Multi-disk filesystems

[Joe Smith]

In article <10174 at brazos.Rice.edu> mp at allegra.tempo.nj.att.com (Mark Plotnick) writes:
>Each filesystem is a full disk in size, but even these 1GB filesystems are
>inconveniently small.  I'd like to know of any products that let filesystems
>span more than one physical drive.  What we'd like to do is have 3 to 6
>disks treated as one filesystem.  Ideally, the failure of one disk would not
>necessitate restoring the others from tape.

I don't know of a software solution, but Interphase Corporation displayed
a hardware solution at the Sun Expo '90 in San Jose.  They can provide a
box that sits at the end of a SCSI bus that uses several disks to create
the illusion of one giant disk.  They have an article in "The Sun
Observer", page 60 of the July 1990 issue.  It mentions how "spanning"
allows a single partition to be larger than a full disk.

They also had a session describing the advantages of "striping", where the
host's I/O request is split into several parts, each going to a different
drive.  For instance, a SunOS request for 8K bytes could be split into 4
requests of two 1K byte blocks each, and read/written from/to 4 drives in
parallel.  Sustained thruput is 4 times that of a single disk.

Another way to go is to separate the individual bits.  Interphase
described one product has five 1GB disks but acts like a single 4GB disk.
Incoming data is split, with 1/4th of the bits going to drive 1, 1/4th to
drive 2, 1/4th to drive 3, and the rest to drive 4.  Drive 5 provides
parity for the other 4 bits.  There is enough redundancy that it can
survive the complete failure of a single disk.  As long as a replacement
disk can be hooked up and made up-to-date before a second disk fails, then
no data will be lost.

I'm sure other companies are working on similar products.  Interphase
Corporation is at 13800 Senlac, Dallas, TX 75234 (214)919-9000.  I am not
a customer, I just listened to them at Sun Expo.

Joe Smith (408)922-6220 | SMTP: jms at tardis.tymnet.com or jms at gemini.tymnet.com
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