Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!asuvax!ncar!mephisto!mcnc!rti!bcw From: bcw@rti.UUCP (Bruce Wright) Newsgroups: comp.sys.ibm.pc Subject: Re: Speeding up disk accesses - Food for thought. Summary: Disks and partitions Message-ID: <3474@rti.UUCP> Date: 19 Jan 90 16:56:59 GMT References: <361@marvin.moncam.co.uk> <1316@maytag.waterloo.edu> Organization: Research Triangle Institute, RTP, NC Lines: 39 In article <1316@maytag.waterloo.edu>, dmurdoch@watstat.waterloo.edu (Duncan Murdoch) writes: > In article <25B49A6D.23757@maccs.dcss.mcmaster.ca> cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn) writes: > > Yup, all the administrative stuff (except for subdirectories) is > >at the front of a DOS disk - the FAT and the root directory, primarily. > >This is why it makes sense on DOS to have the most important files at the > >front of the disk. > > No, all the administrative stuff is at the front of a DOS _partition_. It > sounds (from the original post, about putting it in the middle of the disk) > as though it would be worthwhile setting up the most heavily used partition > about halfway through a disk, so that its FAT ends up in the middle tracks. It really doesn't make much difference whether you talk about putting the most heavily used partition in the middle of the disk, or the administrative overhead within each partition into the middle of each partition. Both techniques will tend to reduce head movement time; in fact, a strong argument could be made that they are essentially the _same_ technique, just employed at different levels of the disk structure. An optimal file placement strategy is difficult and unique to each user; however in the absence of other information, placing things like the administrative overhead for each partition in the middle of the partition means that the heads have to move at most 1/2 the width of the partition in order to get from the directory/FAT area to the file being opened/closed/ whatever; the expected distance will usually be significantly shorter (and will depend on file placement and frequency of reference). Likewise, if a program was using files in the most-heavily-used partition, and then opened a file in another partition, the expected head movement would usually be smaller if the most-heavily-used partition were in the middle of the disk. Most high-end operating systems use techniques like this, to squeeze as much as possible out of the disks. The effect is somewhat incremental compared to defragmenting a disk drive or careful design of the on-disk data structures, but it is quite real and is a fairly easy way of increasing the performance of just about _any_ file system design. For what it's worth OS/2's new HPFS (High Performance File System) uses such techniques, amoung others, to achieve its performance. Bruce C. Wright