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DM-Clone Target Added To Linux 5.4 For Efficient Remote Replication Of A Block Device

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  • DM-Clone Target Added To Linux 5.4 For Efficient Remote Replication Of A Block Device

    Phoronix: DM-Clone Target Added To Linux 5.4 For Efficient Remote Replication Of A Block Device

    Added to the device mapper (DM) code with the Linux 5.4 kernel is an interesting addition that benefits those wanting to carry out some interesting use-cases around remote replication of block devices...

    http://www.phoronix.com/scan.php?pag...M-Clone-Target

  • #2
    So it's like a local cache of sorts?(that persists between reboots?)

    What is the write bit about, when the remote was said to be read-only? Do you write to the local cache copy and that syncs to the remote? Or the remote/origin that's being cloned is always read-only sort of like a snapshot/overlay?

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    • #3
      Originally posted by polarathene View Post
      So it's like a local cache of sorts?(that persists between reboots?)
      No it's a "clone drive and use the clone immediately", so any write goes to the new drive while most reads will come from the original image, until the process finishes and the clone is complete.

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      • #4
        Originally posted by starshipeleven View Post
        No it's a "clone drive and use the clone immediately", so any write goes to the new drive while most reads will come from the original image, until the process finishes and the clone is complete.
        Still sort of sounds like a local cache of the remote? So you make a new partition or something allocated locally to match the same size as the dm-cloned target? And rather than sequentially copying/syncing it over and waiting for that before you can use it, when performing the copy, any user I/O requests are given priority?

        And the writes only persist to that local clone, not also applied to the remote source?

        What sort of use cases does this have? What happens after the remote is cloned locally? The dm-cloned layer is still there?

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        • #5
          Originally posted by polarathene View Post
          What sort of use cases does this have? What happens after the remote is cloned locally? The dm-cloned layer is still there?
          From what I understand from the docs, this is a clone-specific feature for the dm subsystem, not a cache (there is dm-cache and dm-snapshot and dm-thin device mapper targets already, see Device Mapper documentation from Gentoo https://wiki.gentoo.org/wiki/Device-mapper ).

          So yes it's just useful for "instant cloning" from a source, all changes and when the clone is complete you can remove the dm-clone device.

          This is all backend stuff for things like LVM, mdadm or Stratis

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          • #6
            Originally posted by polarathene View Post
            Still sort of sounds like a local cache of the remote?
            it's not a cache because it doesn't propagate writes (origin is readonly)
            Originally posted by polarathene View Post
            And the writes only persist to that local clone, not also applied to the remote source?
            yes
            Originally posted by polarathene View Post
            What sort of use cases does this have?
            disaster recovery
            Originally posted by polarathene View Post
            What happens after the remote is cloned locally? The dm-cloned layer is still there?
            it can be replaced with direct mapping

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