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

[Markus Hentsch]

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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