A computer-implemented method according to one embodiment includes, for each portion of data in a write cache: determining whether a given portion of data was added to the write cache prior to completion of a most recent flash copy operation. In response to determining that the given portion of data was not added to the write cache prior to completion of a most recent flash copy operation, a determination is made of whether the given portion of data has a clock bit value corresponding thereto. In response to determining that the given portion of data does not have a clock bit value corresponding thereto, a clock bit value calculated for the given portion of data based on a current amount of unused storage capacity in the write cache. Moreover, in response to determining that the given portion of data has a clock bit value corresponding thereto, it is decremented.