Neil Brown wrote:
> On Friday June 15, [email protected] wrote:
>
> > As I understand the way
> > raid works, when you write a block to the array, it will have to read all
> > the other blocks in the stripe and recalculate the parity and write it out.
>
> Your understanding is incomplete.
> For raid5 on an array with more than 3 drive, if you attempt to write
> a single block, it will:
>
> - read the current value of the block, and the parity block.
> - "subtract" the old value of the block from the parity, and "add"
> the new value.
> - write out the new data and the new parity.
>
> If the parity was wrong before, it will still be wrong. If you then
> lose a drive, you lose your data.
I see, I didn't know that the MD's raid5 did this.
> And why is it such a big deal anyway? The initial resync doesn't stop
> you from using the array. I guess if you wanted to put an array into
> production instantly and couldn't afford any slowdown due to resync,
> then you might want to skip the initial resync.... but is that really
> likely?
When I've had an unclean shutdown on one of my systems (10x 50gb raid5) it's
always slowed the system down when booting up. Quite significantly I must
say. I wait until I can login and change the rebuild max speed to slow it
down while I'm using it. But that is another thing.
Thanks for the clarification on raid5.
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