Why is SQL Azure and Index Fragmentation a Bad Combination?
I’ve been thinking through and experimenting a bit more with some of the concepts in SQL Azure – specifically I was considering the impact of fragmentation on both the storage (in terms of the storage limit) as well as the maintenance. This is not a new issue, DBA’s face fragmentation regularly and can deal with it in a variety of ways, but with SQL Azure the problem looks magnified by a lack of tools and working space. Whilst looking into this, I then realised that there is an unfortunate consequence of not knowing how much data space your index is actually using.
Each table in SQL Azure has to have a clustered index if data is going to be inserted into it and clustered indexes can suffer from fragmentation if chosen poorly. The combination of SQL Azure and the time-honoured fragmentation provides three consequences about it, fragmentation:
- will occur and you have no way in which to measure it due to the lack of DMV support.
- will create wasted space within your space allocation limit.
- will reduce your performance.
You could work it out if you knew how much space you had actually used vs. what the size of the data held is, but we are unable to measure either of those values. If you have chosen the data compression option on the index then even those values would not give you a fragmentation ratio.
This leaves us with a situation in which you can not know how much you are fragmented, meaning:
- You schedule a regular index rebuild.
- Hope SQL Azure performs index rebuilds for you.
I’m not aware of SQL Azure doing this for you – and you do not have SQL Agent facilities either.
So this seems very wrong, the concept of SQL Azure is to take away a lot of the implementation details and hassle from the subscriber – DR and failover is handled etc. But there looks to be as gap in which certain items such as fragmentation is falling – I have not seen any documentation saying SQL Azure handles it (but there could be some hidden somewhere and I hope there is!) and neither are you given the right tools in which to program and handle it yourself.
What happens when you hit that size limit?
Msg 40544, Level 20, State 5, Line 1 The database has reached its size quota. Partition or delete data, drop indexes, or consult the documentation for possible resolutions. Code: 524289
That took a lot of time to get to, (SQL Azure is not fast), but was generated using a simple example that would also demonstrate fragmentation.
Create Table fragtest ( id uniqueidentifier primary key clustered, padding char(3000) )
Very simple stuff, deliberately using a clustered key on a GUID to cause a decent level of fragmentation, and also using the padding fixed with character field to ensure 2 rows per page only, maximising the page splits.
insert into fragtest values (newid(), replicate('a',1000)) go 200000
Because of the randomness of the newid() function, the level of fragmentation is not predictable but will certainly occur – in my test I hit the wall on 196,403 records and failed with an out of space message.
Given the 2 rows per page and the number of rows, with ~0% fragmentation the data should be able ~767Mb – that is considerably short of 1 Gb – so there is a significant level of fragmentation in there wasting space, about 23% of it. If you include the 2k per page being wasted by the awkward row size then the actual raw data stored is roughly ~60% of the overall size allowing for row overheads etc.
So there are two important points from this contrived example:
- You can lose significant space from bad design.
- Doing this backs you into a corner that you will not be able to get out of – this is the worst part.
How are you cornered? well, try work out how to get out of the situation and defrag the clustered index / free up the space, you could:
- Attempt an index rebuild.
- Try to rebuild it with SORT_IN_TEMP.
- Drop the index.
- Delete data.
The first three fail, the SORT_IN_TEMP is not supported and would not of rescued the situation either since you have no working space in which to write the newly sorted rows prior to removing the old ones. So do you really want to delete data? I don’t think we can consider that an option for now.
This all seems like a ‘rock’ and a ‘hard place'; whilst SQL Azure can support these data quantities, it seems prudent that you never consider actually going close to them at all – and that you equally are going to find it difficult to understand if you are close to them, since there is no way of measuring the fragmentation. The alternative is that you manually rebuild indexes on a regular basis to control fragmentation, but then enough free space is going to have to be left to allow you to rebuild your largest index without running out of space – reducing your data capacity significantly.
The corner is not entirely closed off, the way out of the corner would be to create another SQL Azure database within my account and select the data from database1.fragtest to database2.fragtest and then drop the original table and transfer it back – not ideal but it would work in an emergency.
I think the key is to design to make sure you do not have to face this issue; keep your data quantities very much under the SQL Azure size limits, and watch for the potential of tables being larger than the remaining space and preventing an re-indexing from occurring.
Interested to know your thoughts on this one, and what other consequences of being close to the limit will come out.