Michael Tokarev <mjt@xxxxxxxxxx> wrote: > Luca Berra wrote: > > On Fri, Mar 18, 2005 at 02:42:55PM +0100, Lars Marowsky-Bree wrote: > > > >> The problem is for multi-nodes, both sides have their own bitmap. When a > >> split scenario occurs, and both sides begin modifying the data, that > >> bitmap needs to be merged before resync, or else we risk 'forgetting' > >> that one side dirtied a block. > > > > on a side note i am wondering what would the difference be on using this > > approach within the md driver versus DRBD? > > DRBD is more suitable for the task IMHO. Several points: > > o For md, all drives are equal, that is, for example, raid1 > code will balance reads among all the available drives a-la Not necessarily so. At least part of the FR1 patch is dedicated to timing the latencies of the disks, and choosing the fastest disk to read from. > striping, while DRBD knows one mirror is remote and hence > will not try to read from it. Well, todays GigE is fast, > but it is yet another layer between your data and the memory, > and we also have such a thing as latency. > > o We all know how md "loves" to kick off "faulty" array components > after first I/O error, don't we? DRBD allows "temporary" failures Again, the FR1 patch contains the "Robust Read" subpatch, which stops this happening. It's not a patch that intersects with the bitmap functionality at all, by the way. > of remote component, and will recover automatically when the > remote comes back. Well, so will FR1 (at least when run over ENBD, because FR1 contains a machanism that allows the disks to tell it when they have come back into their full gleam of health again). > o DRBD allows local drive to be a bit ahead compared to remote one > (configurable), while md will wait for all drives to complete a write. The FR1 patch allows asynchronous writes too. This does need the bitmap. I presume Paul's latest patches to raid1 for 2.6.11 also add that to kernel raid1. Incidentally, async writes are a little risky. We can more easily imagine a tricky recovery situation with them than without them! > There's a case which is questionable in the first place: what to > do if local part of the mirror fails? Md will happily run on > single remote component in degraded mode, while DRBD will probably > fail... Dunno which behaviour is "better" or "more correct" > (depends on the usage scenario I think). It's clearly correct to run on the remote if that's what you have. Peter - 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
