In a message dated 2/9/2008 5:34:12 A.M. Central Standard Time, [EMAIL PROTECTED] writes: >Mate, you have to update your disk drive knowledge a bit. It's been a long time since a single 56664 track would fit on 3.5 inch disk. ... largest track size can be estimated as ... nearly a full CKD CYL in a single track. You're right. I was assuming that one RAID track holds only one emulated 3390 track. >in most cases the backend pre-fetch can stay ahead of a single threaded sequential read. And having unlimited CCW prefetch now as an option in the LPAR helps get multiple tracks' data into central storage faster than single threading. >I think it is a long time since backups operated as single track IO. I didn't mean to imply they only read one track per I/O. I meant they read the volume sequentially. They can chain up 1,000 tracks per I/O request if they want and if they have enough real storage. But they still go through the whole volume sequentially. The first such I/O reads tracks 0-999 while maybe a second buffer is reading in tracks 1000-1999. When 0-999 are written to tape, tracks 2000-2999 start being read (if BUFNO is only 2). Etc. Multiple tracks per I/O, but still reading sequentially through the volume with only one process (task) that may be doing multiple simultaneous channel programs (as in SAME's BUFNO>1). With BUFNO=huge you would not to have multiple tasks copying different parts of the volume simultaneously. At one cylinder per BUFNO, you could get 100 such I/Os running together with a mere 150MB of real storage tied up (75 for DASD in and another 75 for tape out). That's not very much real these days. A shop with really huge volumes is also likely to have a really huge amount of central storage. Bill Fairchild Rocket Software
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