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Everything about Table Scans including offload to Exadata, Big Data SQL, and External Tables

  • November 9, 2017

Random thoughts on block sizes

Roger Macnicol
Software Architect

I heard "Oracle only tests on 8k and doesn't really test 16k"

I heard someone assert that one reason you should only use 8k block sizes is that, and I quote, "Oracle only tests on 8k and doesn't really test 16k". I tried googling that rumour and tracked it back to AskTom. Also as disks and memory get bigger and CPUs get faster it is natural to ask if 8k is now "too small".

So here are my thoughts:

1. A quick scan of the data layer regression tests showed a very large number running on 16k blocks

2. Oracle typically runs it DW stress tests on 16k blocks

So, clearly, the assertion is untrue but I did spot some areas where 32k testing could be improved. I also added a note to AskTom clarifying testing on 16k.

Does this mean I should look at using 16k blocks for me DW

Whoa, not so fast. Just because table scans typically run slightly faster on 16k blocks and compression algorithms typically get slightly better compression on 16k blocks does not mean your application will see improvements

1. Multiple block sizes mean added work for the DBA

2. Databases do a lot more than just scan tables - e.g. row based writes and reads could perform worse

3. Most apps have low hanging fruit that will give you far better ROI on your time than worrying about block sizes (if you don't believe me, attend one of Jonathan Lewis's excellent index talks).

Most applications run fine on 8k because it is a good trade off between different access paths and, in general, 8k is still the right choice for most applications. 

What about 'N' row pieces

In general Oracle's block layout tries to ensure row pieces are split on column boundaries but in the case of very wide columns we will split in the middle of a column if too much space would be wasted by aligning with column boundaries. When a column is split in the middle it creates what is known as an 'N' row piece.

Rows are split by default at 255 column boundaries assuming the row piece fits in the block If you have a table with very wide rows or some very wide inline columns, smaller block sizes will result both in rows being split more often and in columns being split in the middle. At a minimum the number of block gets to retrieve a single row will likely increase. This is one case to consider where a 16k block size may be worth investigating. 

The curious case of 70X compression

We had case of a customer legitimately point out that ZLIB would give much better than 70X compression on his very repetitive data but COMPRESS FOR QUERY HIGH was stuck at 70X. And, as it turns out, this is a factor of block size.

Hybrid Columnar Compression (HCC) takes up to 8 MB of raw data and compresses it down to one CU (Compression Unit) subject to row limits depending on compression type. For example COMPRESS FOR ARCHIVE LOW (which is also ZLIB) is limited to 8,000 rows in a CU. But there is another limit which you may deduce from block dumps which is that HCC will only allow one head piece row per block because HCC rowids are the RDBA of the head piece plus the row offset into the CU. So if you have incredibly compressible data where 8,000 rows compress down to less 16k moving to a larger block size can be bad news because you will end up with wasted space at the end of each block.

Summary

There are typically better things to spend time on than worrying about block size but if you wish to run your DW on 16k blocks they are thoroughly tested just like 8k blocks are.

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