Re: backup archive format saved to disk

[Ron Johnson <ron.l.johnson@cox.net>]

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On 12/07/06 18:52, Mike McCarty wrote:
> Ron Johnson wrote:
>>>> 
[snip]
>>> One way of doing RAID 5 with three discs is to write data to
>>> each of two discs, and write the bitwise XOR of the data on
>>> the two discs to a third. This requires no special 
>>> controller, it simply requires a tiny program. Here's one in
>>> completely portable C:
>> 
>> 
>> Separate parity disk is RAID-4, not -5.
> 
> Not according to my information.
> 
> See http://www.sohoconsult.ch/raid/raid.html

http://en.wikipedia.org/wiki/RAID#RAID_4

    A RAID 4 uses block-level striping with a dedicated
    parity disk. RAID 4 looks similar to RAID 5 except
    that it does not use distributed parity,

http://en.wikipedia.org/wiki/RAID#RAID_5

    A RAID 5 uses block-level striping with parity data
    distributed across all member disks.

This is how it is implemented on the "enterprise" systems we have at
work.

What your code looks like is RAID-3.

http://en.wikipedia.org/wiki/RAID#RAID_3

    A RAID 3 uses byte-level striping with a dedicated
    parity disk. RAID 3 is very rare in practice. One
    of the side-effects of RAID 3 is that it generally
    cannot service multiple requests simultaneously. This
    comes about because any single block of data will, by
    definition, be spread across all members of the set
    and will reside in the same location. So, any I/O
    operation requires activity on every disk.

[snip]
>> What you've done is create a parity disk.  Useful, but not
>> RAID.
> 
> Pardon? Perhaps you are using a definition of RAID I'm not 
> familiar with. "Redundant Array of Inexpensive Discs" (some have
> changed that to "Independent".)
> 
> I'll admit that this is not the usual way it is implemented, but
> it is certainly RAID to my way of thinking, and conforms to the 
> definitions I've seen, and uses the same kind of redundant data.

Integral to the implementation of RAID is that the disk is divided
into large blocks, and there are parity blocks for data blocks.

- --
Ron Johnson, Jr.
Jefferson LA  USA

Is "common sense" really valid?
For example, it is "common sense" to white-power racists that
whites are superior to blacks, and that those with brown skins
are mud people.
However, that "common sense" is obviously wrong.
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