Neil Brown <neilb@xxxxxxx> writes:
> On Monday November 12, davidsen@xxxxxxx 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
We are talking about 12-24h resync times here under idle
conditions. Syncing only once per minute is perfectly acceptable.
MfG
Goswin
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