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

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:
> 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).
>   

Sounds right, that part of it is pretty user friendly.
> 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.
>   

Easy to implement and understand is always a strong point, and a user 
can make an informed decision. Thanks for the discussion.

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979