--On Friday, August 22, 2003 8:37 PM -0600 Jim McAtee <[EMAIL PROTECTED]> 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.
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