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From: ron@topaz.rutgers.edu (Ron Natalie)
Newsgroups: comp.unix.wizards
Subject: Re: Disk striping? (4.3 BSD)
Message-ID: <16829@topaz.rutgers.edu>
Date: Fri, 4-Dec-87 12:22:38 EST
Article-I.D.: topaz.16829
Posted: Fri Dec  4 12:22:38 1987
Date-Received: Wed, 9-Dec-87 04:44:47 EST
References: <2369@emory.uucp>
Organization: Rutgers Univ., New Brunswick, N.J.
Lines: 38

First, striping is not defined precisely as you indicate.

There are several ways one might use multiple drives on a UNIX
system.

1.  First method (currently in use).  Chop your filesystem into
    part and place them on separate drives.  Provided you have
    a controller that allows you to do overlapped seeks you can
    arrange for better access times by care placement of filesystems.
    Note that / and /tmp are the heaviest usage filesystems on most
    systems.  If you have multiple controllers, then you can actually
    do two transfers at a time, providing for better througput.

2.  Second method, span file system accross multiple physical devices.
    This was done in some Unixes, especially on disk drives like RS04's
    whose size is prohibitedly small.  Of course, this isn't that hot
    of an idea as it guarantees that only one drive will be active at
    a time.

3.  Stripe the drives.  Interleave the filesystems between two drives
    so that you can transfer data requests from both drives at once.
    This only beats out method #1 if you have the following:
	1.  A controller capabale of transferring to/from each drive
	    simoultaneously (or doing the same with multiple controllers).
	2.  A controller with "zero latency" feature.  Most controllers
	    even when the I/O is a full track (or a sustantial fraction
	    of it) wait until the beginning of the track comes under the
	    head before starting the transfer.  Using these controllers
	    causes you to head towards the maximum latency as you must
	    wait for the maximum latency on each drives which will be
	    statistically higher than the single drive feature.  Newer
	    controllers will start transferring as soon as they realize
	    that there is valid data under the head that is part of the
	    request.

-Ron