Inline LOBs (Large Objects)

Transcription

1 Inline LOBs (Large Objects) Jeffrey Berger Senior Software Engineer DB2 Performance Evaluation

2 Disclaimer/Trademarks THE INFORMATION CONTAINED IN THIS DOCUMENT HAS NOT BEEN SUBMITTED TO ANY FORMAL IBM TEST AND IS DISTRIBUTED AS IS. THE USE OF THIS INFORMATION OR THE IMPLEMENTATION OF ANY OF THESE TECHNIQUES IS A CUSTOMER RESPONSIBILITY AND DEPENDS ON THE CUSTOMER S ABILITY TO EVALUATE AND INTEGRATE THEM INTO THE CUSTOMER S OPERATIONAL ENVIRONMENT. WHILE IBM MAY HAVE REVIEWED EACH ITEM FOR ACCURACY IN A SPECIFIC SITUATION, THERE IS NO GUARANTEE THAT THE SAME OR SIMILAR RESULTS WILL BE OBTAINED ELSEWHERE. ANYONE ATTEMPTING TO ADAPT THESE TECHNIQUES TO THEIR OWN ENVIRONMENTS DO SO AT THEIR OWN RISK. ANY PERFORMANCE DATA CONTAINED IN THIS DOCUMENT WERE DETERMINED IN VARIOUS CONTROLLED LABORATORY ENVIRONMENTS AND ARE FOR REFERENCE PURPOSES ONLY. CUSTOMERS SHOULD NOT ADAPT THESE PERFORMANCE NUMBERS TO THEIR OWN ENVIRONMENTS AS SYSTEM PERFORMANCE STANDARDS. THE RESULTS THAT MAY BE OBTAINED IN OTHER OPERATING ENVIRONMENTS MAY VARY SIGNIFICANTLY. USERS OF THIS DOCUMENT SHOULD VERIFY THE APPLICABLE DATA FOR THEIR SPECIFIC ENVIRONMENT. Trademarks IBM, the IBM logo, and ibm.com are trademarks or registered trademarks of International Business Machines Corp., registered in many jurisdictions worldwide. Other product and service names might be trademarks of IBM or other companies. A current list of IBM trademarks is available on the Web at Copyright and trademark information at 2

3 Inline LOBs Allows LOBs to be stored with the base table row Depending on the page length and LOB length it may be stored fully or partially in the base row CREATE TABLE. clobcol CLOB(x) INLINE LENGTH(y) Maximum inline LOB length is bytes, following the same rules as VARCHAR Requires DB2 10 new function mode Requires Universal Table Space (UTS) and Reordered Row Format (RRF) If a LOB cannot be fully inline, the LOB is split. The first y bytes are stored inline and the remaining bytes are stored in the LOB table space 3

4 Advantages of inline LOBs Save disk space Use fewer synchronous I/Os to access data Use less CPU time to access LOB CPU time is within 10% of VARCHAR for simple SQL, but much less than out-of-line LOBs Prefetch I/Os can read multiple LOBs in a single I/O Improve FETCH CONTINUE when scanning rows Can use index on expression for LOB data 4

5 Potential Disadvantages of inline LOBs If most LOBs will be split, inline LOBs may hurt performance If the LOBs are accessed rarely, inline LOBs may hurt performance If the LOBs don t compress well, having to compress or decompress them adds CPU time Buffer hit ratio for base table may decrease Image copies of the base table space will become larger If LOBs weren t otherwise logged, inlining could cause more logging 5

6 Migration considerations If your existing table space is non-uts, you may need 3 Alter/Reorg steps to convert existing LOBs to inline LOBs 1. Alter/Reorg to UTS 2. Alter/Reorg to increase the page size of the base table space 3. Alter/Reorg to convert existing LOBs to inline This step is not necessary to inline new LOBs 6

7 Why use a LOB for a small column instead of VARCHAR? For at least one row, the column is simply too big to fit in a single page A column is large but infrequently accessed, while other columns are frequently accessed Separation avoids polluting the base table space, possibly maximizing the buffer hit ratio and improving the performance of table scans LOB table spaces can use different attributes (such as logging) from the base table space 7

8 LOB Size Distribution Know your LOB size distribution See DB2 10 for z/os Performance Topics (Chapter: Queries for LOB size distribution) for an SQL statement that computes a histogram of the LOB size distribution If the LOB column will fit entirely in a page of the base table 90% of the time, then inline it 8

9 Inline LOB Tuning Considerations Need to pick an inline quantity and possibly a new page size for your base table space Compression considerations You need to know how well the LOBs will compress in order to know what the row size will be after you inline them No tools are provided to predict how well an inline LOB will compress You may need to do some Alter/Reorg iterations (or copy the data to a temp table) to determine the compressed row size with inlined LOBs Be prepared to retune your buffer pools Are you willing to pollute your base table space with split LOBs? Are you willing to invest the time that it will take to retune your buffer pools? Either choose a different buffer pool for the base table space or resize the buffer pool 9

10 Rules of thumb If fewer than 50% of the LOBs can be fully inlined, then don t inline the LOB If less than 10% of the LOBs will be split Table space pollution is not a concern If the table space is likely to be buffer hostile, then use a large page size to avoid accessing the LOB table space as much as possible If the table space is not buffer hostile.? Increase to the next larger page size if doing so will reduce the number of split LOBs by at least 10% Increase from 4K to 8K to 16K to 32K, as needed 10

11 Cumulative LOB size distribution 100 Small LOBs Percent Case 1 Case 2 Case 3 Case LOB Size Nearly all of the LOBs are less than 32K Conclusion: Use inlining with a 32K page size for all 4 cases 11

12 Cumulative LOB size distribution 100 Table space pollution Percent Case 5 Case 6 Case 7 Case LOB Size If a 32K page size were used, case 8 is polluted by 40% of the LOBs, case 7 by 30%, and so on. If a 4K page size were used, case 8 is polluted by 51% of the LOBs, case 7 by 61%, and so on. Conclusion: use inlining but inline at most 4K 12

13 Cumulative LOB size distribution Large LOBs Percent Case 9 Case 10 Case LOB Size Rule of thumb: If increasing the page size reduces split LOBs by at least 10%, then increase to the next larger page size. Conclusions: Case 9: Choose 32K Case 10: Choose 16K Case 11: Choose 8K 13

14 Splitting LOBs Splitting LOBs is generally bad, but there are two exceptions When you want to enable index on expression SUBSTR is the only function allowed in the index expression This allows a query to do an index search for a text string within the LOB data. When you want to enable faster FETCH CONTINUE for LOB searches In these cases, the goal is not to avoid access to the aux table when you need to fetch the entire LOB Use the smallest inline LOB quantity that enables the index creation or to enable FETCH CONTINUE 14

15 Summary Inline LOBs in DB2 10 can have clear advantages Match the proper base table page size to your LOB length More analysis is needed if more than 50% of the LOBs will be split Review the Rules of Thumb and scenarios and apply them to your environment 15