On Sun, Aug 12, 2007 at 07:03:44PM +0200, Jan Engelhardt wrote: > > On Aug 12 2007 09:39, david@xxxxxxx wrote: > > > > now, I am not an expert on either option, but three are a couple things that I > > would question about the DRDB+MD option > > > > 1. when the remote machine is down, how does MD deal with it for reads and > > writes? > > I suppose it kicks the drive and you'd have to re-add it by hand unless done by > a cronjob. >From my tests, since NBD doesn't have a timeout option, MD hangs in the write to that mirror indefinitely, somewhat like when dealing with a broken IDE driver/chipset/disk. > > 2. MD over local drive will alternate reads between mirrors (or so I've been > > told), doing so over the network is wrong. > > Certainly. In which case you set "write_mostly" (or even write_only, not sure > of its name) on the raid component that is nbd. > > > 3. when writing, will MD wait for the network I/O to get the data saved on the > > backup before returning from the syscall? or can it sync the data out lazily > > Can't answer this one - ask Neil :) MD has the write-mostly/write-behind options - which help in this case but only up to a certain amount. In my experience DRBD wins hands-down over MD+NBD because of MD doesn't know (or handle) a component that never returns from a write, which is quite different from returning with an error. Furthermore, DRBD was designed to handle transient errors in the connection to the peer due to its network-oriented design, whereas MD is mostly designed with local or at least high-reliability disks (where disk can be SAN, SCSI, etc.) and a failure is not normal for MD. Thus the need for manual reconnect in MD case and the automated handling of reconnects in case of DRBD. I'm just a happy user of both MD over local disks and DRBD for networked raid. regards, iustin - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
