Re: Thought about delayed sync

[NeilBrown <neilb@suse.de>]

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On Sat, 8 Oct 2011 14:03:10 -0400 Wakko Warner <wakko@animx.eu.org> 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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