dd on entire disk, but do not want empty portion

Solution 1:

Back in the day I ran into a similar problem with embedded Linux distributions - get rid of all the junk before compressing the image.

dd if=/dev/zero of=asdf.txt. Wait until it dies. Delete asdf.txt.

You've just written zeros to all free space on the device.

Now take a disk image and run it through gzip. Voila, sparse image.

Probably doesn't scale very well and could cause problems if you actually need to write to the disk, but hey.

You could take an rsync snapshot of the disk to another volume, zero that, and then take that disk image.

Note: Could be hazardous for SSD, user should consider this operation befor committing.

Solution 2:

Assuming you want to save /dev/sdXN to /tgtfs/image.rawand you are root:

1. mkdir /srcfs && mount /dev/sdXN /srcfs

2. Use zerofill or just:
   dd if=/dev/zero of=/srcfs/tmpzero.txt
   to fill unused blocks with zero; wait for it to fill the file system completely then:
   rm /srcfs/tmpzero.txt

3. Take the image with dd and use conv=sparse to punch zeros on-the-fly:
   dd conv=sparse if=/dev/sdxn of=/tgtfs/image.raw

If you want to use compression you don't need to punch the zeros with dd as zero blocks are highly compressible:

dd if=/dev/sdxn | gz -c | dd of=/tgtfs/image.raw

PS: You should note that it is not a good idea to this (filling the file system with zeros) on a flash memory based storage media (i.e. your source file system being a SSD) on a regular basis, as it will cause extensive write to your SSD and reduce its lifespan. (but it's alright for occasional transfer of data)

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Solution 3:

Use dd, with the count option.

In your case you were using fdisk so I will take that approach. Your "sudo fdisk -l "produced:

    Disk /dev/sda: 64.0 GB, 64023257088 bytes
    255 heads, 63 sectors/track, 7783 cylinders
    Units = cylinders of 16065 * 512 = 8225280 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disk identifier: 0x0000e4b5

    Device Boot      Start         End      Blocks   Id  System
    /dev/sda1   *           1          27      209920   83  Linux
    Partition 1 does not end on cylinder boundary.
    /dev/sda2              27         525     4000768    5  Extended
    Partition 2 does not end on cylinder boundary.
    /dev/sda5              27         353     2621440   83  Linux
    /dev/sda6             353         405      416768   83  Linux
    /dev/sda7             405         490      675840   83  Linux
    /dev/sda8             490         525      282624   83  Linux

The two things you should take note of are 1) the unit size, and 2) the "End" column. In your case you have cylinders that are equal to 8225280 Bytes. In the "End" column sda8 terminates at 525 (which is 525[units]*16065*512 = ~4.3GB)

dd can do a lot of things, such as starting after an offset, or stopping after a specific number of blocks. We will do the latter using the count option in dd. The command would appear as follows:

    sudo dd if=/dev/sda of=/your_directory/image_name.iso bs=8225280 count=526

Where -bs is the block size (it is easiest to use the unit that fdisk uses, but any unit will do so long as the count option is declared in these units), and count is the number of units we want to copy (note that we increment the count by 1 to capture the last block).

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Solution 4:

While /dev/zeroing the free-disk-space and use dd conv=sparse/gz -c is possible, on huge disks with empty space running in 100s of GBs, /dev/zeroing is painfully slow - not to mention that as other answers noted, /dev/zeroing an SDD till EOF.

Here's what I did when I ran into this situation:

• On a lubuntu live CD, used gparted to 'shrink' the disk to minimum possible size, leaving rest of the space unallocated

• Used
  dd bs=1M count=<size_in_MBs> if=/dev/sdX | gzip -c --fast| dd of=/path/to/image.gz to create the fast-compressed image (needless to say, you may want to skip the compression if you have sufficient space to store raw data (or are otherwise inclined to reduce CPU loading)

• Used
  dd if=/path/to/image.gz | gunzip -c | dd bs=1M of=/dev/sdY to copy the data back onto different disk
• Used gparted again to 'expand' the partition

I haven't tried it for multiple partitions but I believe the process above can be adapted to copy 'partitions' if partition-table on destination disk is created first and only the data contained in the partition is copied via dd - reading/writing offsets (skip/seek option of dd, respectively) would be required as appropriate.

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Solution 5:

You can't. dd is a very low level tool and it has no means of distinguishing between files and empty space.

On the other hand the empty space will compress very, very nicely so if you are only concerned about storage space, not for example write time, then just pipe it through gzip.