[CentOS] imaging a drive with dd

Fri Mar 3 13:34:25 UTC 2017
Robert Moskowitz <rgm at htt-consult.com>

On 03/03/2017 07:50 AM, Fred Smith wrote:
> 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.
Well, I only wanted to copy the used part of the drive which I try to 
keep small so I can still copy the image to an mSD card if I wish.  So I 
have to supply the amount of the drive to copy.  The bs=512 went fast 
enough, but then I was only copying 3.2GB.

thanks for the help.