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I have a dd copy of a filesystem from a SSD I want to restore back to the same SSD filesystem due to a bad OS crash, putting it back into a known good state. 95% of the blocks will be the same, so to avoid wear on the SSD I don't want to have ddrescue write blocks to the SSD that are identical to the blocks in the dd copy.

Is there a tool equivalent to dd or ddrescue or command line flags to the tools that will not write identical blocks on the destination? It'll be slower since it'll do a read and optionally write, but I'm ok with the slowdown.

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    There's no way with standard tools to know what bits are written to which storage location. The controller abstracts this away. Go with Mechanical snail's suggestion.
    – user3463
    Commented Aug 6, 2012 at 7:19

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It should be possible to do this using rsync, the differential data transfer tool.

See the man page, and use the --inplace option because you want to write directly into the device file.

That said, a single write pass across the disk is a trivial amount of wear, so consider just dding it over.

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  • Thanks, I use rsync all the time, but wasn't aware that --inplace works with device files.
    – Blair Zajac
    Commented Aug 6, 2012 at 16:18
  • It's not working with rsync 3.0.9 on RHEL5:bash-3.2# $ rsync -P --inplace md0.image /dev/md0 md0.image 18883313664 2% 118.28MB/s 1:26:10 rsync: writefd_unbuffered failed to write 4 bytes to socket [sender]: Broken pipe (32) rsync: write failed on "/dev/md0": No space left on device (28) rsync error: error in file IO (code 11) at receiver.c(322) [receiver=3.0.9] rsync: connection unexpectedly closed (28 bytes received so far) [sender] rsync error: error in rsync protocol data stream (code 12) at io.c(605) [sender=3.0.9]
    – Blair Zajac
    Commented Aug 6, 2012 at 16:30
  • Turns out that even with --inplace, rsync 3.0.9 clobbers /dev/md0 first.
    – Blair Zajac
    Commented Aug 6, 2012 at 16:57
  • Is it trying to append?
    – Mechanical snail
    Commented Aug 6, 2012 at 17:28
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e2image from e2fstools can do this with the -c option, but it can't read an input image from stdin as it seems to need to seek around.

The "old" dd_rescue tool (which is not the same as GNU ddrescue) has a -W option to avoid writes and can read from stdin: https://sourceforge.net/projects/ddrescue/ You might want -a as well to skip writing blocks of zeros, depending on the situation (I'm currently looking at restoring images to LVM thin pools while preserving the CoW characteristics.)

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