Re: Building a new raid6 with bitmap does not clear bits during resync

[Neil Brown <neilb@suse.de>]

On Monday November 12, davidsen@tmr.com wrote:
> Neil Brown wrote:
> >
> > However there is value in regularly updating the bitmap, so add code
> > to periodically pause while all pending sync requests complete, then
> > update the bitmap.  Doing this only every few seconds (the same as the
> > bitmap update time) does not notciable affect resync performance.
> >   
> 
> I wonder if a minimum time and minimum number of stripes would be 
> better. If a resync is going slowly because it's going over a slow link 
> to iSCSI, nbd, or a box of cheap drives fed off a single USB port, just 
> writing the updated bitmap may represent as much data as has been 
> resynced in the time slice.
> 
> Not a suggestion, but a request for your thoughts on that.

Thanks for your thoughts.
Choosing how often to update the bitmap during a sync is certainly not
trivial.   In different situations, different requirements might rule.

I chose to base it on time, and particularly on the time we already
have for "how soon to write back clean bits to the bitmap" because it
is fairly easy to users to understand the implications (if I set the
time to 30 seconds, then I might have to repeat 30second of resync)
and it is already configurable (via the "--delay" option to --create
--bitmap).

Presumably if someone has a very slow system and wanted to use
bitmaps, they would set --delay relatively large to reduce the cost
and still provide significant benefits.  This would effect both normal
clean-bit writeback and during-resync clean-bit-writeback.

Hope that clarifies my approach.

Thanks,
NeilBrown