu, Mar 02, 2017 at 09:06:52PM -0500, fred roller wrote:
> >>On Thu, Mar 2, 2017 at 8:36 PM, Robert Moskowitz <rgm at htt-consult.com>
> >>wrote:
> >>
> >>>dd if=/dev/sdb of=os.img bs=1M count=3210
> >>>
> >>I would recommend bs=512 to keep the block sizes the same though not a huge
> >>diff just seems to be happier for some reason and add status=progress if
> >>you would like to monitor how it is doing.  Seems the command you have
> >>should work otherwise.
> >The dd blocksize has nothing to do with the disk sector size.
> >
> >the disk sector size is the number of bytes in a minimal read/write
> >operation (because the physical drive can't manipulate anything smaller).
> >
> >the dd blocksize is merely the number of bytes read/written in a
> >single read/write operation. (or not bytes, but K, or Kb, or other
> >depending on the options you use.)
> >
> >It makes sense for the bs option in dd to be a multiple of the actual
> >disk block/sector size, but isn't even required. if you did dd with a
> >block size of, e.g., 27, it would still work, it'd just be stupidly slow.
> 
> Kind of wondered about that.
> 
> So the blocks reported by fdisk is what I should use as the count,
> as that matches the drive's real block size?
> 
> thanks

if you're copying the entire device, you do not need to tell it how
many blocks. just use a large-ish blocksize and let 'er rip.

for a single partition, you could use the blocksize and block number
you get from fdisk. you would then need to say /dev/sda4, e.g., instead
of /dev/sda.

when copying an entire drive I tend to use 10M as the blocksize. using
a large blocksize just reduces the number of read operations that are
needed. that's why a very small blocksize could slow down the copy, as
it would require a whole lot more read operations.

-- 
---- Fred Smith -- fredex at fcshome.stoneham.ma.us -----------------------------
    "Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of
     heaven, but only he who does the will of my Father who is in heaven."
------------------------------ Matthew 7:21 (niv) -----------------------------