Hal, What operating system are you using? The z/OS I use reads multiple blocks per start sub-channel so that blocks are pre-fetched into buffers ahead of the program request.
For sequential caching algorithms, in HDS anyway, having two jobs reading the same dataset does not mean the trailing job will get cache hits. That implies that sequential IO is "walking" the cache - this is very bad. Sequential reads are assigned a pre-fetch area in cache that is re-used by the sequential pre-fetch algorithm. This prevents cache walking, but it also means that most sequential read processes are doing their own pre-fetch reads from disk. Even if they are reading from the same volume or dataset. There are some exceptions to this when there are two sequences of sequential reads in the same general area, but this is an extraordinary situation. For chained writes, a similar explanation except it is the parity generation and destage process that are optimized when sequential writes are detected. Note that detection can be through a bit setting in the channel command, or access pattern detection. I don't think you are really doing sequential IO one block at a time, except perhaps for VSAM using default BUFSP. Ron > -----Original Message----- > From: IBM Mainframe Discussion List [mailto:[EMAIL PROTECTED] On > Behalf Of Hal Merritt > Sent: Wednesday, April 09, 2008 9:00 AM > To: [email protected] > Subject: Re: [IBM-MAIN] Sequential Data Striping > > I'm confused. If a program is reading or writing a dataset > sequentially, > then it does so one I/O (block) at a time. > > If another program is concurrently reading, then it will be doing I/O's > more or less concurrently and the likelihood of cache hits rise. > > If you are writing, you write only one block at a time. With 'fast > write', the operation ends once the block is in the device cache. What > happens physically is not that relevant. > > ---------------------------------------------------------------------- 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
