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Hello,<br>
<br>
is it possible for the dm-era metadata to survive reboots?<br>
<br>
The official dm-era documentation states under "Resilience":<br>
<blockquote>
<pre>Metadata is updated on disk before a write to a previously unwritten
block is performed. As such dm-era should not be effected by a hard
crash such as power failure.
</pre>
</blockquote>
That's why I initially assumed that I may continue (re)using the
metadata after reboots without wiping it (i.e. resetting the eras
and block tracking). But is that actually the case?<br>
<br>
<br>
Using a Ubuntu trusty x64 VM with 2 additional virtual HDDs (<tt>sdb</tt>
and <tt>sdc</tt>, 256MB each), I set up dm-era after bootup like
this:<br>
<blockquote>
<pre>root@alice:~# dmsetup create era-meta --table "0 256 linear /dev/sdb 0"
root@alice:~# dmsetup create era --table "0 524288 era /dev/mapper/era-meta /dev/sdc 4096"
root@alice:~# dmsetup create era-access --table "0 256 linear /dev/mapper/era-meta 0"</pre>
</blockquote>
<br>
When I reboot and repeat the commands above, the layout is recreated
correctly (according to <tt>lsblk</tt>). However, taking a metadata
snapshot and trying to access the metadata using <tt>era_dump</tt>
or <tt>era_invalidate</tt> fails, e.g.:<br>
<blockquote>
<pre>root@alice:~# dmsetup message era 0 take_metadata_snap
root@alice:~# era_dump /dev/mapper/era-access
<superblock uuid="" block_size="4096" nr_blocks="128" current_era="3">
metadata contains errors (run era_check for details).
perhaps you wanted to run with --repair ?
root@alice:~# era_check /dev/mapper/era-access
examining superblock
missing eras from writeset tree
value size mismatch: expected 12, but got 8. This is not the btree you are looking for.
era_check: /usr/include/boost/optional/optional.hpp:1004: boost::optional<T>::reference_const_type
boost::optional<T>::get() const [with T = unsigned int; boost::optional<T>::reference_const_type =
const unsigned int&]: Assertion `this->is_initialized()' failed.
Aborted
</pre>
</blockquote>
<br>
It doesn't matter if I take and drop any metadata snapshots or write
to the era device before the reboot or not.<br>
Do I have to start over and wipe the metadata device directly after
reboot or is there some way to continue using it, i.e. recovering
its state when recreating the previous dmsetup layout?<br>
<br>
<br>
Best regards,<br>
<br>
Markus Hentsch<br>
Cloud&Heat Technologies<br>
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