Am 11.08.2018 um 11:20 schrieb Danil Kipnis: > In the attachment you can find a script that compares md5 sum of a > _file_ on top of ext4 on top of raid on each leg after running fio. > You will see that after a couple of minutes the file itself where fio > is writing to, becomes different on both legs, because ext4 enforced > consistency of its metadata, but not data inside files. > > What we see is that consistency of raid1 is lower as that of a single > disk, as per definition of raid1. Raid1 itself can't prevent writes to > be reordered inside its legs/disks. The only way for an application to > enforce ordering is wait for each write to return, journaling, > barriers, etc. Am I right? i still don't get it: DIRECT_IO and whatever - on the md-layer i see no single valid reason that there is a difference on disk as long there was no hard crash on the machine i also must not matter if ext4 has writeback enabled becahsue ext4 rites something to the blockdevice below or not and anything above is the md-layer which is the only one knowing how much disks are pyhiscally involved or to say it short: it must not matter if DIRECT_IO or some filesystem optimizations are in place - the block-layer reperesented by mdraid is one layer below all of that
