[dm-devel] Usage of dm-era

[Markus Hentsch]

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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