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This page discusses some of the problems with DB2 performance on a z/OS mainframe, from a Storage perspective. It is split into
Unix and Linux use system memory cache to speed up IO operations. A well tuned system cache can service up to 85% of IO requests from memory without having to go out to disk. At first sight, this seems to be a good way to improve application performance, but it might not be best for DB2 databases. If you are having problems with DB2 performance it is worth taking a look at how system buffering is working. There are two kinds of system cache, Cached IO and Concurrent IO.
Data access to disk tends to be localised and repeated, so it is possible to keep frequently accessed data in memory buffers and retain it there. Then when an application requests some data from disk, the system first checks the cache to see if it has the data already. If the data exists in memory cache, it can be retrieved from there, so avoiding a time consuming disk access. If the data is not in cache, then the system goes out to disk for the data. This process is often called 'Cached I/O'. If the I/Os are sequential, then the cache can 'read ahead' and pull the data off disk before it is needed.
Cached I/O also works for writes. Applicationa can write data to the memory cache and continue processing once that write is complete, while the I/O is destaged asynchronously to disk later. Data intergrity is maintained by an Inode locking mechanism, which imposes write serialisation to ensure the writes are sequenced correctly.
The problem is that DB2 has its own caching buffer pools, so if it is used in conjuction with Cached I/O, the data is cached twice and passed between the two cache areas. This has an increased CPU and memory overhead and can actually degrade performance. There is another overhead with Write I/Os, because DB2 has its own locking mechanisms to maintain data consistency and integrity.
Direct I/O bypasses cache buffers and copies data directly from disk to the appropriate application buffers. However, direct I/O preserves data integrity by using write serialisation and Inode locking, which conflicts with DB2 locking. Concurrent I/O works just like direct I/O, but without the locking mechanism. Vendors often advise that database performance can be improved by placing data on raw disks without file systems, to eliminate the file system processes. The problem then is that the raw disks are difficult to manage. Concurrent I/O lets you use the management facilities of file systems, without the performance issues caused by caching and Inode locking so that multiple threads can read and write concurrently to the same file.
The problem is that you must have some way of enforcing data consistency between reads and attempts at concurrent writes, so Concurrent I/O is only suitable for applications that have their own methods for enforcing data consistency and data integrity. DB2, like most relational database systems, uses its own application cache, and has sophisticated locking mechanisms to manage data access and consistency. Therefore Concurrent I/O is very suitable for DB2 databases.
Concurrent I/O can be implemented at the disk level by mounting file systems with the -o cio option specified. However it can also be implemeted within DB2 (for versions 9.1 or higher) at database level, if a tablespace has the NO FILESYSTEM CACHING option applied. This is a better way to do it, as then the DB2 Database Manager will decide which files are to be opened with Concurrent I/O.
Some DB2 files work better with file system caching, for example, temporary table spaces that use SMS storage or a table containing LOB data that is not stored inline and is queried frequently.
The issue is that you will need to ask your DBAs how they have defined their tablespaces, whereas you can check the status of file system mount points using the 'mount' command.
This section is intended to give Storage people a flavour of how DB2 buffering works. As such, it is too simplistic for DBAs, who will need a lot more detail. In particular, note how the buffers introduce asynchronous IO. This buffers up IO and processes them in bulk, so they appear as long response times. This can appear to be a performance problem, but in fact this is very efficient. The bottom line is that the prefetch buffering gets data into the CEC memory before the customer needs it. That makes the applications run a lot faster
DB2 uses four kinds of buffer processing
The amount of data transferred in a single IO depends on the buffer pool size and the page size, and will be between 2 and 32 pages. DB2 has several thresholds which control buffering. Two of them are Data Management (DMTH) and Sequential Prefetch (SPTH). If SPTH is exceeded, then prefetch will be cancelled. This is usually a symptom that the bufferpools are too small. Ask your DBAs the check out their DB2 PM reports to see if this is happening. A good benchmark to aim for is to make the buffer pool big enough to hold randomly read pages for five minutes.
It is probable that once a page is prefetched into a buffer, then it is unlikely to be accessed again by a random request. This means that it is best to keep random pages in the buffer for longer than prefetched pages. Prefetched page buffer usage is controlled by the VPSEQT parameter, which defaults to 80%. This means that up to 80 percent of the virtual pool can be used to cache prefetched pages. In general, a value of 20% to 30% is enough to hold prefetched pages for a few seconds, as this is plenty of time for the pages to be used before they are swapped out, and it prevents the buffer pool from being swamped by prefetched pages. You can calculate prefetch page rates from standard buffer pool statistics records. If you are using a hiperpool, HPSEQT should be set to zero so that no prefetched pages are moved to the hiperpool, but VPSEQT may need to be higher than 20-30%. If the buffer pool just contains the work database DSNDB07, VPSEQT should be set to 90% or more, as nearly all of the read activity is prefetch.
Remember that bigger is not necessarily better. If the buffer pools are too big, the z/OS system may start paging, which will adversely affect response times.
DB2 does asynchronous writes unless it needs to free up buffer space, or if is doing a Checkpoint, log switch or system shutdown. The Immediate Write Threshold (IWTH) is set to 97.5% of buffer pool space. if the IWTH is exceeded, DB2 does Synchronous writes and the application waits until they are complete. DB2 will now do a write for every table update until the buffer pool usage drops again. This means that if the IWTH is exceeded, write IOs will be excessive.
Asynchronous writes processes 4-32 pages at once, which is up to 128K. If the buffer pools are large, then DB2 does a lot of write IO at checkpoint time, and this can cause problems. IBM recommends that databases are tuned at table space level. Busy table spaces should be kept in the buffer pool, while table spaces with light usage should not be kept, to reduce the amount of write IO at checkpoint time. Also, make sure the LOGLOAD parameter is not set too high, as this determines frequency of checkpoints.
Just to complicate matters a little more, log handling is also controlled within the application program. If a write is specified as NO WAIT then the log record is written to buffers. If FORCE is specified, then the record is committed, and buffer flushed. In the latter case, the application waits until disk write complete, in the former, the log records write out asynchronously when buffer thresholds are met. Ideally, keep the log buffers as big as possible, and keep the WRITE THRESHOLD at less than 20% of the no. of buffers.
A good place to start, if you have DB2 performance problems, is to collect SMS records. Three SMF types specifically apply to DB2. The SMF section describes how to collect SMF data, and also details general SMF Storage records.
SMF 100 records have minimal overhead, and in general, all classes can be turned on. They contain DB2 system-wide information statistics.
SMF 101 records contain individual DB2 thread-related information. They are a good place to start to investigate DB2 performance. You will usually need sub types 1, 2, and 3, and also 7 and 8 if you're would like DB2 package level information.
SMF 102 records contain detailed performance-related information. These records collect lots of data, use lots of resource, and can seriously affect DB2 performance. Use with caution. It could be worth investing in a product like Spectrum SMF Writer that reads and simplifies SMF 102 records.