[mythtv-users] Full or Partial Database restore?

John Haywood john at jhit.com.au
Tue Sep 14 18:29:15 UTC 2010


On 15/09/2010, at 4:05 AM, Michael T. Dean wrote:
> On 09/14/2010 01:41 PM, John Haywood wrote:
>> On 15/09/2010, at 3:18 AM, Michael T. Dean wrote:
>>> On 09/14/2010 09:04 AM, John Haywood wrote:
>>>> After much instability going from Mythbuntu 9.10 to 10.04, even trying a database backup, clean install and restore the inevitable finally happened:
>>>> 0.23.1 fixes, PPA repo
>>>> 
>>>> I was experiencing several freezes and lockups, usually one or two per day.
>>>> 
>>>> Finally the whole box just started up to a setup screen - and refused to update the schema from<blank>  to 1025 (i.e. no value to current)
>>>> I ran the db optimise script to check, then the mysqlcheck, as per the wiki
>>>> mysqlcheck found problems which it repaired
>>>> 
>>>> I then thought I'd check the xfs filesystem on which the recordings are stored and found quite a few orphaned files, which it placed in Lost + Found
>>>> 
>>>> I'm now faced with reinstalling the OS, and restoring the db from a backup. I'd like to keep my database of recordings, videos and schedules if possible, but if I have to lose the rest, I can live with setting the combined fe/be up again
>>>> 
>>>> Should I try a full restore from a couple of days before the final crash first - or am I just prolonging the agony given the previous issues?
>>>> 
>>>> Or should I just do a partial restore from the last backup
>>> 
>>> There's really no benefit to doing a partial restore if you have a good backup of a non-corrupt schema.
>> 
>> This I understand, and thanks for the advice. What i'm concerned about is the quality of the backup,  and hence the restore I guess ....
> 
> Yeah, that question is a bit more difficult to answer.
> 
> Based on the symptoms you've described, the most-likely problem you were encountering was that your settings table (where the current DB schema version is stored) was corrupt.  In this case, optimize_mythdb.pl will be unable to run, and mythtv-setup and mythbackend would both start up and think that no database exists and try to update from <blank> to current DB schema version.
> 
> If that's the case, the most-likely worst-case result is that you lost a single setting value from the DB when settings was repaired.  That setting value will be replaced with its default value the next time you run the program that uses the setting.  Therefore, if the above is true (and is the /only/ problem), a backup from the repaired DB is usable (and, IMHO, you should use the full restore).
> 
> However, it's also possible that you had other crashed tables, too.  If that's the case, you may have lost more data.  Even so, the worst-case scenario is likely that you lost a row from each of those tables.  And, if any of those tables is in the list of tables restored with a partial restore, you lost the row whether you do a full or partial restore.  And, if the tables aren't in the list of tables restored by a partial restore, then you still have to re-create the lost data--as well as all the data you're throwing away with a partial restore--so, the partial restore really doesn't do anything other than force you to re-create data now (rather than when you find it's missing--i.e. doing http://www.gossamer-threads.com/lists/mythtv/users/264034#264034 when you find out one or more capture card isn't working; or going into mythtv-setup and fixing Storage Groups when you find out one or more directories is missing; or running mythcommflag --rebuild on a recording when you find it isn't seeking properly; or ...).
> 
> Note, also, that you can do a full restore, now, run for a couple weeks, and then change your mind (and do a partial restore from the new system's backup).  If you do a partial restore, now, you can't change your mind--you'll lose all the recording information from the point of the backup until you change your mind.
> 
> It's completely up to you, but if it were me, I'd do a full restore, and if things don't work, then I'd likely fix the issue(s), as above.  If a bunch of things don't work--and I fix a few of them over a period of a few weeks--and I stop trusting my data, then I'd consider a partial restore.  That said, I doubt that you'll have any problems with a full restore (it will likely "just work").
> 
> FWIW, I used to do partial restores all the time to "clean up" my DB.  Then I started doing a lot of work with the DB-related code in MythTV and with the database itself, and I found that partial restores are almost always a lot of work with no benefit.
> 
> Mike
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Mike
This is exactly the direction towards which I was leaning, now its just up to me whether I put my trust in the distro which gave me grief...
...And you have laid it out extremely logically

thanks as aways for your patience and advice

John Haywood

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