2008/12/1 Takahiro Yasui <tyasui@xxxxxxxxxx>: > Phillip Susi wrote: [...] >> Right... and when recording a log on every disk in the mirror, each copy >> of the log may not contain exactly the same information at any given >> time. If you write to one disk in the mirror first, then you need to >> mark the region as dirty on that disk first, so that if the system >> crashes before you can copy the data to the other mirror, you can see >> that the first disk is more up to date than the second disk. >> >> In other words, knowing that a region is or is not synchronized across >> each disk is not enough; if they are out of sync you need to figure out >> which disk has the most current information so it can be replicated to >> the others, don't you? >> >> Or do you just always write to the first disk first, and assume it has >> the most recent data if the region was marked as dirty in ANY of the logs? > > log disks are updated in parallel and we do not know which disk has the > latest and correct data if the system crashes during write operations > on log devices. But there is no problem about it. > > There are two cases we need to think about. > > 1) Some log devices contain "clean", but mirror devices are not synchronized > > This case is problematic, but never happens, because data is written on > mirror devices after marking log devices "dirty", and make it "clean" > after write I/Os on mirror devices completed and mirrors get synchronized. > > 2) Some log devices contain "dirty", but mirror devices are synchronized > > This case may happen but is not problematic. Just data replication of > the region among mirror devices will be done when the mirror is resumed. > This case would also happen on the system with the current single log if > the system crashes after marking a log device "dirty" and before marking > it back to "clean". What happens if some log devices contain "dirty" and not all mirrors were written yet before a crash? How do you know which mirror has the most recent data? Are the writes to mirrors ordered somehow? Best Regards, Michal Miroslaw -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
