[mythtv-users] Disable Automatic Database Backup

Michael T. Dean mtdean at thirdcontact.com
Wed Nov 11 20:39:04 UTC 2009

On 11/11/2009 02:25 PM, Eloy Paris wrote:
> On Tue, Nov 10, 2009 at 08:30:10PM -0500, Michael T. Dean wrote:
> [...]
>> Sorry, I haven't gotten that far, yet.  We need a /lot/ more
>> infrastructure to prevent automatic backups from corrupting the DB
>> or recordings.  It's on my TODO--just might be below a much bigger
>> job that will actually make the auto-backups /much/ easier.  :)
> Hhmmm. Dumb question: why would a backup corrupt the database or
> recordings?

The recordings could easily be corrupted when the system becomes I/O or 
even CPU bound, depending on system resources and capture configuration.

The database corruption is mainly possible when making Myth perform 
automatic maintenance--which is required to make sure the backups are 
actually useful.  If we tell users we do automatic backups, then when a 
user needs a backup, she finds out that some of the tables were crashed 
when the backup(s) was(were) created, so those tables and their data is 
not in the backup, it's actually worse than telling users to maintain 
and backup their databases themselves.

For both of these issues, we need an approach to make sure that Myth 
isn't doing anything (recording, playing back, transcoding, commercial 
flagging, ...) at the point when we do the backup.  And, to make things 
work properly, we really need to ensure that no one (/none/ of the 
frontends/backends/other clients (like MythWeb)) is changing the data in 
the database when the backup is created.

>  I mean, a backup just reads data, right? Or that's not the
> case here? Is the issue that a backup cannot be made while there are
> clients (backends, frontends) connected to the database?

That could easily cause corruption of the data in the backup, itself, so 
another reason we need the infrastructure discussed above.

FWIW, you should be fine creating a cron job to back up your database 
daily (or whatever).  Just do your best to pick a time when recordings 
are least likely to occur.  If you're running optimize_mythdb.pl in a 
cron job, you may want to just add mythconverg_backup.pl right after 
it.  However, we're currently leaving it up to the user to decide 
whether or not to do so.



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