Usage of dm-era

Markus Hentsch <markus.hentsch@xxxxxxxxxxxxxxxx> · Thu, 22 Sep 2016 16:10:18 +0200



    Dear dm-devel community,

    
    I'm trying to get familiar with dm-era but I'm stuck.

    
    What I'm trying to achieve is to have a dm-era device which keeps
    track of changed blocks on a seperate hard drive or logical
    volume/mapping in order to create a live backup solution.

    
    The documentation on dm-era is very sparse. So far I found a Gentoo
    wiki entry, a blog entry by Rackcorp and the official kernel doc.

    I tried to come up with a basic setup using a virtual machine
    running Ubuntu 16.04.

    
    So far I was able to create a setup that looks like this: http://i.imgur.com/LNCPima.png

    To set this up, I came up with the following script: http://pastebin.com/Y3B2m7wL

    It is inspired by the setup done by Rackcorp [1]. The resulting
    'lsblk' looks like this:

    sdb                       8:16   0  256M  0 disk 
    `-metadev-era           252:2    0  276K  0 dm   
      |-era                 252:4    0  276K  0 dm   
      `-era-access          252:5    0  276K  0 dm   
    sdc                       8:32   0  256M  0 disk 
    `-metadev-data          252:3    0  256M  0 dm   
      |-era                 252:4    0  276K  0 dm   
      `-metadev-data-access 252:6    0  256M  0 dm   /media/sdc-data
    

    In comparison to Rackcorp's setup, I did the following adjustment:

    
      i) mounting /dev/mapper/metadev-data (or the underlying sdc)
        or ii) using it as an origin device for era, seemed to be
        mutually exclusive
      that's why I created an additional linear mapping called
        'metadev-data-access' inspired by the 'era-access' device to be
        able to mount and write to the sdc device again
    
    Basically this setup is supposed to track changed blocks on /dev/sdc
    and uses a mounted /dev/mapper/metadev-data-access at
    /media/sdc-data to access /dev/sdc on top of the dm setup.

    
    However, when reading the era metadata via 'era_dump
    /dev/mapper/era-access', I only get this result:

    <superblock uuid="" block_size="4096" nr_blocks="1" current_era="1">
      <era_array>
        <era block="0" era="0"/>
      </era_array>
    </superblock>
    

    Why is there only one block listed? Shouldn't that list exactly as
    much blocks as my 'metadev-data' device has, since it is the origin
    for the era target?

    Or is there some basic misunderstanding in my interpretation of how
    dm-era is supposed to work and be used?

    
    Please be so kind and shed some light on this!

    
    [1] http://blog.rackcorp.com/2016/03/dm-era-device-for-backups/

    
    Best Regards,

    
    Markus Hentsch

    Cloud&Heat Technologies

    
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