Re: [PATCH 2/6] nilfs2: add new timestamp to seg usage and function to change su_nblocks

[Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org>]

On 2014-03-16 14:34, Vyacheslav Dubeyko wrote:
> 
>> On 16 марта 2014 г., at 16:24, Andreas Rohner <andreas.rohner-hi6Y0CQ0nG0@public.gmane.org> wrote:
>>
>>> On 2014-03-16 14:00, Vyacheslav Dubeyko wrote:
>>>
>>>> On Mar 16, 2014, at 1:47 PM, Andreas Rohner wrote:
>>>>
>>>> This patch adds an additional timestamp to the segment usage
>>>> information that indicates the last time the usage information was
>>>> changed. So su_lastmod indicates the last time the segment itself was
>>>> modified and su_lastdec indicates the last time the usage information
>>>> itself was changed.
>>>
>>> What will we have if user changes time?
>>> What sequence will we have after such "malicious" action?
>>> Did you test such situation?
>>
>> The timestamp is just a hint for the userspace GC. If the hint is wrong
>> the result would be that the GC is less efficient for a while. After a
>> while it would go back to normal. You have the same problem with the
>> already existing su_lastmod timestamp.
>>
> 
> But I worry about such thing. Previously, we had complains of users about
> different issues with timestamp policy of GC. And I had hope that namely
> new GC policies can resolve such GC disadvantage. So, what have we again?
> The same issue of GC?

Yes but I have to compare it to the protection period, which is a
timestamp. Maybe I could use the current checkpoint number instead...

Regards,
Andreas Rohner

> Thanks,
> Vyacheslav Dubeyko.
> 
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