[openstack-dev] [oslo][oslo.db] MySQL Cluster support
mbayer at redhat.com
Fri Feb 3 15:34:11 UTC 2017
On 02/03/2017 10:21 AM, Doug Hellmann wrote:
> Excerpts from Mike Bayer's message of 2017-02-03 09:41:11 -0500:
>> On 02/02/2017 05:28 PM, Octave J. Orgeron wrote:
>>> That refers to the total length of the row. InnoDB has a limit of 65k
>>> and NDB is limited to 14k.
>>> A simple example would be the volumes table in Cinder where the row
>>> length goes beyond 14k. So in the IF logic block, I change columns types
>>> that are vastly oversized such as status and attach_status, which by
>>> default are 255 chars.
>> let me give you a tip on IF blocks, that they are a bit of an
>> anti-pattern. If you want a column type to do one thing in one case,
>> and another in another case, create an object that does the thing you want:
>> some_table = Table(
>> 'some_table', metadata,
>> Column('my_column', VARCHAR(255).with_variant(VARCHAR(50), 'ndb'))
> I wonder if we want to do either, though. Shouldn't we try to use
> the same (smaller) column size all the time? Otherwise we end up
> with another incompatibility between different deployments, since
> sometimes things like names might have different sizes in different
in that case you have to do a migration which as you know these days
means the "old" column remains for a whole release cycle and the
application must undergo significant complexity, either at the app level
or in triggers, to keep data between "old" and "new" columns
simultaneously. So one advantage to keeping this at the "create for
NDB" level is that we don't need to get into schema migrations.
Unless we changed the value in the application and its migration files
completely, and *didnt* migrate old applications, and just hope/ensure
that they aren't writing larger data values. Maybe that's possible
though it seems a little scary. Perhaps some kind of annotated type
like VARCHAR(50, unmigrated=255) to note what's going on.
>> I think we might want to look into creating a stub dialect called 'ndb'
>> that subclasses mysql+pymysql. Treating ndb as a whole different
>> database means there's no longer the need for a flag in oslo.db, the
>> 'ndb' name would instead be interpreted as a new backend - the main
>> thing would be ensuring all the mysql-appropriate hooks in oslo.db are
>> also emitted for ndb, but this also gives us a way to pick and choose
>> which hooks apply. It seems like there may be enough different about
>> it to separate it at this level.
>> Not sure if people on the list are seeing that we are simultaneously
>> talking about getting rid of Postgresql in the efforts to support only
>> "one database", while at the same time adding one that is in many ways a
>> new database.
> Yes, that does seem a bit ironic. That's also why I was pointing
> out that we're going to want to have people lined up to support the
> work before starting. The lack of help with Postresql testing
> resulted in removing it from the gate, and possibly to dropping
> support entirely.
> For reference, the discussion in  led to this proposed TC
> resolution .
>  http://lists.openstack.org/pipermail/openstack-dev/2017-February/thread.html#111357
>  https://review.openstack.org/427880
>> So to determine a more appropriate size, I look
>>> through the Cinder code to find where the possible options/states are
>>> for those columns. Then I cut it down to a more reasonable size. I'm
>>> very careful when I cut the size of a string column to ensure that all
>>> of the possible values can be contained.
>>> In cases where a column is extremely large for capturing the outputs of
>>> a command, I will change the type to Text or TinyText depending on the
>>> length required. A good example of this is in the agents table of
>>> Neutron where there is a column for configurations that has a string
>>> length of 4096 characters, which I change to Text. Text blobs are stored
>>> differently and do not count against the row length.
>>> I've also observed differences between Kilo, Mitaka, and tip where even
>>> for InnoDB some of these tables are getting wider than can be supported.
>>> So in the case of Cinder, some of the columns have been shifted to
>>> separate tables to fit within 65k. I've seen the same thing in Neutron.
>>> So I fully expect that some of the services that have table bloat will
>>> have to cut the lengths or break the tables up over time anyways. As
>>> that happens, it reduces the amount of work for me, which is a good thing.
>>> The most complicated database schemas to patch up are cinder, glance,
>>> neutron, and nova due to the size and complexity of their tables. Those
>>> also have a lot of churn between releases where the schema changes more
>>> often. Other services like keystone, heat, and ironic are considerably
>>> easier to work with and have well laid out tables that don't change much.
>>> On 2/2/2017 1:25 PM, Mike Bayer wrote:
>>>> On 02/02/2017 02:52 PM, Mike Bayer wrote:
>>>>> But more critically I noticed you referred to altering the names of
>>>>> columns to suit NDB. How will this be accomplished? Changing a column
>>>>> name in an openstack application is no longer trivial, because online
>>>>> upgrades must be supported for applications like Nova and Neutron. A
>>>>> column name can't just change to a new name, both columns have to exist
>>>>> and logic must be added to keep these columns synchronized.
>>>> correction, the phrase was "Row character length limits 65k -> 14k" -
>>>> does this refer to the total size of a row? I guess rows that store
>>>> JSON or tables like keystone tokens are what you had in mind here, can
>>>> you give specifics ?
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