Neil Brown wrote: > On Friday June 15, wakko@xxxxxxxxxxxx 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. -- Lab tests show that use of micro$oft causes cancer in lab animals Got Gas??? - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
