[dm-devel] dm-era: metadata reuse after reboot possible?

[Markus Hentsch]

Dear Somu,

thanks for your quick response!

I applied your patch to the 4.4 kernel of a Ubuntu 16.04.2 VM and can
now confirm that the dm-era metadata survived every subsequent reboot so
far. Thank you!


Best regards,

Markus Hentsch
Cloud&Heat Technologies


On Apr 19, 2017 at 18:18 Somasundaram Krishnasamy wrote:
> I have sent a patch recently for review.
>
> https://www.redhat.com/archives/dm-devel/2017-April/msg00138.html
>
> It should fix this issue.
>
> Regards,
> Somu.
>
> On 4/19/2017 6:32 AM, Markus Hentsch wrote:
>> Hello, it's been a while. I've also been busy with other stuff in the
>> last months but now I'm back stuck at this issue. Has there been any
>> progress on this?
>>
>> On Mo, Oct 31, 2016 at 12:37, Edward Thornber wrote:
>>> On Fri, Oct 21, 2016 at 11:29:26AM +0200, Markus Hentsch wrote:
>>>> Hello,
>>>>
>>>> is it possible for the dm-era metadata to survive reboots?
>>> No, you certainly should be able to continue using the metadata.  I
>>> don't have time to look at this for the next couple of weeks, but
>>> after that will definitely fix.
>>>
>>> - Joe
>>
>> For reference, my original message from Fri, Oct 21, 2016 was:
>>> Hello,
>>>
>>> is it possible for the dm-era metadata to survive reboots?
>>>
>>> The official dm-era documentation states under "Resilience":
>>>
>>>     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.
>>>
>>> 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?
>>>
>>>
>>> Using a Ubuntu trusty x64 VM with 2 additional virtual HDDs (sdb and
>>> sdc, 256MB each), I set up dm-era after bootup like this:
>>>
>>>     root at alice:~# dmsetup create era-meta --table "0 256 linear
>>> /dev/sdb 0"
>>>     root at alice:~# dmsetup create era --table "0 524288 era
>>> /dev/mapper/era-meta /dev/sdc 4096"
>>>     root at alice:~# dmsetup create era-access --table "0 256 linear
>>> /dev/mapper/era-meta 0"
>>>
>>>
>>> When I reboot and repeat the commands above, the layout is recreated
>>> correctly (according to lsblk). However, taking a metadata snapshot
>>> and trying to access the metadata using era_dump or
>>> era_invalidate fails, e.g.:
>>>
>>>     root at alice:~# dmsetup message era 0 take_metadata_snap
>>>     root at 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 at 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
>>>
>>>
>>> It doesn't matter if I take and drop any metadata snapshots or write
>>> to the era device before the reboot or not.
>>> 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?
>>
>> Best regards,
>>
>> Markus Hentsch
>> Cloud&Heat Technologies
>>
>>
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