[OSM-talk] Database Maintenance

Lester Caine lester at lsces.co.uk
Mon Jul 30 08:52:07 BST 2007


Tom Hughes wrote:
>> I thought you had said we'd be able to read data? If I try to download
>> the area I surveyed today into JOSM, I get a "Database offline for
>> maintenance" message.
> 
> I turned off the reads at lunchtime today because (a) the disks on
> the database server were so busy that the reads were taking forever
> so there was a hug queue and (b) I was hoping that reducing the reads
> would speed up the schema changes.

Since this sort of tidy up will be necessary again no doubt
MySQL is simply not ready for heavyweight use ;)

Then the procedure SHOULD be -
Make the on-line server read only.
Run a backup of the on-line data to a separate machine.
Run the updates off line.
Then switch the updated data to the on line machine.
Switch back to read/write.

The on line machine could not cope simply because of the load, and the updated 
data can be tested prior to the 'live' update. Since the time taken for the 
second machine was known already, that time was never going to be improved on 
and as has been found, any other approach takes a lot longer :(

Of cause what would help would be proper replication to multiple sites. If 
implemented properly this would also allow structural changes to be made via 
the replication process.

-- 
Lester Caine - G8HFL
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