Re: [RFC] add ovp valid_blocks check for bg gc victim to fg_gc

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 02/17, Hou Pengyang wrote:

...

> > > On 2017/2/17 7:48, Jaegeuk Kim wrote:
> > > > +	ovp_count = SM_I(sbi)->ovp_segments << sbi->log_blocks_per_seg;
> > > 
> > > About the ovp_count calculation,
> > > 
> > > in mkfs.f2fs, we get ovp_segment by
> > > 
> > >      set_cp(overprov_segment_count, (get_sb(segment_count_main) -
> > >             get_cp(rsvd_segment_count)) *
> > >              config.overprovision / 100);
> > > 
> > >      set_cp(overprov_segment_count, get_cp(overprov_segment_count) +
> > >              get_cp(rsvd_segment_count));
> > > 
> > > where the overprov calculation is based on the space excluding the
> > > rsvd segment, and the final overprov_segment is sum of the REAL
> > > overprov segments and the rsvd ones.
> > > 
> > > So, when to calculate the overprov ratio, the rsvd segments should
> > > be subtracted from the ckpt->overprov_semgents?
> > 
> > I just got calculation from fresh mounted image. What I could confirm was that
> > user can see (main_segments - ovp_segments).
> 
> I add some log into mkfs.f2fs, and get the config.overprovision,
> rsvd/ovrp segments, user_block_count, and use the mkfs.f2fs to formatting a
> 512M disk, where:
> 
> config.overprovision: 10
> 
> ovrp : 50 segments(25600 blocks)
> rsvd : 28 segments(14336 blocks)
> user_block_count: 101376 blocks
> 
> Use ovrp/rsvd/user_block_count to get the ovrp ratio info reversely :
> 
> 1) user_block_count/(user_block_count + ovrp blocks)
> 
>   101376/(101376+25600) = 79%
> 
> 2) user_block_count/(user_block_count + ovrp_blocks - rsvd_blocks)
> 
>   101376/(101376+25600-14336) = 90% = 1-10%
> 
> By 2), 1-90%, we can get the original config.overprovion when mkfs.f2fs
> So I think we should use 2) to calculate ovrp ratio. Anything I miss
> in 2)?

Yup, indeed 2) is correct, which I found my calcuation was wrong, and tried
to verify with:

threshold = (main_segments - ovp_segments) / (main_segments - reserved_segments)
            ------------------------------    ---------------------------------
                User-visible space                  F2FS-writable space
              (user_block_count) / (user_block_count + ovp_blocks - rsvd_blocks)

For example,

1. 8GB
 ovp reported by mkfs = 2.22%
 main_segments = 4065
 ovp_segments = 186
 reserved_segments = 98
 threshold = 500 (mine) vs. 502 (yours)

 -> Actual ovp = (512-500) / 512 = 2.34%
 -> reserved segments = 98 / 2 = 49 can cover 2.34%

2. 64GB
 ovp reported by mkfs = 0.78%
 main_segments = 32581
 ovp_segments = 516
 reserved_segments = 264
 threshold = 508 (mine) vs. 512 (yours)

 -> Actual ovp = (512-508) / 512 = 0.78%
 -> reserved segments = 264 / 2 = 132 can cover 0.78%

Thanks,

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