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- Newsgroups: comp.sys.ibm.pc.hardware
- Path: sparky!uunet!spool.mu.edu!agate!boulder!kinglear!drew
- From: drew@kinglear.cs.colorado.edu (Drew Eckhardt)
- Subject: Re: IDE, RLL, SCSI, EDSI, MFM, SCSI-2 ?
- Message-ID: <1992Nov8.113759.25562@colorado.edu>
- Sender: news@colorado.edu (The Daily Planet)
- Nntp-Posting-Host: kinglear.cs.colorado.edu
- Organization: University of Colorado at Boulder
- References: <92308.202512U35334@uicvm.uic.edu> <2227@sdrc.COM>
- Date: Sun, 8 Nov 1992 11:37:59 GMT
- Lines: 66
-
- In article <2227@sdrc.COM> scjones@thor.sdrc.com (Larry Jones) writes:
- >In article <92308.202512U35334@uicvm.uic.edu>, U35334@uicvm.uic.edu (Juan A. Varela F.) writes:
- >> I have been reading in this group a lot about the different types of hard
- >> disks. What is the difference among all these? Which one is technologically
- >> superior? Why? How are they compatible with each other? I basically
- >> understand nothing about hard drives and any information would be greatly
- >> appreciated.
- >
- >
- >Advantages and Disadvantages
- >----------------------------
- >Recording technologies are easily summed up -- higher densities give
- >you higher capacity and speed and somewhat lower reliability because of
- >the tighter tolerances required. As long as the drive is designed for
- >use with the recording technology, you shouldn't have any problems.
-
- Today's high density disks are much more reliable than yesterdays
- low-density disks.
-
- >Drives with integrated controllers increase reliability since the
- >low-level signals from the disk don't have nearly as far to go. Thus,
- >a high density drive with an integral controller should be as reliable
- >as a lower density drive with a separate controller.
- >
- >Drives with integral controllers are more expensive than drives without
- >(for obvious reasons), which can be important if you're buying more than
- >one. ESDI is a nice compromise here since some of the controller logic
- >is shared. SCSI is nice if you want to support lots of devices (SCSI
- >allows up to 7 devices on the bus, the other interfaces support only two
- >disks) or a number of different devices.
-
- Drives without integral controllers limit the number of sectors per
- track to whatever the standard says - ie 17 or 26 sectors per track with
- MFM / RLL. Double the capacity of a drive conforming to one of these
- standards means doubleing the platers (expensive) or accuracy of the
- headpositioning mechanism (expensive). Double the density, using
- tighter timing, and you've douled capacity without a significant
- increase in cost (to the manufacturer). So, since the higher-density
- drive uses fewer expensive mechanical components to achieve the
- same capacity, it can be cheaper.
-
- >Performance is very difficult to generalize. It depends as much on
- >what you are doing and what kind of software is driving the hardware
- >as it does on the actual hardware. As a very rough rule of thumb,
- >ST506 interface drives are slowest, IDE, SCSI, and ESDI fastest. For
- >multiple disks on a multitasking operating system, nothing beats a
- >top-of-the-line bus mastering SCSI controller.
-
- The command overhead on some SCSI devices can be significant.
- If your operating system isn't caching, and doing reads/writes
- with a reasonable number of sectors, you'll suffer. IDE lacks
- this command overhead, and all things equal, is probably
- better in a single disk system in terms of both performance and
- price.
-
- However, with IDE / MFM / RLL / ESDI drives, you can only talk to
- one at a time. In a multidisk system, when one disk is seeking,
- you can't touch the other disk. SCSI devices will disconnect from the
- bus when they're seeking, etc, so that other devices can use it. This
- will let you keep a number of disks busy at the same time...
-
- --
- Microsoft is responsible for propogating the evils it calls DOS and Windows,
- IBM for AIX (appropriately called Aches by those having to administer it), but
- marketing's sins don't come close to those of legal departments.
- Boycott AT&T for their absurd anti-BSDI lawsuit.
-