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,
