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From:Michael Loftis Date:September 2 2003 4:15am
Subject:Re: RAID or not?
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--On Friday, August 22, 2003 8:37 PM -0600 Jim McAtee 
<jmcatee@stripped> wrote:

> I don't quite understand the need to read data before any write.  Why
> wouldn't it just calculate the parity of whatever is being written and
> just write it to disk?  Wouldn't there be slack space, as with any disk
> system?  Write a 1 byte file and it uses an N byte block on one disk,
> plus an N byte parity block on another.

This wholly depends on the RAID subsystem, but better than 80% will need to 
either read the entire stripe, or hold off until they're writing the whole 
stripe at once.  Remember the RAID is below the filesystem layer, and 
*separate* from it, esp. in the case of a hardware controller.  Really big 
systems may (do) keep 'maps' of the space so they can cheat by not reading 
a strip when it knows it hasn't been written since (destructive) 
initialization and is thus all 0's.
RAID or not?Jackson Miller21 Aug
  • Re: RAID or not?Jon Drukman21 Aug
    • Re: RAID or not?Jackson Miller21 Aug
      • RE: RAID or not?Steven) (Lefevre21 Aug
        • Re: RAID or not?David Griffiths21 Aug
        • Re: RAID or not?Per Andreas Buer21 Aug
        • Re: RAID or not?Jon Drukman22 Aug
          • Re: RAID or not?Dan Nelson22 Aug
          • RE: RAID or not?Steven) (Lefevre22 Aug
            • RE: RAID or not?Michael Loftis2 Sep
  • Re: RAID or not?Colbey21 Aug
  • Re: RAID or not?Per Andreas Buer21 Aug
RE: RAID or not?Alec.Cawley22 Aug
  • Re: RAID or not?Jim McAtee23 Aug
    • Re: RAID or not?Michael Loftis2 Sep
RE: RAID or not?Alec.Cawley22 Aug
Re: RAID or not?Alec.Cawley26 Aug