Re: Reservation amount

[Chao Yu <chao2.yu@samsung.com>]

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> Sent: Wednesday, October 21, 2015 2:32 AM
> To: Chao Yu
> Cc: 'Chao Yu'; linux-f2fs-devel@lists.sourceforge.net
> Subject: Re: [f2fs-dev] Reservation amount
> 
> On Tue, Oct 20, 2015 at 11:31:45PM +0800, Chao Yu wrote:
> > Hi Jaegeuk,
> >
> > > -----Original Message-----
> > > From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> > > Sent: Tuesday, October 20, 2015 2:59 AM
> > > To: Chao Yu; Chao Yu
> > > Cc: linux-f2fs-devel@lists.sourceforge.net; linux-f2fs-devel@lists.sourceforge.net
> > > Subject: Re: [f2fs-dev] Reservation amount
> > >
> > > Hello,
> > >
> > > On Mon, Oct 19, 2015 at 06:44:40PM +0800, Chao Yu wrote:
> > > > Hi Jaegeuk,
> > > >
> > > > > -----Original Message-----
> > > > > From: Josh Dener [mailto:denerjosh@yahoo.com]
> > > > > Sent: Sunday, October 18, 2015 12:58 PM
> > > > > To: linux-f2fs-devel@lists.sourceforge.net
> > > > > Subject: [f2fs-dev] Reservation amount
> > > > >
> > > > > Hi,
> > > > >
> > > > > Where does this formula come from?
> > > > >
> > > > >
> > > > > config.reserved_segments =
> > > > >
> > > > >     (2 * (100 / config.overprovision + 1) + 6)
> > >
> > > What I've intended here was to handle the worst case of foreground gc.
> > > If a data segment is selected as a victim, all the valid data blocks would be
> > > migrated, resulting in producing a same number of dirty node blocks in the worse
> > > case. Then, checkpoint should write such the node blocks in order to reclaim the
> > > prefree segments accordingly. Therefore, we require x2 segments.
> > > 6 means the number of active segments, which is actually not a exact number
> > > though. :)
> >
> > Ah, I see, but one more thing is why using (100 / config.overprovision) as
> > the base segment number of reserved segments calculation? Let me guess, is
> > that not accurate too? the intention here is to balance in between ovp and
> > rsv segments number, not leading the (ovp + rsv) number higher? :)
> 
> For example, if ovp is 20%, we select 5 victim segments to reclaim one free
> segment in the worst case. During this migration, we need additional 4 free
> segments to write valid blocks in the victim segments.
> Other remaining added segments are just to keep as a buffer to prepare any
> abnormal situation.

I can understand now, if ovp is 20%, in worst case, each potential victim
segment may contain ~4/5 * blk_per_seg valid blocks, so we will write all
blocks (4/5 * blk_per_seg * 5) of 5 selected victim segments to reserved
4 segments, eventually we can reclaim one free segment.

So actually '100 / config.overprovision' is used to calculate the victim
segment number which we can reclaim one free segment from in the worst case.

Thanks a lot for your explanation! :)

Thanks,

> 
> Thanks,
> 
> >
> > Thanks,
> >
> > >
> > > Thanks,
> > >
> > > >
> > > > I'm interest in this too, could you sharing more detail about policy
> > > > of overprovision and reserved space calculation?
> > > >
> > > > Thanks,
> > > >
> > > > >
> > > > >
> > > > > Is there a formal justification for it?
> > > > >
> > > > > Thanks.
> > > > >
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