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Re: More on vinum woes

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Tue, 13 Sep 2005 23:37:10 -0700 (PDT)

:Given some experience here, I'd say that software RAID is far, far 
:better than pseudo-RAID.  I've had a Promise card do some horrible 
:things to a RAID-1 array...
:Never had a problem with software RAID (either Linux or Vinum).  This is 
:on personal workstations, though, and not a server supporting a 
:workgroup or anything enterprise class.  Of course, in an enterprise 
:environment (or any how-many-9's pissing match :-), you should be 
:considering nothing less than a SCSI RAID setup.

    I don't agree re: SCSI RAID.  It used to be true that SCSI was 
    superior not just in the reliability of the bus protocol but also
    in the actual hardware.  I remember back in the day when seagate 
    waxed poetic about all the work they did to make their SCSI drives
    more robust, and I gladly paid for SCSI drives.  But the hardware
    differences have steadily eroded over time (1) to the point now where
    there isn't any significant difference as far as I can tell, and
    the vendors charge such ridiculous premiums for SCSI drives over SATA
    that it's just throwing money down the drain.  SATA is clearly just
    as reliable a bus protocol.

    note 1: the erosion is due to consumer demand for more robustness,
    like running-shock protection, and vendor demand for more robustness,
    such as for use in laptops and DVRs.  Most of these technologies do
    not require any actual hardware to implement, or very little, and that
    being the case drive vendors have migrated them out of their specialty
    lines and into their mainstream brands.  Also, modern drives have far
    fewer moving parts and far smaller (and fewer) heads, and its hard
    to differentiate the robustness for commercial vs consumer models
    by using (e.g.) more robust materials because of that.

    Software raid is a fine solution as long as your computer doesn't
    crash and as long as you have the extra cpu and bandwidth to spare.
    i.e. it can handle drive faults just fine, but it isn't so good handling
    operating system faults or power failures due to the lack of
    battery-backed cache, and it costs a lot of extra cpu to do something
    like RAID-5 in software.   So if you intend to run your system full-out
    a hardware raid solution is what you want not so much because it is
    higher performing on its own, but because it offloads the system's
    main bus and cpu from having to perform bandwidth-intensive multiple 
    I/O's to support the RAID configuration.

					Matthew Dillon 

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