Mike Snitzer wrote:
I tracked down the thread you referenced and these posts (by you) seems to summarize things well: http://marc.theaimsgroup.com/?l=linux-raid&m=111116563016418&w=2 http://marc.theaimsgroup.com/?l=linux-raid&m=111117515400864&w=2 But for clarity's sake, could you elaborate on the negative implications of not merging the bitmaps on the secondary server? Will the previous primary's dirty blocks get dropped on the floor because the secondary (now the primary) doesn't have awareness of the previous primary's dirty blocks once it activates the raid1?
Right. At the time of the failover, there were (probably) blocks that were out of sync between the primary and secondary. Now, after you've failed over to the secondary, you've got to overwrite those blocks with data from the secondary in order to make the primary disk consistent again. This requires that either you do a full resync from secondary to primary (if you don't know what differs), or you merge the two bitmaps and resync just that data.
Also, what is the interface one should use to collect dirty bits from the primary's bitmap?
Whatever you'd like. scp the bitmap file over or collect the ranges into a file and scp that over, or something similar.
This bitmap merge can't happen until the primary's dirty bits can be collected right? Waiting for the failed server to come back to
Right. So, when the primary fails, you start the array on the secondary with a _clean_ bitmap, and just its local disk component. Now, whatever gets written while the primary is down gets put into the bitmap on the secondary. When the primary comes back up, you take the dirty bits from it and add them into the secondary's bitmap. Then, you insert the primary's disk (via nbd or similar) back into the array, and begin a resync.
That's the whole reason for this interface. We have to modify the bitmap while the array is active (modifying the bitmap while the array is down is trivial, and certainly doesn't require sysfs :).
harvest the dirty bits it has seems wrong (why failover at all?); so I must be missing something.
We fail over immediately. We wait until later to combine the bitmaps and resync the data.
Hope that helps. -- Paul - 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
