Re: [PATCH v3 1/4] f2fs: Lower threshold for disable_cp_again

[Chao Yu]

On 2019/5/30 8:49, Daniel Rosenberg wrote:
> The existing threshold for allowable holes at checkpoint=disable time is
> too high. The OVP space contains reserved segments, which are always in
> the form of free segments. These must be subtracted from the OVP value.
> 
> The current threshold is meant to be the maximum value of holes of a
> single type we can have and still guarantee that we can fill the disk
> without failing to find space for a block of a given type.
> 
> If the disk is full, ignoring current reserved, which only helps us,
> the amount of unused blocks is equal to the OVP area. Of that, there
> are reserved segments, which must be free segments, and the rest of the
> ovp area, which can come from either free segments or holes. The maximum
> possible amount of holes is OVP-reserved.
> 
> Now, consider the disk when mounting with checkpoint=disable.
> We must be able to fill all available free space with either data or
> node blocks. When we start with checkpoint=disable, holes are locked to
> their current type. Say we have H of one type of hole, and H+X of the
> other. We can fill H of that space with arbitrary typed blocks via SSR.
> For the remaining H+X blocks, we may not have any of a given block type
> left at all. For instance, if we were to fill the disk entirely with
> blocks of the type with fewer holes, the H+X blocks of the opposite type
> would not be used. If H+X > OVP-reserved, there would be more holes than
> could possibly exist, and we would have failed to find a suitable block
> earlier on, leading to a crash in update_sit_entry.
> 
> If H+X <= OVP-reserved, then the holes end up effectively masked by the OVP
> region in this case.
> 
> Signed-off-by: Daniel Rosenberg <drosen@xxxxxxxxxx>

Reviewed-by: Chao Yu <yuchao0@xxxxxxxxxx>

Thanks,