On Sat, 8 Oct 2011 14:03:10 -0400 Wakko Warner <wakko@xxxxxxxxxxxx> wrote: > A few days ago, I thought about creating raid arrays w/o syncing. I > understand why sync is needed. Please correct me if I'm wrong in any of my > statements. > > Currently, if someone uses large disks (1tb or larger), the initial sync can > take a long time and until it has completed, the array isn't fully > protected. I noted on a raid1 of a pair of 1tb disks took hours to complete > when there was no activity. > > Here is my thought. There is already a bitmap to indicate which blocks are > dirty. Thus by using that, a drop of a disk (accidental or intentional), a > resync only syncs those blocks that the bitmap knows were dirtied. > > What if another bitmap could be utilized. This would be an "in use" bitmap. > The purpose of this could be that there would never be an initial sync. > When data is written to an area that has not been synced, a sync will happen > of that region. Once the sync is complete, that region will be marked as > synced in the bitmap. Only the parts that have been written to will be > synced. The other data is of no consequence. As with the current bitmap, > this would have to be asked for. > > Lets say someone has been using this array for some time and a disk dropped > out and had to be replaced. Lets also say that the actual usage was about > 25-30% of the array (of course, that would be wasted space). With the "in > use" bitmap, they would replace the disk and only the areas that had been > written to would be resynced over to the new disk. The rest, since it had > not been used, would not need to be. > > A side effect of this would be that a check or a resync could use this to > check the real data (IE on a weekly basis) and take less time. > > Over all, depending on the usage, this can keep the wear and tear on a disk > down. I'm speaking of personal experience with my systems. I have arrays > that are not 100% or even 80% used. I have some production servers that > have extra space for expansion and not fully used. > > I'm sure this would take some time to implement if someone does this. As I > mentioned at the beginning, this was just a thought, but I think it could > benefit people if it were implemented. > > I am on the list, but feel free to keep me in the CC. > I think you are suggesting this: http://neil.brown.name/blog/20110216044002#5 ?? Patches welcome :-) NeilBrown
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