There are 2 different issues here - 1. the MVS catalog that contains the definitions for DB2 utilitized datasets DB2 does not override anything just issues normal OPEN. 2. the actual DB2 datasets that contain table data and indexes. DB2 overrides everything for this datasets because it uses media manager for this IO. DB2 can and does have multiple IO active for the same dataset. It is handled by DB2 if there are multiple requests for the same block, both requestors wait for the single IO. DB2 applications wait for the completion of either async or sync IO (the DB2 terms are PREFETCH and synchronous read). The only difference is that DB2 will initiate prefetch requests as soon as the previous IO completed so the data will be available in less elapsed time.
Mike On 12/2/05, Avram Friedman <[EMAIL PROTECTED]> wrote: > > I would think asking this question on the IDUG DB2-L list would be the > best bet. However a few comments. > > 1. STRNO specified in the VSAM Catalog may not be in use. I could be > over- > ridden at open time. > 2. The number of concurrent physical I/O's to a DB2 object needs to be > limited espically for reads, this insures that only one copy of a CI is in > the pool at a time. > 3. DB2 whenever possible does async I/O that is applications expect to > find the required data in the buffers with out I/O waits. The method for > doing this is called 'PREFETCH' on the read side and 'DEFERED I/O' on the > write side. > 4. DB2 has a limited number of prefetch and write engines, usually far > fewer than the total number of objects. > > ---------------------------------------------------------------------- > For IBM-MAIN subscribe / signoff / archive access instructions, > send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO > Search the archives at http://bama.ua.edu/archives/ibm-main.html > -- Mike ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
