From: utzoo!decvax!wivax!dyer Newsgroups: net.unix-wizards Title: Re: disk block interleave methods Article-I.D.: wivax.5431 Posted: Sun Feb 6 11:41:05 1983 Received: Mon Feb 7 00:39:59 1983 References: tekmdp.1754 The key lies in the meaning of "interleave". With most UNIX systems, physical sectors are contiguous at the device driver level. This assumes that multi-sector I/O can be performed by the hardware, given only a starting disk address and a count. Non-file system operations (swapping, raw I/O) are accomplished in this way. If the underlying disk hardware cannot support multi-sector I/O, then it is common to implement a device-driver level interleaving scheme to minimize latency. But, most mini-type systems don't require this. However, it's another matter when performing I/O within the UNIX file system abstraction. Remember that logically contiguous blocks are not physically contiguous on the disk. Each disk block requires a separate I/O request. The so-called "interleaving" that mkfs performs is actually a sort on the list of free blocks, so that a program reading or writing a file sequentially will not suffer unnecessary latency between logical I/O requests. So, you see, this "interleaving" is quite orthogonal to device-driver interleaving. As I alluded before, the actual interleaving factors are crucially dependent on rotational speed, seek time, and overall throughput of your disk subsystem. As delivered for V7, they're set up for a PDP11-70/RP03. Figures for other processor/ hardware combinations have been published in ";login"; I don't have them on-hand. Maybe others can submit them. We had to discover them empirically for our C/60 and C/70 systems. Steve Dyer