On Monday March 14, lmb@xxxxxxx wrote: > On 2005-03-14T21:22:57, Neil Brown <neilb@xxxxxxxxxxxxxxx> wrote: > > > > Hi there, just a question about how the bitmap stuff works with > > > 1++-redundancy, say RAID1 with 2 mirrors, or RAID6. > > I assume you mean RAID1 with 3 drives (there isn't really one main > > drive and all the others are mirrors - all drives are nearly equal). > > Yeah, that's what I meant. > > (BTW, if they are all equal, how to you figure out where to sync > from? It arbitrarily chooses one. It doesn't matter which. The code currently happens to choose the first, but this is not a significant choice. > Isn't the "first" one also the first one to receive the writes, so > unless it's somehow identified as bad, it's the one which will have the > "best" data?) Data is written to all drives in parallel (the request to the first might be launched slightly before the second, but the difference is insignificant compared to the time it takes for the write to complete). There is no such thing as "best" data. Consider the situation where you want to make a transactional update to a file that requires writing two block. If the system dies while writing the first, the "before" data is better. If it dies while writing the second, the "after" data is better. > > > We haven't put any significant work into bitmap intent logging for > > levels other than raid1, so some of the answer may be pure theory. > > OK. > > (Though in particular for raid5 with the expensive parity and raid6 with > the even more expensive parity this seems desireable.) Yes. We will get there. We just aren't there yet so I cannot say with confidence how it will work. > > I think each disk needs to have it's own bitmap in the long run. On > start, we need to merge them. I think any scheme that involved multiple bitmaps would be introducing too much complexity. Certainly your examples sound very far fetched (as I think you admitted yourself). But I always try to be open to new ideas. NeilBrown - 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
