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

[Markus Hentsch]

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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